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Opamp offset question

Started by Klaus Kragelund November 28, 2021
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
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
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
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.
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.
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
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.
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.
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
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/&#2013266096;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/&#2013266096;C ,The Vos of the LMV358 is a bit more than 1&#2013266096;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/&#2013266096;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 &#2013266096;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