Opamp offset question...

[Klaus Kragelund]

Hi

I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inhireted

Problem is the large offset voltage of the opamp is amplified producing large errors

We are contemplating production calibration, but I am worried that the offset isn\'t stable after the calibration has been done

In litterature the offset comes from mismatch of the long tailed pair. Is that expected to be stable, so a calibration done in production also cancels out after 10 years operation?

By the way, my suggestion is to ditch the opamp and feed the signal directly into the ADC. All the opamp errors disappears then

I just need a big sample cap to reduce charge injection problems from ADC channel switching and sample/hold effects

Regards

Klaus

--
Klaus

 

[Klaus Kragelund]

28.11.21 22:35, Klaus Kragelund wrote:

Hi

I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inhireted

Problem is the large offset voltage of the opamp is amplified producing large errors

We are contemplating production calibration, but I am worried that the offset isn\'t stable after the calibration has been done

In litterature the offset comes from mismatch of the long tailed pair. Is that expected to be stable, so a calibration done in production also cancels out after 10 years operation?

By the way, my suggestion is to ditch the opamp and feed the signal directly into the ADC. All the opamp errors disappears then

I just need a big sample cap to reduce charge injection problems from ADC channel switching and sample/hold effects

Regards

Klaus

I have been doing some tests. Quite odd, but hitting the circuit with a hot airflow of 60 Degrees creates 20mV offset on the output of the opamp. The specs define temperature drift of 5uV/K, so something weird is going on...

--
Klaus

 

[Jeroen Belleman]

On 2021-11-28 22:44, Klaus Kragelund wrote:

28.11.21 22:35, Klaus Kragelund wrote:
Hi

I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inhireted

Problem is the large offset voltage of the opamp is amplified producing large errors

We are contemplating production calibration, but I am worried that the offset isn\'t stable after the calibration has been done

In litterature the offset comes from mismatch of the long tailed pair. Is that expected to be stable, so a calibration done in production also cancels out after 10 years operation?

By the way, my suggestion is to ditch the opamp and feed the signal directly into the ADC. All the opamp errors disappears then

I just need a big sample cap to reduce charge injection problems from ADC channel switching and sample/hold effects

Regards

Klaus

I have been doing some tests. Quite odd, but hitting the circuit with a hot airflow of 60 Degrees creates 20mV offset on the output of the opamp. The specs define temperature drift of 5uV/K, so something weird is going on...

--
Klaus

The offset and its drift are specified at the input.

Jeroen Belleman

 

[server]

On Sun, 28 Nov 2021 22:44:27 +0100, Klaus Kragelund
<klauskvik@hotmail.com> wrote:

28.11.21 22:35, Klaus Kragelund wrote:
Hi

I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inhireted

Problem is the large offset voltage of the opamp is amplified producing large errors

We are contemplating production calibration, but I am worried that the offset isn\'t stable after the calibration has been done

In litterature the offset comes from mismatch of the long tailed pair. Is that expected to be stable, so a calibration done in production also cancels out after 10 years operation?

By the way, my suggestion is to ditch the opamp and feed the signal directly into the ADC. All the opamp errors disappears then

I just need a big sample cap to reduce charge injection problems from ADC channel switching and sample/hold effects

Regards

Klaus

I have been doing some tests. Quite odd, but hitting the circuit with a hot airflow of 60 Degrees creates 20mV offset on the output of the opamp. The specs define temperature drift of 5uV/K, so something weird is going on...

Got a schematic?

Are you seeing resistor tempcos maybe? An RTD is about 4000 ppm/k, so
you\'d need some very good resistors to keep their error contribution
down. If the pullup is 1K, that kills half of the gain too.

Would your RTD make enough voltage to drive the ADC directly? Too much
excitation current could self-heat the RTD.

What about the RTD+resistor voltage reference? Is it the same as the
ADC ref?

Can you auto-zero?

I would expect opamp offset to be pretty stable over time, so you
could cal it out.





--

Father Brown\'s figure remained quite dark and still;
but in that instant he had lost his head. His head was
always most valuable when he had lost it.

 

[Chris Jones]

On 29/11/2021 08:35, Klaus Kragelund wrote:

Hi

I have a PT1000 circuit where a LMV358 is used in a differential
coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inhireted

Problem is the large offset voltage of the opamp is amplified producing
large errors

We are contemplating production calibration, but I am worried that the
offset isn\'t stable after the calibration has been done

In litterature the offset comes from mismatch of the long tailed pair.
Is that expected to be stable, so a calibration done in production also
cancels out after 10 years operation?

