Is this op-amp uV/C that high?

[John Fields]

On Wed, 10 Sep 2008 12:43:54 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 10 Sep 2008 14:31:42 -0500, John Fields
jfields@austininstruments.com> wrote:

On Wed, 10 Sep 2008 13:13:06 -0400, Spehro Pefhany
speffSNIP@interlogDOTyou.knowwhat> wrote:

On Wed, 10 Sep 2008 11:56:37 -0500, John Fields
jfields@austininstruments.com> wrote:

On Wed, 10 Sep 2008 09:35:51 -0700 (PDT), Paul <energymover@gmail.com
wrote:

On Sep 10, 9:07 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
You could temperature control an opamp, or put a temperature sensor
nearby and tweak a compensation factor.

Thanks for the good advice. I just have a bit of experience in temp
controlling op-amps, but it sounds worth it. I nearly have a truck
load
of thermistors ranging from 402 SMD to the typical large round
versions. As far as I know it would take a bit of trial and error to
calibrate such a thermistor offset circuit for the op-amp? Also I
suppose there's no reason to add more thermal insulation to prevent
the op-amp temperature from varying as much.

---
"No reason _not_ to add", yes? Also,:

http://www.isotemp.com/product.html

JF

One reason not to put an electrometer amplifier in an oven is that
you'll greatly increase the input bias current at normal operating
temperatures (since the oven has to run somewhat hotter than the
hottest possible Ta to be effective).

---
OK, then, how about in a little Peltier fridge?

JF

Just be very careful about condensation.

---
Evacuated, back-filled with dry N2, and hermetically sealed oughta do
it.

JF

 

[George Herold]

I had a possibly good idea. By placing a double switch on the input
DUT, in this case a mechanical switch would be most appropriate IMO, I
could flip the polarity of the DUT across the electrometer input and
the change in DC voltage output divided by gain and divided by 2
equals the true DC voltage input.

So far example, say the DUT is +10uV DC, and the electrometer output
is 120mV DC. The switch now reverses the polarity of the DUT such that
the DUT is now -10uV DC relative to the electrometer. So now the
electrometer output is say 100mV DC. Given a gain of 1000 the DUT is
((120mV - 100mV) / 2) / 1000 = 10uV.

Is that a conventional method?

Thanks,
Paul- Hide quoted text -

- Show quoted text -

I had a possibly good idea. By placing a double switch on the input
DUT, in this case a mechanical switch would be most appropriate IMO, I
could flip the polarity of the DUT across the electrometer input and
the change in DC voltage output divided by gain and divided by 2
equals the true DC voltage input.

Hi Paul, This sounds like you're "inventing" a lock-in technique as
suggested by John. Can you instead modulate the bias current to the
diodes? or some other parameter? Then you look at the signal at the
modulation frequency and the unwanted DC offsets go away. (Well they
don't go away but they are stuck there at DC.)
George

 

[mpm]

On Sep 10, 11:21�am, Paul <energymo...@gmail.com> wrote:

I'm try to figure what the offset voltage temp drift for the Ina116P
op-amp,

Ina116P datasheet:http://focus.ti.com/lit/ds/symlink/ina116.pdf

At the bottom left side of page 5 is a chart. Looks like there's an
appreciable chance of getting an op-amp that near 20uV/C drift! �I
would like to build an electrometer that has extremely low input bias
current and low drift, but that kind of drift seems far to much. If
it's 20uV/C and a gain of 1000 then it's 20mV drift per C, correct?
I'm more used to op-amps like the AD8551, which has a typical drift of
0.005uV/C.

AD8551 datasheet:http://www.analog.com/static/imported-files/Data_Sheets/AD8551_8552_8...

I appreciate any help or advice.
Paul

What exactly is an "electrometer"?

