Is this op-amp uV/C that high?

P

Paul

Guest
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_8554.pdf


I appreciate any help or advice.
Paul
 
On Wed, 10 Sep 2008 08:46:00 -0700 (PDT), Paul <energymover@gmail.com>
wrote:

Thanks for the reply George. That's very disappointing to read that it
truly has that much drift. The DUT voltage is near 10uV DC, so I need
that sensitivity. Also I would like to eliminate as much external bias
current flowing through the DUT as possible, hence my big attraction
to electrometer type op-amps with their femto level bias current. The
AD8551 is a great chopper op-amp, but it has far too much bias
current. Also I'm not to fond of the possibility of the chopper pulses
getting past the filter.

Uggg, must I scrap my entire circuit and find another electrometer
type op-amp?

Regards,
Paul
Click to expand...
What is the DUT?

John
 
On Wed, 10 Sep 2008 08:57:23 -0700 (PDT), Paul <energymover@gmail.com>
wrote:

On Sep 10, 8:50 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 10 Sep 2008 08:46:00 -0700 (PDT), Paul <energymo...@gmail.com
wrote:

Thanks for the reply George. That's very disappointing to read that it
truly has that much drift. The DUT voltage is near 10uV DC, so I need
that sensitivity. Also I would like to eliminate as much external bias
current flowing through the DUT as possible, hence my big attraction
to electrometer type op-amps with their femto level bias current. The
AD8551 is a great chopper op-amp, but it has far too much bias
current. Also I'm not to fond of the possibility of the chopper pulses
getting past the filter.

Uggg, must I scrap my entire circuit and find another electrometer
type op-amp?

Regards,
Paul

What is the DUT?

John



Hi John,

Special high frequency low signal diodes used to pickup weak signals.
The diodes are used as a passive method.

Would you happen to have any recommendations on a low bias current op-
amp that has a max Ib of 100fA?

Paul
Click to expand...
Can you chop the input signal (light? RF?) and use a lock-in
technique? Or occasionally auto-zero, with a relay maybe?

National has some fA rated opamos, but any fA part must be jfet or
cmos, so don't expect great DC specs. As you note, choppers have
charge-induced bias current and are usually noisy.

You could temperature control an opamp, or put a temperature sensor
nearby and tweak a compensation factor.

John
 
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.
Click to expand...
---
"No reason _not_ to add", yes? Also,:

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

JF
 
On Wed, 10 Sep 2008 09:44:28 -0700 (PDT), Paul <energymover@gmail.com>
wrote:

On Sep 10, 8:35 am, George Herold <gher...@teachspin.com> wrote:
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

Paul,  It looks like you are reading the spec sheet correctly.  You'll
notice that the INA116 instrument amp has a Vos of 2mV (max) to start
with..... so at a gain of 1000 you have 2 volts to deal with
already... (Why worry about a measly 20 mV?)  With that much offset
it's hard to ask the opamp to be much more temperature stable than is
listed.
What's wrong with the AD8551?  You could always look into chopper
stabilized opamps for low offset and drift.


I'm curious though. In chart (page 5) in the datasheet seems to
indicate that the drift per typical op-amp decreases with higher
gains? Does anyone know how to interpret that chart in terms of gain?
For example at gain of 100 they show that 19% of the parts have a
drift near 10uV/C, but is that 10uV/C per gain of 1 or per gain of
100? If the gain is set to 100 and the *output* only varies by 10uV/C
then that's not so bad. Somehow I doubt that's correct though, no?
It's probably 10uV/C * 100. :-(

Regards,
Paul
Click to expand...
Your interpretation is correct.

As you suspect, the offset is referred to the input-- that's why it's
called "Input Offset Voltage" on the graph, and that's what the "RTI"
means in the Specifications. At the output, you'll see G * delta-Vos.
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 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
Click to expand...
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).

Of course the variation will be reduced.

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, 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
Click to expand...
Paul, It looks like you are reading the spec sheet correctly. You'll
notice that the INA116 instrument amp has a Vos of 2mV (max) to start
with..... so at a gain of 1000 you have 2 volts to deal with
already... (Why worry about a measly 20 mV?) With that much offset
it's hard to ask the opamp to be much more temperature stable than is
listed.
What's wrong with the AD8551? You could always look into chopper
stabilized opamps for low offset and drift.
 
On Wed, 10 Sep 2008 10:17:20 -0700 (PDT), Paul <energymover@gmail.com>
wrote:

On Sep 10, 10:07 am, Spehro Pefhany
speffS...@interlogDOTyou.knowwhat> wrote:
On Wed, 10 Sep 2008 09:44:28 -0700 (PDT), Paul <energymo...@gmail.com
wrote:



On Sep 10, 8:35 am, George Herold <gher...@teachspin.com> wrote:
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
[snip]
I'm curious though. In chart (page 5) in the datasheet seems to
indicate that the drift per typical op-amp decreases with higher
gains? Does anyone know how to interpret that chart in terms of gain?
For example at gain of 100 they show that 19% of the parts have a
drift near 10uV/C, but is that 10uV/C per gain of 1 or per gain of
100? If the gain is set to 100 and the *output* only varies by 10uV/C
then that's not so bad. Somehow I doubt that's correct though, no?
It's probably 10uV/C * 100. :-(

Regards,
Paul

Your interpretation is correct.

