LM339 Comparator input limits?

B

Bill Bowden

Guest
The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill
 
On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden
<bperryb@bowdenshobbycircuits.info> wrote:

The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill
Click to expand...
The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
 
On Thu, 24 May 2012 07:29:05 -0700 John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in Message id:
<34hsr7p3tp0msi7g1om00edd6m0m08to95@4ax.com>:

On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:

The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill

The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.
Click to expand...
National was the worst eh? That explains why they don't work so well in a
HP 8116A function generator's reset circuit. Which manufacturer's part is
the least sensitive?
 
On Thu, 24 May 2012 11:08:31 -0400, JW <none@dev.null> wrote:

On Thu, 24 May 2012 07:29:05 -0700 John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in Message id:
34hsr7p3tp0msi7g1om00edd6m0m08to95@4ax.com>:

On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:

The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill

The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.

National was the worst eh? That explains why they don't work so well in a
HP 8116A function generator's reset circuit. Which manufacturer's part is
the least sensitive?
Click to expand...
I don't know. JT claimed that the Motorola parts were better, but I
don't know if they can still be had... perhaps from OnSemi. There are
probably drop-ins from someone else, with a different process. A
honkin' schottky clamp diode will probably help.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
 
On Thu, 24 May 2012 08:23:12 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 24 May 2012 11:08:31 -0400, JW <none@dev.null> wrote:

On Thu, 24 May 2012 07:29:05 -0700 John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in Message id:
34hsr7p3tp0msi7g1om00edd6m0m08to95@4ax.com>:

On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:

The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill

The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.

National was the worst eh? That explains why they don't work so well in a
HP 8116A function generator's reset circuit. Which manufacturer's part is
the least sensitive?

I don't know. JT claimed that the Motorola parts were better, but I
don't know if they can still be had... perhaps from OnSemi. There are
probably drop-ins from someone else, with a different process. A
honkin' schottky clamp diode will probably help.
Click to expand...
When I last used them in production, late '80's, Motorola's 339 was
best because they had split up the bias; but that same bias structure
implemented in their 324 had the worst cross-over distortion.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
On Thu, 24 May 2012 18:00:20 -0700 (PDT), Bill Bowden
<bperryb@bowdenshobbycircuits.info> wrote:

On May 24, 7:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden

bper...@bowdenshobbycircuits.info> wrote:
The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill

The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.


How about using a diode across the input to limit the negative
voltage? The comparator input reads about 1000 using a DMM to check
diode drops. A silicon diode reads about 800, so it seems a diode
across the comparator input to ground would absorb all the reverse
current?

-Bill
Click to expand...
As I recall, a silicon PN diode isn't good enough. The chip screws up
at lower voltages than you'd expect from the ESD diode drop. Both the
Fairchild and National data sheets rate the abs max input voltage
range as -0.3 to +36. That -0.3 number seems to be serious.

A power schottky diode might work, if it keeps the negative swing
below -0.3.


--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
 
On May 24, 7:29 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden

bper...@bowdenshobbycircuits.info> wrote:
The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill

The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.
Click to expand...
How about using a diode across the input to limit the negative
voltage? The comparator input reads about 1000 using a DMM to check
diode drops. A silicon diode reads about 800, so it seems a diode
across the comparator input to ground would absorb all the reverse
current?

-Bill
 
On Thu, 24 May 2012 18:14:18 -0700, John Larkin
<jlarkin@highlandtechnology.com> wrote:

On Thu, 24 May 2012 18:00:20 -0700 (PDT), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:

On May 24, 7:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden

bper...@bowdenshobbycircuits.info> wrote:
The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill

The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.


How about using a diode across the input to limit the negative
voltage? The comparator input reads about 1000 using a DMM to check
diode drops. A silicon diode reads about 800, so it seems a diode
across the comparator input to ground would absorb all the reverse
current?