 

[Jim Thompson]

On Wed, 10 Sep 2008 15:36:52 -0700 (PDT), miso@sushi.com wrote:

On Sep 10, 8:21 am, Paul <energymo...@gmail.com> wrote:
I'm try to figure what the offset voltage temp drift for the Ina116P
op-amp,

Ina116P datasheet:http://focus.ti.com/lit/ds/symlink/ina116.pdf

At the bottom left side of page 5 is a chart. Looks like there's an
appreciable chance of getting an op-amp that near 20uV/C drift!  I
would like to build an electrometer that has extremely low input bias
current and low drift, but that kind of drift seems far to much. If
it's 20uV/C and a gain of 1000 then it's 20mV drift per C, correct?
I'm more used to op-amps like the AD8551, which has a typical drift of
0.005uV/C.

AD8551 datasheet:http://www.analog.com/static/imported-files/Data_Sheets/AD8551_8552_8...

I appreciate any help or advice.
Paul

You got some good posts below. My question is can you contact the
factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience
that drift gets worse when you add hooks to the circuit to correct for
offset. That is, a part with high offset (untrimmed) may have lower
drift than the same part that has been trimmed.

Yep. Nothing has been said, AFAICR, about bandwidth. In similar
cases I've made a zero-measure-zero-measure... system. Then you don't
give a hoot about offset or offset drift.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Obama snickers, claiming McCain is running for Bush's third term,
While he thinks running for Carter's second term is a good thing?

 

[George Herold]

What exactly is an "electrometer?

A Voltmeter with infinite input impedance.

 

[George Herold]

---
Evacuated, back-filled with dry N2, and hermetically sealed oughta do
it.

Way to much work, just servo the temperature to stay at Room temp.
George

 

[Spehro Pefhany]

On Wed, 10 Sep 2008 14:31:42 -0500, the renowned John Fields
<jfields@austininstruments.com> wrote:

On Wed, 10 Sep 2008 13:13:06 -0400, Spehro Pefhany
speffSNIP@interlogDOTyou.knowwhat> wrote:

....

One reason not to put an electrometer amplifier in an oven is that
you'll greatly increase the input bias current at normal operating
temperatures (since the oven has to run somewhat hotter than the
hottest possible Ta to be effective).

---
OK, then, how about in a little Peltier fridge?

JF

Maybe something like this? The thermal mass of the liquid-filled
cylinder would help stabilize the temperature, acting as a sort of
output bypass capacitor (provided the cylinder did not somehow lose
its seal).

http://www.avantgardegadgets.com/mr-103.html



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

On Sep 10, 8:21 am, Paul <energymo...@gmail.com> wrote:

I'm try to figure what the offset voltage temp drift for the Ina116P
op-amp,

Ina116P datasheet:http://focus.ti.com/lit/ds/symlink/ina116.pdf

At the bottom left side of page 5 is a chart. Looks like there's an
appreciable chance of getting an op-amp that near 20uV/C drift!  I
would like to build an electrometer that has extremely low input bias
current and low drift, but that kind of drift seems far to much. If
it's 20uV/C and a gain of 1000 then it's 20mV drift per C, correct?
I'm more used to op-amps like the AD8551, which has a typical drift of
0.005uV/C.

AD8551 datasheet:http://www.analog.com/static/imported-files/Data_Sheets/AD8551_8552_8...

I appreciate any help or advice.
Paul

You got some good posts below. My question is can you contact the
factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience
that drift gets worse when you add hooks to the circuit to correct for
offset. That is, a part with high offset (untrimmed) may have lower
drift than the same part that has been trimmed.

 

[whit3rd]

On Sep 10, 3:36 pm, m...@sushi.com wrote:

... can you contact the
factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience
that drift gets worse when you add hooks to the circuit to correct for
offset.

Don't pay for screened devices. It's unlikely to be easy for the
factory (unless they already do it-and charge extra for 'B' versions).
It's also unlikely to survive shipping and soldering and aging.
Your best bet is to trim it after all assembly is complete.

Heck, thermal gradients in the printed circuit board can
create thermocouple voltages, you HAVE to trim it
after assembly and warmup.

It's possible to trim a uA741 for minimum Voffset (the usual way),
or to sense the input stage current at the adjust pins, and
trim for matched emitter current. Zeroing the emitter-current
imbalance in the input stage results in minimum offset
voltage temperature coefficient. Many precision amplifiers
(the technique was pioneered in the uA725) are designed
so that the offset voltage and the thermal drift are simultaneously
minimized. That's true, at least, for bipolar-input precision
amplifiers...