As you suspect, the offset is referred to the input-- that's why it's
called "Input Offset Voltage" on the graph, and that's what the "RTI"
means in the Specifications.  At the output, you'll see G * delta-Vos.
Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
sp...@interlog.com             Info for manufacturers:http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

OK, I was at least hopeful.:) Why would using a higher gain
resistor, RG, narrow the chart width. The chart makes it appear as if
using a high gain R could be helpful in decreasing uV/C per gain.
Click to expand...
A portion of the offset drift is actually related to the output, and
the total effect (referred to the input) is reduced by increasing the
gain. The drift in mV/K or whatever at the output only goes up as gain
increases, of course, there's no free lunch. Same deal with
input-referred noise at lower frequencies.

Regards,
Paul
Best regards,
Click to expand...
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 Wed, 10 Sep 2008 10:34:14 -0700 (PDT), Paul <energymover@gmail.com>
wrote:

On Sep 10, 10:13 am, Spehro Pefhany
speffS...@interlogDOTyou.knowwhat> wrote:
On Wed, 10 Sep 2008 11:56:37 -0500, John Fields



jfie...@austininstruments.com> wrote:
On Wed, 10 Sep 2008 09:35:51 -0700 (PDT), Paul <energymo...@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).  

Of course the variation will  be reduced.

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


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
Click to expand...
Sure, it works if the DUT is resistive. What exactly is it?
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
 
Thanks for the reply George. That's very disappointing to read that it
truly has that much drift. The DUT voltage is near 10uV DC, so I need
that sensitivity. Also I would like to eliminate as much external bias
current flowing through the DUT as possible, hence my big attraction
to electrometer type op-amps with their femto level bias current. The
AD8551 is a great chopper op-amp, but it has far too much bias
current. Also I'm not to fond of the possibility of the chopper pulses
getting past the filter.

Uggg, must I scrap my entire circuit and find another electrometer
type op-amp?

Regards,
Paul
 
On Sep 10, 8:50 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 10 Sep 2008 08:46:00 -0700 (PDT), Paul <energymo...@gmail.com
wrote:

Thanks for the reply George. That's very disappointing to read that it
truly has that much drift. The DUT voltage is near 10uV DC, so I need
that sensitivity. Also I would like to eliminate as much external bias
current flowing through the DUT as possible, hence my big attraction
to electrometer type op-amps with their femto level bias current. The
AD8551 is a great chopper op-amp, but it has far too much bias
current. Also I'm not to fond of the possibility of the chopper pulses
getting past the filter.

Uggg, must I scrap my entire circuit and find another electrometer
type op-amp?

Regards,
Paul

What is the DUT?

John
Click to expand...


Hi John,

Special high frequency low signal diodes used to pickup weak signals.
The diodes are used as a passive method.

Would you happen to have any recommendations on a low bias current op-
amp that has a max Ib of 100fA?

Paul
 
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.
Click to expand...
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.

Regards,
Paul
 
On Sep 10, 8:35 am, George Herold <gher...@teachspin.com> wrote:
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

Paul,  It looks like you are reading the spec sheet correctly.  You'll
notice that the INA116 instrument amp has a Vos of 2mV (max) to start
with..... so at a gain of 1000 you have 2 volts to deal with
already... (Why worry about a measly 20 mV?)  With that much offset
it's hard to ask the opamp to be much more temperature stable than is
listed.
What's wrong with the AD8551?  You could always look into chopper
stabilized opamps for low offset and drift.
Click to expand...

I'm curious though. In chart (page 5) in the datasheet seems to
indicate that the drift per typical op-amp decreases with higher
gains? Does anyone know how to interpret that chart in terms of gain?
For example at gain of 100 they show that 19% of the parts have a
drift near 10uV/C, but is that 10uV/C per gain of 1 or per gain of
100? If the gain is set to 100 and the *output* only varies by 10uV/C
then that's not so bad. Somehow I doubt that's correct though, no?
It's probably 10uV/C * 100. :-(

Regards,
Paul
 
On Sep 10, 8:35 am, George Herold <gher...@teachspin.com> wrote:
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

Paul, It looks like you are reading the spec sheet correctly. You'll
notice that the INA116 instrument amp has a Vos of 2mV (max) to start
with..... so at a gain of 1000 you have 2 volts to deal with
already... (Why worry about a measly 20 mV?) With that much offset
it's hard to ask the opamp to be much more temperature stable than is
listed.
What's wrong with the AD8551? You could always look into chopper
stabilized opamps for low offset and drift.
Click to expand...

