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.
Opamp offset question
Started by ●November 28, 2021
Reply by ●November 29, 20212021-11-29
Reply by ●November 29, 20212021-11-29
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
Reply by ●November 29, 20212021-11-29
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.
Reply by ●November 29, 20212021-11-29
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?
Reply by ●November 29, 20212021-11-29
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.
Reply by ●November 29, 20212021-11-29
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
Reply by ●November 29, 20212021-11-29
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)
Reply by ●November 29, 20212021-11-29
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. > > > > >
Reply by ●November 29, 20212021-11-29
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
Reply by ●November 29, 20212021-11-29
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