By the way, my suggestion is to ditch the opamp and feed the signal
directly into the ADC. All the opamp errors disappears then

I just need a big sample cap to reduce charge injection problems from
ADC channel switching and sample/hold effects

Regards

Klaus

--
Klaus

If the offset has a temperature dependence then you would not be able to
take it out without doing calibrations at multiple temperatures. If the
opamp and the RTD are not always at the same temperature then it may be
impossible to fix by calibration.

Can you swap the opamp for a type that has less offset? There are ones
that would have the same pinout.

 

[Anthony William Sloman]

On Monday, November 29, 2021 at 9:46:07 AM UTC+11, Chris Jones wrote:

On 29/11/2021 08:35, Klaus Kragelund wrote:
Hi

I have a PT1000 circuit where a LMV358 is used in a differential
coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inherited .

Problem is the large offset voltage of the opamp is amplified producing
large errors

We are contemplating production calibration, but I am worried that the
offset isn\'t stable after the calibration has been done

In literature the offset comes from mismatch of the long tailed pair.
Is that expected to be stable, so a calibration done in production also
cancels out after 10 years operation?

By the way, my suggestion is to ditch the opamp and feed the signal
directly into the ADC. All the opamp errors disappears then

I just need a big sample cap to reduce charge injection problems from
ADC channel switching and sample/hold effects

If the offset has a temperature dependence then you would not be able to
take it out without doing calibrations at multiple temperatures. If the
opamp and the RTD are not always at the same temperature then it may be
impossible to fix by calibration.

Can you swap the opamp for a type that has less offset? There are ones
that would have the same pinout.

https://www.analog.com/media/en/technical-documentation/data-sheets/LT1013-LT1014.pdf

The LT1013 is quite nice, but not all that cheap. You can run a Pt1000 bridge with more volts across the platinum sensor than you can with a nominally one hundred ohm sensor - those are normally rated for 1mA through the sensor, which is only 100mV and 0.3% of that per degree Celcius is only 30uV per degree.

At that level you start seeing thermocouple voltages in the copper to invar to alumium-on-silicon junctions you\'ve got around the op amp, and you rapidily start thinking that the complexities of AC bridge excitation might be worth the effort.

--
Bill Sloman, Sydney

 

[Jasen Betts]

On 2021-11-28, Klaus Kragelund <klauskvik@hotmail.com> wrote:

Hi

I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inhireted

Problem is the large offset voltage of the opamp is amplified producing large errors

Can you drive it with AC instead?


--
Jasen.

 

[whit3rd]

On Sunday, November 28, 2021 at 1:44:33 PM UTC-8, Klaus Kragelund wrote:

28.11.21 22:35, Klaus Kragelund wrote:
Hi

I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC

I have been doing some tests. Quite odd, but hitting the circuit with a hot airflow of 60 Degrees creates 20mV offset on the output of the opamp. The specs define temperature drift of 5uV/K, so something weird is going on...

It is the input bias current that is most temperature-sensitive. It can vary by quite a lot (even change sign
at high temperature), so if the Thevenin input resistance of the two inputs mismatches by
more than a few kOhms, you get thermal trends as a result. Figure 11 nA nominal, but plan for
an order of magnitude more than that as range; a 20 mV change could result from 200k ohms
disparity in the input feeds.

 

[Tauno Voipio]

We jettisoned the amplifier and used a minimal component
count solution: A 20 bit sigma-delta ADC and a precision
10k resistor to the ADC reference and the sensor between
the input and ground.

This will lose 90% of the range of the converter, but we
still have more than 16 bits of resolution left. With
suitable microprocessor linearization, it is more than
needed for the sensor tolerances in a range of say, -50C
to +200C.

--

-TV


On 28.11.21 23.35, Klaus Kragelund wrote:

Hi

I have a PT1000 circuit where a LMV358 is used in a differential
coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inhireted

Problem is the large offset voltage of the opamp is amplified producing
large errors

We are contemplating production calibration, but I am worried that the
offset isn\'t stable after the calibration has been done

In litterature the offset comes from mismatch of the long tailed pair.
Is that expected to be stable, so a calibration done in production also
cancels out after 10 years operation?