I'm curious though. In chart (page 5) in the datasheet seems to
indicate that the drift per typical op-amp decreases with higher
gains? Does anyone know how to interpret that chart in terms of gain?
For example at gain of 100 they show that 19% of the parts have a
drift near 10uV/C, but is that 10uV/C per gain of 1 or per gain of
100? If the gain is set to 100 and the *output* only varies by 10uV/C
then that's not so bad. Somehow I doubt that's correct though, no?
It's probably 10uV/C * 100. :-(

Regards,
Paul
 
On Sep 10, 9:07 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 10 Sep 2008 08:57:23 -0700 (PDT), Paul <energymo...@gmail.com
wrote:



On Sep 10, 8:50 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 10 Sep 2008 08:46:00 -0700 (PDT), Paul <energymo...@gmail.com
wrote:

Thanks for the reply George. That's very disappointing to read that it
truly has that much drift. The DUT voltage is near 10uV DC, so I need
that sensitivity. Also I would like to eliminate as much external bias
current flowing through the DUT as possible, hence my big attraction
to electrometer type op-amps with their femto level bias current. The
AD8551 is a great chopper op-amp, but it has far too much bias
current. Also I'm not to fond of the possibility of the chopper pulses
getting past the filter.

Uggg, must I scrap my entire circuit and find another electrometer
type op-amp?

Regards,
Paul

What is the DUT?

John

Hi John,

Special high frequency low signal diodes used to pickup weak signals.
The diodes are used as a passive method.

Would you happen to have any recommendations on a low bias current op-
amp that has a max Ib of 100fA?

Paul

Can you chop the input signal (light? RF?) and use a lock-in
technique? Or occasionally auto-zero, with a relay maybe?

National has some fA rated opamos, but any fA part must be jfet or
cmos, so don't expect great DC specs. As you note, choppers have
charge-induced bias current and are usually noisy.

You could temperature control an opamp, or put a temperature sensor
nearby and tweak a compensation factor.

John
Click to expand...

What about using a thermistor for the Rg resistor (gain resistor)? Of
course I'd have to hope that the drift is in the correct direction,
otherwise the thermistor would only add to the drift. Perhaps a
thermistor and metal film combo.

Paul
 
On Sep 10, 9:56 am, John Fields <jfie...@austininstruments.com> wrote:
On Wed, 10 Sep 2008 09:35:51 -0700 (PDT), Paul <energymo...@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
Click to expand...

Thanks for link! Similar idea, if not the same as John's suggestion.
I could place a small metal place near the op-amp. Attached to the
metal place is a heating element and a thermistor. A circuit maintains
the temperature. Once concern is the heating element inducing a
voltage on the input device / op-amp. I think if the heating element
reacts slow enough then the induced voltage caused by a change in
heating element current should be low enough.

Paul
 
On Sep 10, 10:07 am, Spehro Pefhany
<speffS...@interlogDOTyou.knowwhat> wrote:
On Wed, 10 Sep 2008 09:44:28 -0700 (PDT), Paul <energymo...@gmail.com
wrote:



On Sep 10, 8:35 am, George Herold <gher...@teachspin.com> wrote:
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
[snip]
I'm curious though. In chart (page 5) in the datasheet seems to
indicate that the drift per typical op-amp decreases with higher
gains? Does anyone know how to interpret that chart in terms of gain?
For example at gain of 100 they show that 19% of the parts have a
drift near 10uV/C, but is that 10uV/C per gain of 1 or per gain of
100? If the gain is set to 100 and the *output* only varies by 10uV/C
then that's not so bad. Somehow I doubt that's correct though, no?
It's probably 10uV/C * 100. :-(

Regards,
Paul

Your interpretation is correct.

As you suspect, the offset is referred to the input-- that's why it's
called "Input Offset Voltage" on the graph, and that's what the "RTI"
means in the Specifications.  At the output, you'll see G * delta-Vos.
Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
sp...@interlog.com             Info for manufacturers:http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Click to expand...
OK, I was at least hopeful.:) Why would using a higher gain
resistor, RG, narrow the chart width. The chart makes it appear as if
using a high gain R could be helpful in decreasing uV/C per gain.

Regards,
Paul
 
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).
Click to expand...
---
OK, then, how about in a little Peltier fridge? :)

JF
 
On Sep 10, 10:13 am, Spehro Pefhany
<speffS...@interlogDOTyou.knowwhat> wrote:
On Wed, 10 Sep 2008 11:56:37 -0500, John Fields



jfie...@austininstruments.com> wrote:
On Wed, 10 Sep 2008 09:35:51 -0700 (PDT), Paul <energymo...@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).  

Of course the variation will  be reduced.

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

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
 
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
Click to expand...
Just be very careful about condensation.

John
 
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