 

[John Larkin]

On Fri, 12 Sep 2008 09:00:01 -0700 (PDT), Paul <energymover@gmail.com>
wrote:

On Sep 12, 1:28 am, whit3rd <whit...@gmail.com> wrote:
On Sep 10, 3:36 pm, m...@sushi.com wrote:



... can you contact the
factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience
that drift gets worse when you add hooks to the circuit to correct for
offset.

Don't pay for screened devices.  It's unlikely to be easy for the
factory (unless they already do it-and charge extra for 'B' versions).
It's also unlikely to survive shipping and soldering and aging.
Your best bet is to trim it after all assembly is complete.

Heck, thermal gradients in the printed circuit board can
create thermocouple voltages, you HAVE to trim it
after assembly and warmup.

It's possible to trim a uA741 for minimum Voffset (the usual way),
or to sense the input stage current at the adjust pins, and
trim for matched emitter current.  Zeroing the emitter-current
imbalance in the input stage results in minimum  offset
voltage temperature coefficient.   Many precision amplifiers
(the technique was pioneered in the uA725) are designed
so that the offset voltage and the thermal drift are simultaneously
minimized.  That's true, at least, for bipolar-input precision
amplifiers...


Thanks for all the helpful advice! BTW is there a better site to post
than from google groups? A few days ago I typed a good size post,
clicked Send, and google said I made too many posts in a given time
period.

If you want to do this regularly, get a real newsreader program and a
server subscription; Forte Agent can furnish both.

Anyhow, I'll try to add more insulation around the DUT to dampen out
temp changes, and will implement a mechanical switch that when pulled
will flip the DUT polarity across the electrometer circuit, which
should eliminate the concern for temp drift, hopefully. Sometimes
designs doesn't work as good as on paper, but I'll see.

The circuit was build yesterday with the exception of filtering noise.
I placed a fixed resistor as the input for initial testing. Oddly
enough when the resistor is flipped the electrometer output changes
from 30mV to 500mV DC! For now all I can think of is that there's a
bit more wiring in one position. So perhaps stray noise from radio
station for instance being rectified by the op-amp? Today I'll try to
move the circuit around to see if it's due to RF noise, and perhaps
place the circuit inside the shield. I have extensive shielding
waiting on the side, but it's a pain to work with a circuit inside the
shield. ... Ah, could it be from lack of grounding? It's battery
operated, so while the circuit is outside the shield there's not much
ground material. Not sure, as this is my first real electrometer op-
amp.

Even 30 mV means something goofy is going on.

But be careful about the relay: it can inject charge when it switches,
and it can have many microvolts of thermal EMF with ugly time
constants. The tiny telecom-type latching relays are great thermally,
since you only have to pulse the coil for a millisecond to get them to
flip.

John

 

[Paul]

On Sep 12, 1:28 am, whit3rd <whit...@gmail.com> wrote:

On Sep 10, 3:36 pm, m...@sushi.com wrote:



... can you contact the
factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience
that drift gets worse when you add hooks to the circuit to correct for
offset.

Don't pay for screened devices.  It's unlikely to be easy for the
factory (unless they already do it-and charge extra for 'B' versions).
It's also unlikely to survive shipping and soldering and aging.
Your best bet is to trim it after all assembly is complete.

Heck, thermal gradients in the printed circuit board can
create thermocouple voltages, you HAVE to trim it
after assembly and warmup.

It's possible to trim a uA741 for minimum Voffset (the usual way),
or to sense the input stage current at the adjust pins, and
trim for matched emitter current.  Zeroing the emitter-current
imbalance in the input stage results in minimum  offset
voltage temperature coefficient.   Many precision amplifiers
(the technique was pioneered in the uA725) are designed
so that the offset voltage and the thermal drift are simultaneously
minimized.  That's true, at least, for bipolar-input precision
amplifiers...