-Bill

As I recall, a silicon PN diode isn't good enough. The chip screws up
at lower voltages than you'd expect from the ESD diode drop. Both the
Fairchild and National data sheets rate the abs max input voltage
range as -0.3 to +36. That -0.3 number seems to be serious.

A power schottky diode might work, if it keeps the negative swing
below -0.3.
Click to expand...
The "quality" way to do it, avoiding leaky Schottky's or finding a
Germanium diode, is an active clamp, as I demonstrated several years
ago in a 4-20mA circuit for Spehro...

http://www.analog-innovations.com/SED/SpehroLoopProtect2.pdf

The 324/339 input stage architecture is a Darlington differential pair
made up of lateral PNP's, with an NPN mirror load on the inner pair of
PNP's.

This "screws up" if driven below ground by a Vbe, because a lateral
device doesn't know its emitter from its collector, never mind a hole
in the ground.

Thus a positive input pulled below ground causes a phase reversal and
the output rises instead of continuing to fall.

Driven hard, this could inject current into the substrate and upset
things, but I've never seen that... rational designers limit current
;-)

Another effect is what I think Larkin confuses with the front-end
below ground... take inputs too high and lateral PNP mirrors saturate.
Cheapy approaches, in certain brands, used split collector
arrangements rather than separate-well devices, so one mirror
saturating screws all sections of the device.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Jim Thompson wrote:
On Thu, 24 May 2012 18:14:18 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:


On Thu, 24 May 2012 18:00:20 -0700 (PDT), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:


On May 24, 7:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden

bper...@bowdenshobbycircuits.info> wrote:

The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill

The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.


How about using a diode across the input to limit the negative
voltage? The comparator input reads about 1000 using a DMM to check
diode drops. A silicon diode reads about 800, so it seems a diode
across the comparator input to ground would absorb all the reverse
current?

-Bill

As I recall, a silicon PN diode isn't good enough. The chip screws up
at lower voltages than you'd expect from the ESD diode drop. Both the
Fairchild and National data sheets rate the abs max input voltage
range as -0.3 to +36. That -0.3 number seems to be serious.

A power schottky diode might work, if it keeps the negative swing
below -0.3.


The "quality" way to do it, avoiding leaky Schottky's or finding a
Germanium diode, is an active clamp, as I demonstrated several years
ago in a 4-20mA circuit for Spehro...

http://www.analog-innovations.com/SED/SpehroLoopProtect2.pdf

The 324/339 input stage architecture is a Darlington differential pair
made up of lateral PNP's, with an NPN mirror load on the inner pair of
PNP's.

This "screws up" if driven below ground by a Vbe, because a lateral
device doesn't know its emitter from its collector, never mind a hole
in the ground.

Thus a positive input pulled below ground causes a phase reversal and
the output rises instead of continuing to fall.

Driven hard, this could inject current into the substrate and upset
things, but I've never seen that... rational designers limit current
;-)

Another effect is what I think Larkin confuses with the front-end
below ground... take inputs too high and lateral PNP mirrors saturate.
Cheapy approaches, in certain brands, used split collector
arrangements rather than separate-well devices, so one mirror
saturating screws all sections of the device.

...Jim Thompson
Click to expand...
VCC
|
-
^ high speed diode
+
lowest R possible |
___ |
+------+|___|+--+---------------+ To Comp input
Unknown signal |+
|
+
-
^ high speed diode
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I am just throwing shit again the wall !

Jamie
 
On Sat, 02 Jun 2012 16:50:10 -0400, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

Jim Thompson wrote:
On Thu, 24 May 2012 18:14:18 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:


On Thu, 24 May 2012 18:00:20 -0700 (PDT), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:


On May 24, 7:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden

bper...@bowdenshobbycircuits.info> wrote:

The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill

The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.


How about using a diode across the input to limit the negative
voltage? The comparator input reads about 1000 using a DMM to check
diode drops. A silicon diode reads about 800, so it seems a diode
across the comparator input to ground would absorb all the reverse
current?

