On 29/11/2021 02.52, Anthony William Sloman wrote:> 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. >The LT1013 is good. It was preferred when I did space stuff. But it's too expensive
Opamp offset question
Started by ●November 28, 2021
Reply by ●November 29, 20212021-11-29
Reply by ●November 29, 20212021-11-29
On 29/11/2021 08.12, Jasen Betts wrote:> 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? > >That is not possible I am affraid
Reply by ●November 29, 20212021-11-29
On 29/11/2021 10.52, Tauno Voipio 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 have a 12bit ADC, so raw performance is not good: We do use oversampling, so that can get the performance back on track
Reply by ●November 29, 20212021-11-29
On 29/11/2021 15.04, Spehro Pefhany wrote:> 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. > >Good comments About the ADC reference it is taken from the SMPS with post LC filter. So there is a little difference between the reference voltage ripple and the voltage fed to the pullup resistor for the PT1000 One option to get better performance is to sync the PT1000 sampling to a time slice when the microcontroller has steady current consumption, but that makes the SW more complicated and susceptible if changes are done in other places in the SW>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.Yeah. problem is that we can have wide temperature range, so I am afraid the calibration is not valid
Reply by ●November 29, 20212021-11-29
tirsdag den 30. november 2021 kl. 01.20.00 UTC+1 skrev Klaus Kragelund:> On 29/11/2021 08.12, Jasen Betts wrote: > > On 2021-11-28, Klaus Kragelund <klau...@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? > > > > > That is not possible I am affraidcould you toggle between two currents ?
Reply by ●November 29, 20212021-11-29
Klaus Vestergaard Kragelund wrote:> 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)Probably some horrible temperature gradient times a huge thermocouple coefficient. Silicon has huge TC coeffs versus any metal, for instance. It should be much smaller in an isothermal test. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●November 29, 20212021-11-29
On Tue, 30 Nov 2021 01:12:55 +0100, Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:>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 PT1000If you can turn off the pullup, maybe you can autozero. Turn it off and measure offset.>> >> 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. >> >> >> >> >>We like to do RTDs like this: https://www.dropbox.com/s/kcb705scwytfzty/RTD-ADC.jpg?raw=1 The measurement is purely ratiometric on the Susumu thinfilm resistor. -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
Reply by ●November 30, 20212021-11-30
tirsdag den 30. november 2021 kl. 02.22.04 UTC+1 skrev John Larkin:> On Tue, 30 Nov 2021 01:12:55 +0100, Klaus Vestergaard Kragelund > <klau...@hotmail.com> wrote: > > >On 28/11/2021 23.26, jla...@highlandsniptechnology.com wrote: > >> On Sun, 28 Nov 2021 22:44:27 +0100, Klaus Kragelund > >> <klau...@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 > If you can turn off the pullup, maybe you can autozero. Turn it off > and measure offset. > >> > >> 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. > >> > >> > >> > >> > >> > We like to do RTDs like this: > > https://www.dropbox.com/s/kcb705scwytfzty/RTD-ADC.jpg?raw=1 > > The measurement is purely ratiometric on the Susumu thinfilm resistor. >an extra resistor on the ground side puts the differential voltage in the middle of the input range, and makes the noise "impedance" of the two signal wires similar
Reply by ●November 30, 20212021-11-30
On Tue, 30 Nov 2021 01:28:01 +0100, Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:>Yeah. problem is that we can have wide temperature range, so I am afraid >the calibration is not validNot just calibration of the electronics - those cheap sensors have significant hysteresis. -- Best regards, Spehro Pefhany
Reply by ●November 30, 20212021-11-30
On 30/11/2021 01.35, Lasse Langwadt Christensen wrote:> tirsdag den 30. november 2021 kl. 01.20.00 UTC+1 skrev Klaus Kragelund: >> On 29/11/2021 08.12, Jasen Betts wrote: >>> On 2021-11-28, Klaus Kragelund <klau...@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? >>> >>> >> That is not possible I am affraid > > could you toggle between two currents ? >Yes, could be. So you are thinking like syncronous detection? Shifting the signal to a higher frequency avoiding DC errors