By the way, my suggestion is to ditch the opamp and feed the signal
directly into the ADC. All the opamp errors disappears then

I just need a big sample cap to reduce charge injection problems from
ADC channel switching and sample/hold effects

Regards

Klaus

--
Klaus

 

[Spehro Pefhany]

On Sun, 28 Nov 2021 22:35:11 +0100, Klaus Kragelund
<klauskvik@hotmail.com> wrote:

Hi

I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inhireted

Problem is the large offset voltage of the opamp is amplified producing large errors

We are contemplating production calibration, but I am worried that the offset isn\'t stable after the calibration has been done

In litterature the offset comes from mismatch of the long tailed pair. Is that expected to be stable, so a calibration done in production also cancels out after 10 years operation?

By the way, my suggestion is to ditch the opamp and feed the signal directly into the ADC. All the opamp errors disappears then

I just need a big sample cap to reduce charge injection problems from ADC channel switching and sample/hold effects

Regards

Klaus

LMV358 is not a precision op-amp- its a general purpose low-voltage
part with Vos as much as +/-9mV. TCVos is not guaranteed and is
\'typically\' 5uV/°C. You can translate that into degrees error from
whatever circuit you are using.

Not sure about your comment about the resistor, you need at least one
precision resistor somewhere or you won\'t get a voltage as a function
of the sensor resistance. Maybe you\'ve got a bridge circuit given your
\"differential coupling\" comment- in which case all the resistors
affect the accuracy in general and the zero in particular.

If there is 2V across the sensor then the output at the sensor is
about 7.7mV/°C ,The Vos of the LMV358 is a bit more than 1°C error
worst-case, if there is 2V across the sensor. If you have a bridge
configuration a 1% error in one of the resistors represents several
degrees C error (20mV or so). So unless you\'re using precision (like
0.1% resistors) even the LMV358 is not your main issue.

With the series resistor, the excitation should be taken from the ADC
reference voltage, obviously, so the ADC reading is ratiometric. Using
the supply voltage as a reference for both can cause noise issues.

There are advantages and disadvantages to haveing an op-amp in there,
I would probably stay with it in most situations because it allows a
nice low pass filter. Sounds like your ADC has no PGA or buffer
amplifier.

You can get op-amps that are pin-compatible and have offset voltages
in the microvolts and TCVos in low tens of nV/°C. An old-fashioned
non-zerodrift precision type may be more resistant to EMI, though.

That said, you can probably count on the change in offset being within
<100uV at the same temperature far off into the future, even with the
crappy LMV358. If the voltage across the sensor is 2V, that\'s one or
two hundredths of a °C. If the Pt1000 sees much in the way of
temperature swing or mechanical stress that\'s in the wash.

Best regards,
Spehro Pefhany
--
Best regards,
Spehro Pefhany

 

[server]

On Mon, 29 Nov 2021 11:52:48 +0200, Tauno Voipio
<tauno.voipio@notused.fi.invalid> wrote:

We jettisoned the amplifier and used a minimal component
count solution: A 20 bit sigma-delta ADC and a precision
10k resistor to the ADC reference and the sensor between
the input and ground.

This will lose 90% of the range of the converter, but we
still have more than 16 bits of resolution left. With
suitable microprocessor linearization, it is more than
needed for the sensor tolerances in a range of say, -50C
to +200C.

We do that too. One current project has three thermistor Wheatstone
bridges (on the bottom of an optical gadget) feeding a multiplexed
differential-input delta-sigma ADC, using the ADC reference voltage to
power the bridges. It turned out to be fairly linear from 25 to 45c,
with microkelvin resolution and noise. It uses one entire PGA range.

https://www.dropbox.com/s/taqdht5epy4asqi/T500_Br_15K.jpg?dl=0

https://www.dropbox.com/s/05cirtpihrmahpu/T500_Br_15K_2.asc?dl=0



--

Father Brown\'s figure remained quite dark and still;
but in that instant he had lost his head. His head was
always most valuable when he had lost it.

 

[Lasse Langwadt Christensen]

mandag den 29. november 2021 kl. 10.52.58 UTC+1 skrev Tauno Voipio:

We jettisoned the amplifier and used a minimal component
count solution: A 20 bit sigma-delta ADC and a precision
10k resistor to the ADC reference and the sensor between
the input and ground.

a resistor on both top and bottom and a differential input ADC
keeps the two wires the same AC \"impedance\" helps with common mode noise

 

[server]

On Mon, 29 Nov 2021 09:05:15 -0800 (PST), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

mandag den 29. november 2021 kl. 10.52.58 UTC+1 skrev Tauno Voipio:
We jettisoned the amplifier and used a minimal component
count solution: A 20 bit sigma-delta ADC and a precision
10k resistor to the ADC reference and the sensor between
the input and ground.


a resistor on both top and bottom and a differential input ADC
keeps the two wires the same AC \"impedance\" helps with common mode noise

A Wheatstone bridge is better. Lets you crank up the PGA gain.