-Bill

As I recall, a silicon PN diode isn't good enough. The chip screws up
at lower voltages than you'd expect from the ESD diode drop. Both the
Fairchild and National data sheets rate the abs max input voltage
range as -0.3 to +36. That -0.3 number seems to be serious.

A power schottky diode might work, if it keeps the negative swing
below -0.3.


The "quality" way to do it, avoiding leaky Schottky's or finding a
Germanium diode, is an active clamp, as I demonstrated several years
ago in a 4-20mA circuit for Spehro...

http://www.analog-innovations.com/SED/SpehroLoopProtect2.pdf

The 324/339 input stage architecture is a Darlington differential pair
made up of lateral PNP's, with an NPN mirror load on the inner pair of
PNP's.

This "screws up" if driven below ground by a Vbe, because a lateral
device doesn't know its emitter from its collector, never mind a hole
in the ground.

Thus a positive input pulled below ground causes a phase reversal and
the output rises instead of continuing to fall.

Driven hard, this could inject current into the substrate and upset
things, but I've never seen that... rational designers limit current
;-)

Another effect is what I think Larkin confuses with the front-end
below ground... take inputs too high and lateral PNP mirrors saturate.
Cheapy approaches, in certain brands, used split collector
arrangements rather than separate-well devices, so one mirror
saturating screws all sections of the device.

...Jim Thompson

VCC
|
-
^ high speed diode
+
lowest R possible |
___ |
+------+|___|+--+---------------+ To Comp input
Unknown signal |+
|
+
-
^ high speed diode
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I am just throwing shit again the wall !

Jamie
Click to expand...
It's not a speed issue... an LM339 is a micro-second range slug-speed
device. To avoid the problem, you need a clamp that holds at a
voltage less than a silicon Vbe forward.

AND: Your diode to VCC is useless, you will have already exceeded the
allowed common-mode range (about 1.5V BELOW VCC) before it conducts.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Jim Thompson wrote:

On Sat, 02 Jun 2012 16:50:10 -0400, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:


Jim Thompson wrote:

On Thu, 24 May 2012 18:14:18 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:



On Thu, 24 May 2012 18:00:20 -0700 (PDT), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:



On May 24, 7:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:


On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden

bper...@bowdenshobbycircuits.info> wrote:


The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill

The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.


How about using a diode across the input to limit the negative
voltage? The comparator input reads about 1000 using a DMM to check
diode drops. A silicon diode reads about 800, so it seems a diode
across the comparator input to ground would absorb all the reverse
current?

-Bill

As I recall, a silicon PN diode isn't good enough. The chip screws up
at lower voltages than you'd expect from the ESD diode drop. Both the
Fairchild and National data sheets rate the abs max input voltage
range as -0.3 to +36. That -0.3 number seems to be serious.

A power schottky diode might work, if it keeps the negative swing
below -0.3.


The "quality" way to do it, avoiding leaky Schottky's or finding a
Germanium diode, is an active clamp, as I demonstrated several years
ago in a 4-20mA circuit for Spehro...

http://www.analog-innovations.com/SED/SpehroLoopProtect2.pdf

The 324/339 input stage architecture is a Darlington differential pair
made up of lateral PNP's, with an NPN mirror load on the inner pair of
PNP's.

This "screws up" if driven below ground by a Vbe, because a lateral
device doesn't know its emitter from its collector, never mind a hole
in the ground.

Thus a positive input pulled below ground causes a phase reversal and
the output rises instead of continuing to fall.

Driven hard, this could inject current into the substrate and upset
things, but I've never seen that... rational designers limit current
;-)

Another effect is what I think Larkin confuses with the front-end
below ground... take inputs too high and lateral PNP mirrors saturate.
Cheapy approaches, in certain brands, used split collector
arrangements rather than separate-well devices, so one mirror
saturating screws all sections of the device.