Thanks for all the helpful advice! BTW is there a better site to post
than from google groups? A few days ago I typed a good size post,
clicked Send, and google said I made too many posts in a given time
period.

Anyhow, I'll try to add more insulation around the DUT to dampen out
temp changes, and will implement a mechanical switch that when pulled
will flip the DUT polarity across the electrometer circuit, which
should eliminate the concern for temp drift, hopefully. Sometimes
designs doesn't work as good as on paper, but I'll see.

The circuit was build yesterday with the exception of filtering noise.
I placed a fixed resistor as the input for initial testing. Oddly
enough when the resistor is flipped the electrometer output changes
from 30mV to 500mV DC! For now all I can think of is that there's a
bit more wiring in one position. So perhaps stray noise from radio
station for instance being rectified by the op-amp? Today I'll try to
move the circuit around to see if it's due to RF noise, and perhaps
place the circuit inside the shield. I have extensive shielding
waiting on the side, but it's a pain to work with a circuit inside the
shield. ... Ah, could it be from lack of grounding? It's battery
operated, so while the circuit is outside the shield there's not much
ground material. Not sure, as this is my first real electrometer op-
amp.


Thanks for all the help!!
Paul

 

[Paul]

On Sep 12, 1:28 am, whit3rd <whit...@gmail.com> wrote:

On Sep 10, 3:36 pm, m...@sushi.com wrote:



... can you contact the
factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience
that drift gets worse when you add hooks to the circuit to correct for
offset.

Don't pay for screened devices.  It's unlikely to be easy for the
factory (unless they already do it-and charge extra for 'B' versions).
It's also unlikely to survive shipping and soldering and aging.
Your best bet is to trim it after all assembly is complete.

Heck, thermal gradients in the printed circuit board can
create thermocouple voltages, you HAVE to trim it
after assembly and warmup.

It's possible to trim a uA741 for minimum Voffset (the usual way),
or to sense the input stage current at the adjust pins, and
trim for matched emitter current.  Zeroing the emitter-current
imbalance in the input stage results in minimum  offset
voltage temperature coefficient.   Many precision amplifiers
(the technique was pioneered in the uA725) are designed
so that the offset voltage and the thermal drift are simultaneously
minimized.  That's true, at least, for bipolar-input precision
amplifiers...

I'm wondering. The INA116P datasheet shows max Ib as 25fA. So that's
taking such a chip and measuring the *total* bias current, correct? I
mean, they're not subtracting thermoelectric effects are they?

Thanks,
Paul

 

[Paul]

On Sep 12, 9:13 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 12 Sep 2008 09:00:01 -0700 (PDT), Paul <energymo...@gmail.com
wrote:



On Sep 12, 1:28 am, whit3rd <whit...@gmail.com> wrote:
On Sep 10, 3:36 pm, m...@sushi.com wrote:

... can you contact the
factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience
that drift gets worse when you add hooks to the circuit to correct for
offset.

Don't pay for screened devices.  It's unlikely to be easy for the
factory (unless they already do it-and charge extra for 'B' versions).
It's also unlikely to survive shipping and soldering and aging.
Your best bet is to trim it after all assembly is complete.

Heck, thermal gradients in the printed circuit board can
create thermocouple voltages, you HAVE to trim it
after assembly and warmup.

It's possible to trim a uA741 for minimum Voffset (the usual way),
or to sense the input stage current at the adjust pins, and
trim for matched emitter current.  Zeroing the emitter-current
imbalance in the input stage results in minimum  offset
voltage temperature coefficient.   Many precision amplifiers
(the technique was pioneered in the uA725) are designed
so that the offset voltage and the thermal drift are simultaneously
minimized.  That's true, at least, for bipolar-input precision
amplifiers...

Thanks for all the helpful advice!  BTW is there a better site to post
than from google groups? A few days ago I typed a good size post,
clicked Send, and google said I made too many posts in a given time
period.

If you want to do this regularly, get a real newsreader program and a
server subscription; Forte Agent can furnish both.