Use good resistors.



--

Father Brown\'s figure remained quite dark and still;
but in that instant he had lost his head. His head was
always most valuable when he had lost it.

 

[Lasse Langwadt Christensen]

mandag den 29. november 2021 kl. 18.10.49 UTC+1 skrev jla...@highlandsniptechnology.com:

On Mon, 29 Nov 2021 09:05:15 -0800 (PST), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 29. november 2021 kl. 10.52.58 UTC+1 skrev Tauno Voipio:
We jettisoned the amplifier and used a minimal component
count solution: A 20 bit sigma-delta ADC and a precision
10k resistor to the ADC reference and the sensor between
the input and ground.


a resistor on both top and bottom and a differential input ADC
keeps the two wires the same AC \"impedance\" helps with common mode noise

A Wheatstone bridge is better. Lets you crank up the PGA gain.

how would that work when the RTD is at the end of two wires somewhere else?

 

[server]

On Mon, 29 Nov 2021 09:21:59 -0800 (PST), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

mandag den 29. november 2021 kl. 18.10.49 UTC+1 skrev jla...@highlandsniptechnology.com:
On Mon, 29 Nov 2021 09:05:15 -0800 (PST), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 29. november 2021 kl. 10.52.58 UTC+1 skrev Tauno Voipio:
We jettisoned the amplifier and used a minimal component
count solution: A 20 bit sigma-delta ADC and a precision
10k resistor to the ADC reference and the sensor between
the input and ground.


a resistor on both top and bottom and a differential input ADC
keeps the two wires the same AC \"impedance\" helps with common mode noise

A Wheatstone bridge is better. Lets you crank up the PGA gain.

how would that work when the RTD is at the end of two wires somewhere else?

Two wires? You are stuck with the wire error.



--

Father Brown\'s figure remained quite dark and still;
but in that instant he had lost his head. His head was
always most valuable when he had lost it.

 

[Lasse Langwadt Christensen]

mandag den 29. november 2021 kl. 18.50.57 UTC+1 skrev jla...@highlandsniptechnology.com:

On Mon, 29 Nov 2021 09:21:59 -0800 (PST), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 29. november 2021 kl. 18.10.49 UTC+1 skrev jla...@highlandsniptechnology.com:
On Mon, 29 Nov 2021 09:05:15 -0800 (PST), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

mandag den 29. november 2021 kl. 10.52.58 UTC+1 skrev Tauno Voipio:
We jettisoned the amplifier and used a minimal component
count solution: A 20 bit sigma-delta ADC and a precision
10k resistor to the ADC reference and the sensor between
the input and ground.


a resistor on both top and bottom and a differential input ADC
keeps the two wires the same AC \"impedance\" helps with common mode noise

A Wheatstone bridge is better. Lets you crank up the PGA gain.

how would that work when the RTD is at the end of two wires somewhere else?

Two wires? You are stuck with the wire error.

so 4 wires, thought with PT1000 (~3.8 Ohm per C) is much less of an issue

 

[Klaus Vestergaard Kragelund]

On 28/11/2021 23.23, Jeroen Belleman wrote:

On 2021-11-28 22:44, Klaus Kragelund wrote:
28.11.21 22:35, Klaus Kragelund   wrote:
Hi

I have a PT1000 circuit where a LMV358 is used in a differential
coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inhireted

Problem is the large offset voltage of the opamp is amplified
producing large errors

We are contemplating production calibration, but I am worried that
the offset isn\'t stable after the calibration has been done

In litterature the offset comes from mismatch of the long tailed
pair. Is that expected to be stable, so a calibration done in
production also cancels out after 10 years operation?

By the way, my suggestion is to ditch the opamp and feed the signal
directly into the ADC. All the opamp errors disappears then

I just need a big sample cap to reduce charge injection problems from
ADC channel switching and sample/hold effects

Regards

Klaus

I have been doing some tests. Quite odd, but hitting the circuit with
a hot airflow of 60 Degrees creates 20mV offset on the output of the
opamp. The specs define temperature drift of 5uV/K, so something weird
is going on...

--
Klaus

The offset and its drift are specified at the input.