...Jim Thompson

VCC
|
-
^ high speed diode
+
lowest R possible |
___ |
+------+|___|+--+---------------+ To Comp input
Unknown signal |+
|
+
-
^ high speed diode
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I am just throwing shit again the wall !

Jamie


It's not a speed issue... an LM339 is a micro-second range slug-speed
device. To avoid the problem, you need a clamp that holds at a
voltage less than a silicon Vbe forward.

AND: Your diode to VCC is useless, you will have already exceeded the
allowed common-mode range (about 1.5V BELOW VCC) before it conducts.

...Jim Thompson
Click to expand...
How so? the reference isn't passing through 2 diodes, it is only doing
one diode, depending on if it's a low or high signal coming in?

The idea is to have the diodes only conduct when the input exceeds
either way by a margin of 0.7 volts.?

You being the semi engineer, I most likely am over looking something
very obvious to you. I have used that before and it has worked for me.
Maybe I got lucky ?


Jamie
 
On Sat, 02 Jun 2012 18:04:34 -0400, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

Jim Thompson wrote:

On Sat, 02 Jun 2012 16:50:10 -0400, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:


Jim Thompson wrote:

On Thu, 24 May 2012 18:14:18 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:



On Thu, 24 May 2012 18:00:20 -0700 (PDT), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:



On May 24, 7:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:


On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden

bper...@bowdenshobbycircuits.info> wrote:


The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill

The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.


How about using a diode across the input to limit the negative
voltage? The comparator input reads about 1000 using a DMM to check
diode drops. A silicon diode reads about 800, so it seems a diode
across the comparator input to ground would absorb all the reverse
current?

-Bill

As I recall, a silicon PN diode isn't good enough. The chip screws up
at lower voltages than you'd expect from the ESD diode drop. Both the
Fairchild and National data sheets rate the abs max input voltage
range as -0.3 to +36. That -0.3 number seems to be serious.

A power schottky diode might work, if it keeps the negative swing
below -0.3.


The "quality" way to do it, avoiding leaky Schottky's or finding a
Germanium diode, is an active clamp, as I demonstrated several years
ago in a 4-20mA circuit for Spehro...

http://www.analog-innovations.com/SED/SpehroLoopProtect2.pdf

The 324/339 input stage architecture is a Darlington differential pair
made up of lateral PNP's, with an NPN mirror load on the inner pair of
PNP's.

This "screws up" if driven below ground by a Vbe, because a lateral
device doesn't know its emitter from its collector, never mind a hole
in the ground.

Thus a positive input pulled below ground causes a phase reversal and
the output rises instead of continuing to fall.

Driven hard, this could inject current into the substrate and upset
things, but I've never seen that... rational designers limit current
;-)

Another effect is what I think Larkin confuses with the front-end
below ground... take inputs too high and lateral PNP mirrors saturate.
Cheapy approaches, in certain brands, used split collector
arrangements rather than separate-well devices, so one mirror
saturating screws all sections of the device.

...Jim Thompson

VCC
|
-
^ high speed diode
+
lowest R possible |
___ |
+------+|___|+--+---------------+ To Comp input
Unknown signal |+
|
+
-
^ high speed diode
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I am just throwing shit again the wall !

Jamie


It's not a speed issue... an LM339 is a micro-second range slug-speed
device. To avoid the problem, you need a clamp that holds at a
voltage less than a silicon Vbe forward.

AND: Your diode to VCC is useless, you will have already exceeded the
allowed common-mode range (about 1.5V BELOW VCC) before it conducts.

...Jim Thompson

How so? the reference isn't passing through 2 diodes, it is only doing
one diode, depending on if it's a low or high signal coming in?
Click to expand...
After I pressed "Send" I realized that an LM339 will be OK CM-wise if
the reference is 1.5V below rail.