Anyhow, I'll try to add more insulation around the DUT to dampen out
temp changes, and will implement a mechanical switch that when pulled
will flip the DUT polarity across the electrometer circuit, which
should eliminate the concern for temp drift, hopefully. Sometimes
designs doesn't work as good as on paper, but I'll see.

The circuit was build yesterday with the exception of filtering noise.
I placed a fixed resistor as the input for initial testing. Oddly
enough when the resistor is flipped the electrometer output changes
from 30mV to 500mV DC!  For now all I can think of is that there's a
bit more wiring in one position. So perhaps stray noise from radio
station for instance being rectified by the op-amp?  Today I'll try to
move the circuit around to see if it's due to RF noise, and perhaps
place the circuit inside the shield. I have extensive shielding
waiting on the side, but it's a pain to work with a circuit inside the
shield.  ...  Ah, could it be from lack of grounding?  It's battery
operated, so while the circuit is outside the shield there's not much
ground material. Not sure, as this is my first real electrometer op-
amp.

Even 30 mV means something goofy is going on.

But be careful about the relay: it can inject charge when it switches,
and it can have many microvolts of thermal EMF with ugly time
constants. The tiny telecom-type latching relays are great thermally,
since you only have to pulse the coil for a millisecond to get them to
flip.

John

When you say, "be careful about the relay: it can inject charge when
it switches, and it can have many microvolts of thermal EMF with ugly
time constants." Are you referring to a current driven coil type
relay or a mechanical relay/switch? What I had in mind was a small
mechanical switch setup such that the slight tug of a non-conductive
string would flip the mechanical switch. The string would enter the
shield through a pin hole and to the mechanical switch.


Are these the type of relays:

Part #: PB1117-ND
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=PB1117-ND

Part #: PB1091CT-ND
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=PB1091CT-ND


Thanks,
Paul

 

[John Larkin]

On Fri, 12 Sep 2008 09:41:23 -0700 (PDT), Paul <energymover@gmail.com>
wrote:

On Sep 12, 9:13 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 12 Sep 2008 09:00:01 -0700 (PDT), Paul <energymo...@gmail.com
wrote:



On Sep 12, 1:28 am, whit3rd <whit...@gmail.com> wrote:
On Sep 10, 3:36 pm, m...@sushi.com wrote:

... can you contact the
factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience
that drift gets worse when you add hooks to the circuit to correct for
offset.

Don't pay for screened devices.  It's unlikely to be easy for the
factory (unless they already do it-and charge extra for 'B' versions).
It's also unlikely to survive shipping and soldering and aging.
Your best bet is to trim it after all assembly is complete.

Heck, thermal gradients in the printed circuit board can
create thermocouple voltages, you HAVE to trim it
after assembly and warmup.

It's possible to trim a uA741 for minimum Voffset (the usual way),
or to sense the input stage current at the adjust pins, and
trim for matched emitter current.  Zeroing the emitter-current
imbalance in the input stage results in minimum  offset
voltage temperature coefficient.   Many precision amplifiers
(the technique was pioneered in the uA725) are designed
so that the offset voltage and the thermal drift are simultaneously
minimized.  That's true, at least, for bipolar-input precision
amplifiers...

Thanks for all the helpful advice!  BTW is there a better site to post
than from google groups? A few days ago I typed a good size post,
clicked Send, and google said I made too many posts in a given time
period.

If you want to do this regularly, get a real newsreader program and a
server subscription; Forte Agent can furnish both.





Anyhow, I'll try to add more insulation around the DUT to dampen out
temp changes, and will implement a mechanical switch that when pulled
will flip the DUT polarity across the electrometer circuit, which
should eliminate the concern for temp drift, hopefully. Sometimes
designs doesn't work as good as on paper, but I'll see.

The circuit was build yesterday with the exception of filtering noise.
I placed a fixed resistor as the input for initial testing. Oddly
enough when the resistor is flipped the electrometer output changes
from 30mV to 500mV DC!  For now all I can think of is that there's a
bit more wiring in one position. So perhaps stray noise from radio
station for instance being rectified by the op-amp?  Today I'll try to
move the circuit around to see if it's due to RF noise, and perhaps
place the circuit inside the shield. I have extensive shielding
waiting on the side, but it's a pain to work with a circuit inside the
shield.  ...  Ah, could it be from lack of grounding?  It's battery
operated, so while the circuit is outside the shield there's not much
ground material. Not sure, as this is my first real electrometer op-
amp.