Yes. The differential gain is 6 times, so even for a 60 degrees
temperature change, the output would only change by 2mV (60*5uV*6)

 

[Klaus Vestergaard Kragelund]

On 28/11/2021 23.26, jlarkin@highlandsniptechnology.com wrote:

On Sun, 28 Nov 2021 22:44:27 +0100, Klaus Kragelund
klauskvik@hotmail.com> wrote:

28.11.21 22:35, Klaus Kragelund wrote:
Hi

I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inhireted

Problem is the large offset voltage of the opamp is amplified producing large errors

We are contemplating production calibration, but I am worried that the offset isn\'t stable after the calibration has been done

In litterature the offset comes from mismatch of the long tailed pair. Is that expected to be stable, so a calibration done in production also cancels out after 10 years operation?

By the way, my suggestion is to ditch the opamp and feed the signal directly into the ADC. All the opamp errors disappears then

I just need a big sample cap to reduce charge injection problems from ADC channel switching and sample/hold effects

Regards

Klaus

I have been doing some tests. Quite odd, but hitting the circuit with a hot airflow of 60 Degrees creates 20mV offset on the output of the opamp. The specs define temperature drift of 5uV/K, so something weird is going on...


Got a schematic?

It\'s quite close to this one:

https://i.stack.imgur.com/8uwas.png

Except the R1 and R3 is replaced by a fixed voltage reference. Gain is
about 6 of the diff amp. Output feeds into an ADC with RC filter to deal
with charge injection/sample capacitor

Are you seeing resistor tempcos maybe? An RTD is about 4000 ppm/k, so
you\'d need some very good resistors to keep their error contribution
down. If the pullup is 1K, that kills half of the gain too.

The pullup is 3kohm, and the amplifier is used to get full use of the
ADC range (0 to 3.3V)

All resistor are 0.1%/5ppm, so it does account for some error, but does
not explain it all

Would your RTD make enough voltage to drive the ADC directly? Too much
excitation current could self-heat the RTD.

My point about this circuit is to shift to a direct input into the ADC,
then only the pullup has temco. I would then use a pullup with the same
resistance as the PT1000 (1kohm), to maximize the dynamic range. Use a
FET to turn on the pullup to prevent self-heating of the PT1000

What about the RTD+resistor voltage reference? Is it the same as the
ADC ref?

Yes, for ratiometric measurement

Can you auto-zero?

No, it\'s a simple circuit so not an option

I would expect opamp offset to be pretty stable over time, so you
could cal it out.

 

[Klaus Vestergaard Kragelund]

On 29/11/2021 08.42, whit3rd wrote:

On Sunday, November 28, 2021 at 1:44:33 PM UTC-8, Klaus Kragelund wrote:
28.11.21 22:35, Klaus Kragelund wrote:
Hi

I have a PT1000 circuit where a LMV358 is used in a differential coupling to feed it to an ADC

I have been doing some tests. Quite odd, but hitting the circuit with a hot airflow of 60 Degrees creates 20mV offset on the output of the opamp. The specs define temperature drift of 5uV/K, so something weird is going on...

It is the input bias current that is most temperature-sensitive. It can vary by quite a lot (even change sign
at high temperature), so if the Thevenin input resistance of the two inputs mismatches by
more than a few kOhms, you get thermal trends as a result. Figure 11 nA nominal, but plan for
an order of magnitude more than that as range; a 20 mV change could result from 200k ohms
disparity in the input feeds.

Good point, I will try to measure it

 

[Klaus Vestergaard Kragelund]

On 28/11/2021 23.45, Chris Jones wrote:

On 29/11/2021 08:35, Klaus Kragelund wrote:
Hi

I have a PT1000 circuit where a LMV358 is used in a differential
coupling to feed it to an ADC

PT1000 is pull up with a resistor

I would NOT do it like that, this is a design I have inhireted

Problem is the large offset voltage of the opamp is amplified
producing large errors

We are contemplating production calibration, but I am worried that the
offset isn\'t stable after the calibration has been done

In litterature the offset comes from mismatch of the long tailed pair.
Is that expected to be stable, so a calibration done in production
also cancels out after 10 years operation?

By the way, my suggestion is to ditch the opamp and feed the signal
directly into the ADC. All the opamp errors disappears then

I just need a big sample cap to reduce charge injection problems from
ADC channel switching and sample/hold effects

Regards

Klaus

--
Klaus

If the offset has a temperature dependence then you would not be able to
take it out without doing calibrations at multiple temperatures. If the
opamp and the RTD are not always at the same temperature then it may be
impossible to fix by calibration.

Yes, that can be a problem

Can you swap the opamp for a type that has less offset? There are ones
that would have the same pinout.

It is possible, but trying to keep cost low. I would rarther just remove
the opamp and feed the ADC directly