Not so for the LM324...presuming you have feedback ;-)

The idea is to have the diodes only conduct when the input exceeds
either way by a margin of 0.7 volts.?
Click to expand...
You don't need a diode in the positive direction unless you talking
like +30V. The positive direction applies _reverse_ potentials to the
lateral junctions... they breakdown at the same voltage as the NPN
BVcbo, 30-35V

You being the semi engineer, I most likely am over looking something
very obvious to you. I have used that before and it has worked for me.
Maybe I got lucky ?


Jamie
Click to expand...
As I pointed out above, the positive-direction diode isn't needed. In
the negative direction, most discrete diode Vbe's are smaller than
that of the 339 silicon, so silicon diodes are OK... most of the time.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
Jim Thompson wrote:
On Sat, 02 Jun 2012 16:50:10 -0400, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

Jim Thompson wrote:
On Thu, 24 May 2012 18:14:18 -0700, John Larkin
jlarkin@highlandtechnology.com> wrote:


On Thu, 24 May 2012 18:00:20 -0700 (PDT), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:


On May 24, 7:29 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden

bper...@bowdenshobbycircuits.info> wrote:

The LM339 spec sheet indicates the maximum negative input voltage
should not be more than -0.3 volts. The schematic shows the input
connected to the base of a PNP transistor with the collector at the
negative supply terminal. So, it looks like if the input or base of
the transistor goes more negative than about 0.7 volts relative to the
negative supply point, the transistor will conduct like a diode from
base to collector. Is this situation acceptable, and will the
comparator work with one input equal to the negative supply (0 volts)
while the other is -0.7 volts below the negative supply point?

-Bill

The 339, like the LM324, has some sneak paths that squirt current all
around the chip if you pull any input much below ground. One such
pulled-low input, even a tiny current, can freak out all four channels
in weird ways. It's better to not allow any input to go negative. The
severity seems to vary among vendors, with the classic National part
being really, really bad.


How about using a diode across the input to limit the negative
voltage? The comparator input reads about 1000 using a DMM to check
diode drops. A silicon diode reads about 800, so it seems a diode
across the comparator input to ground would absorb all the reverse
current?

-Bill

As I recall, a silicon PN diode isn't good enough. The chip screws up
at lower voltages than you'd expect from the ESD diode drop. Both the
Fairchild and National data sheets rate the abs max input voltage
range as -0.3 to +36. That -0.3 number seems to be serious.

A power schottky diode might work, if it keeps the negative swing
below -0.3.


The "quality" way to do it, avoiding leaky Schottky's or finding a
Germanium diode, is an active clamp, as I demonstrated several years
ago in a 4-20mA circuit for Spehro...

http://www.analog-innovations.com/SED/SpehroLoopProtect2.pdf

The 324/339 input stage architecture is a Darlington differential pair
made up of lateral PNP's, with an NPN mirror load on the inner pair of
PNP's.

This "screws up" if driven below ground by a Vbe, because a lateral
device doesn't know its emitter from its collector, never mind a hole
in the ground.

Thus a positive input pulled below ground causes a phase reversal and
the output rises instead of continuing to fall.

Driven hard, this could inject current into the substrate and upset
things, but I've never seen that... rational designers limit current
;-)

Another effect is what I think Larkin confuses with the front-end
below ground... take inputs too high and lateral PNP mirrors saturate.
Cheapy approaches, in certain brands, used split collector
arrangements rather than separate-well devices, so one mirror
saturating screws all sections of the device.

...Jim Thompson

VCC
|
-
^ high speed diode
+
lowest R possible |
___ |
+------+|___|+--+---------------+ To Comp input
Unknown signal |+
|
+
-
^ high speed diode
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I am just throwing shit again the wall !

Jamie

It's not a speed issue... an LM339 is a micro-second range slug-speed
device. To avoid the problem, you need a clamp that holds at a
voltage less than a silicon Vbe forward.

AND: Your diode to VCC is useless, you will have already exceeded the
allowed common-mode range (about 1.5V BELOW VCC) before it conducts.
Click to expand...

His "lowest R possible" would be zero.


--
You can't have a sense of humor, if you have no sense.
 
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