Even 30 mV means something goofy is going on.

But be careful about the relay: it can inject charge when it switches,
and it can have many microvolts of thermal EMF with ugly time
constants. The tiny telecom-type latching relays are great thermally,
since you only have to pulse the coil for a millisecond to get them to
flip.

John


When you say, "be careful about the relay: it can inject charge when
it switches, and it can have many microvolts of thermal EMF with ugly
time constants." Are you referring to a current driven coil type
relay

Yes.

or a mechanical relay/switch? What I had in mind was a small
mechanical switch setup such that the slight tug of a non-conductive
string would flip the mechanical switch. The string would enter the
shield through a pin hole and to the mechanical switch.


Are these the type of relays:

Part #: PB1117-ND
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=PB1117-ND

Part #: PB1091CT-ND
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=PB1091CT-ND


Thanks,
Paul

We use the Fujitsu FTR-B3GA series for thermocouple switching, similar
to the ones you show. Very nice.

But if you can mechanically move a contact via a string or rod or some
such, from outside the shield, that's ideal.

John

 

[George Herold]

On Sep 12, 12:00 pm, Paul <energymo...@gmail.com> wrote:

On Sep 12, 1:28 am, whit3rd <whit...@gmail.com> wrote:





On Sep 10, 3:36 pm, m...@sushi.com wrote:

... can you contact the
factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience
that drift gets worse when you add hooks to the circuit to correct for
offset.

Don't pay for screened devices.  It's unlikely to be easy for the
factory (unless they already do it-and charge extra for 'B' versions).
It's also unlikely to survive shipping and soldering and aging.
Your best bet is to trim it after all assembly is complete.

Heck, thermal gradients in the printed circuit board can
create thermocouple voltages, you HAVE to trim it
after assembly and warmup.

It's possible to trim a uA741 for minimum Voffset (the usual way),
or to sense the input stage current at the adjust pins, and
trim for matched emitter current.  Zeroing the emitter-current
imbalance in the input stage results in minimum  offset
voltage temperature coefficient.   Many precision amplifiers
(the technique was pioneered in the uA725) are designed
so that the offset voltage and the thermal drift are simultaneously
minimized.  That's true, at least, for bipolar-input precision
amplifiers...

Thanks for all the helpful advice!  BTW is there a better site to post
than from google groups? A few days ago I typed a good size post,
clicked Send, and google said I made too many posts in a given time
period.

Anyhow, I'll try to add more insulation around the DUT to dampen out
temp changes, and will implement a mechanical switch that when pulled
will flip the DUT polarity across the electrometer circuit, which
should eliminate the concern for temp drift, hopefully. Sometimes
designs doesn't work as good as on paper, but I'll see.

The circuit was build yesterday with the exception of filtering noise.
I placed a fixed resistor as the input for initial testing. Oddly
enough when the resistor is flipped the electrometer output changes
from 30mV to 500mV DC!  For now all I can think of is that there's a
bit more wiring in one position. So perhaps stray noise from radio
station for instance being rectified by the op-amp?  Today I'll try to
move the circuit around to see if it's due to RF noise, and perhaps
place the circuit inside the shield. I have extensive shielding
waiting on the side, but it's a pain to work with a circuit inside the
shield.  ...  Ah, could it be from lack of grounding?  It's battery
operated, so while the circuit is outside the shield there's not much
ground material. Not sure, as this is my first real electrometer op-
amp.

Thanks for all the help!!
Paul- Hide quoted text -

- Show quoted text -

from 30mV to 500mV DC!
How are you switching the resistor? What's the gain? What's the

resistnace? 0.5 V is lots!

 

[Paul]

On Sep 12, 8:45 pm, George Herold <gher...@teachspin.com> wrote:

On Sep 12, 12:00 pm, Paul <energymo...@gmail.com> wrote:



On Sep 12, 1:28 am, whit3rd <whit...@gmail.com> wrote:

On Sep 10, 3:36 pm, m...@sushi.com wrote:

... can you contact the
factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience
that drift gets worse when you add hooks to the circuit to correct for
offset.

Don't pay for screened devices.  It's unlikely to be easy for the
factory (unless they already do it-and charge extra for 'B' versions)..
It's also unlikely to survive shipping and soldering and aging.
Your best bet is to trim it after all assembly is complete.

Heck, thermal gradients in the printed circuit board can
create thermocouple voltages, you HAVE to trim it
after assembly and warmup.

It's possible to trim a uA741 for minimum Voffset (the usual way),
or to sense the input stage current at the adjust pins, and
trim for matched emitter current.  Zeroing the emitter-current
imbalance in the input stage results in minimum  offset
voltage temperature coefficient.   Many precision amplifiers
(the technique was pioneered in the uA725) are designed
so that the offset voltage and the thermal drift are simultaneously
minimized.  That's true, at least, for bipolar-input precision
amplifiers...

Thanks for all the helpful advice!  BTW is there a better site to post
than from google groups? A few days ago I typed a good size post,
clicked Send, and google said I made too many posts in a given time
period.

Anyhow, I'll try to add more insulation around the DUT to dampen out
temp changes, and will implement a mechanical switch that when pulled
will flip the DUT polarity across the electrometer circuit, which
should eliminate the concern for temp drift, hopefully. Sometimes
designs doesn't work as good as on paper, but I'll see.

The circuit was build yesterday with the exception of filtering noise.
I placed a fixed resistor as the input for initial testing. Oddly
enough when the resistor is flipped the electrometer output changes
from 30mV to 500mV DC!  For now all I can think of is that there's a
bit more wiring in one position. So perhaps stray noise from radio
station for instance being rectified by the op-amp?  Today I'll try to
move the circuit around to see if it's due to RF noise, and perhaps
place the circuit inside the shield. I have extensive shielding
waiting on the side, but it's a pain to work with a circuit inside the
shield.  ...  Ah, could it be from lack of grounding?  It's battery
operated, so while the circuit is outside the shield there's not much
ground material. Not sure, as this is my first real electrometer op-
amp.

Thanks for all the help!!
Paul- Hide quoted text -

- Show quoted text -
from 30mV to 500mV DC!

How are you switching the resistor?  What's the gain?  What's the
resistnace?  0.5 V is lots!

Hi,

I'm using a dual non-magnetic SPDT mechanical micro-switch to
essentially reverse the DUT across the electrometer input. I couldn't
find a DPDT in this non-magnetic switch. The electrometer gain is 3
and the second stage op-amp (OP07C) gain is 213 for a total gain of
639. I know, the OP07C design is old. It's what I have on hand,
relatively low Vos, nice and slow = good for DC. The OP07C chips are
new. Also they don't have chopper noise. I'd be more than happy for an
improved equivalent recommendation. The 2nd stage op-amp will
eventually be used as a voltage-to-current converter since the
feedback will be an LED, but for now the feedback is a 100K ohm
resistor. Also there's an offset pot for the second stage op-amp. The
input DUT is 850K ohms. Each of the electrometer op-amp input pins is
tied to ground by a 29M ohm resistor. The datasheet shows 100M ohms
resistors, but I don't have any.

An update: I placed the circuit inside the shield and as expected/
hoped the output went down to normal values regardless if the input
was reversed or not. So I guess it was just RF signals (radio stations
and such) causing the problem. Electrometer op-amps are probably more
sensitive.

Regards,
Paul

 

[Paul]

On Sep 14, 12:57 pm, Paul <energymo...@gmail.com> wrote:

On Sep 12, 8:45 pm, George Herold <gher...@teachspin.com> wrote:



On Sep 12, 12:00 pm, Paul <energymo...@gmail.com> wrote:

On Sep 12, 1:28 am, whit3rd <whit...@gmail.com> wrote:

On Sep 10, 3:36 pm, m...@sushi.com wrote:

... can you contact the
factory and pay for screened devices?

I never designed any precision op-amp, but it has been my experience
that drift gets worse when you add hooks to the circuit to correct for
offset.

Don't pay for screened devices.  It's unlikely to be easy for the
factory (unless they already do it-and charge extra for 'B' versions).
It's also unlikely to survive shipping and soldering and aging.
Your best bet is to trim it after all assembly is complete.

Heck, thermal gradients in the printed circuit board can
create thermocouple voltages, you HAVE to trim it
after assembly and warmup.

It's possible to trim a uA741 for minimum Voffset (the usual way),
or to sense the input stage current at the adjust pins, and
trim for matched emitter current.  Zeroing the emitter-current
imbalance in the input stage results in minimum  offset
voltage temperature coefficient.   Many precision amplifiers
(the technique was pioneered in the uA725) are designed
so that the offset voltage and the thermal drift are simultaneously
minimized.  That's true, at least, for bipolar-input precision
amplifiers...

Thanks for all the helpful advice!  BTW is there a better site to post
than from google groups? A few days ago I typed a good size post,
clicked Send, and google said I made too many posts in a given time
period.

Anyhow, I'll try to add more insulation around the DUT to dampen out
temp changes, and will implement a mechanical switch that when pulled
will flip the DUT polarity across the electrometer circuit, which
should eliminate the concern for temp drift, hopefully. Sometimes
designs doesn't work as good as on paper, but I'll see.

The circuit was build yesterday with the exception of filtering noise..
I placed a fixed resistor as the input for initial testing. Oddly
enough when the resistor is flipped the electrometer output changes
from 30mV to 500mV DC!  For now all I can think of is that there's a
bit more wiring in one position. So perhaps stray noise from radio
station for instance being rectified by the op-amp?  Today I'll try to
move the circuit around to see if it's due to RF noise, and perhaps
place the circuit inside the shield. I have extensive shielding
waiting on the side, but it's a pain to work with a circuit inside the
shield.  ...  Ah, could it be from lack of grounding?  It's battery
operated, so while the circuit is outside the shield there's not much
ground material. Not sure, as this is my first real electrometer op-
amp.

Thanks for all the help!!
Paul- Hide quoted text -

- Show quoted text -
from 30mV to 500mV DC!

How are you switching the resistor?  What's the gain?  What's the
resistnace?  0.5 V is lots!

Hi,

I'm using a dual non-magnetic SPDT mechanical micro-switch to
essentially reverse the DUT across the electrometer input. I couldn't
find a DPDT in this non-magnetic switch. The electrometer gain is 3
and the second stage op-amp (OP07C) gain is 213 for a total gain of
639. I know, the OP07C design is old. It's what I have on hand,
relatively low Vos, nice and slow = good for DC. The OP07C chips are
new. Also they don't have chopper noise. I'd be more than happy for an
improved equivalent recommendation. The 2nd stage op-amp will
eventually be used as a voltage-to-current converter since the
feedback will be an LED, but for now the feedback is a 100K ohm
resistor. Also there's an offset pot for the second stage op-amp. The
input DUT is 850K ohms. Each of the electrometer op-amp input pins is
tied to ground by a 29M ohm resistor. The datasheet shows 100M ohms
resistors, but I don't have any.

An update: I placed the circuit inside the shield and as expected/
hoped the output went down to normal values regardless if the input
was reversed or not. So I guess it was just RF signals (radio stations
and such) causing the problem. Electrometer op-amps are probably more
sensitive.

Regards,
Paul

BTW, as much bias current as possible was eliminated by bending the
input pins up in the air and soldering the input pins to the DUT
without touching the board. Also the entire electrometer op-amp is
held in the air by the wires so that no part of the electrometer is
touching the board. This not only produces the least amount of bias
current leakage, but also allows me to easily clean the entire
electrometer top, bottom, and sides of any residue or dust. I've read
about that technique in several femto bias current op-amp datasheets.
Air is considered nearly perfect in terms of leakage.

Paul