high gain ADC modes

 			%AAAA_BBBB_FFF_PPPVVV_OHHHLLL_TT_MMMMM_0
			%VVV = ADC config
			000: GIO, 1x (~5 volt range, centred on VIO/2)
			001: VIO, 1x       "
			010: RevB: PinB, 1x      RevC: high-Z
			011: PinA, 1x      "
			100: PinA, 3.16x (~1.58 volt range, centred on VIO/2)
			101: PinA, 10x   (~0.5 volt range, centred on VIO/2)
			110: PinA, 31.6x (~0.158 volt range, centred on VIO/2)
			111: PinA, 100x  (~0.05 volt range, centred on VIO/2)
I just noticed that there are ADC modes with a gain higher than 1x what means they have a smaller input range that doesn't include GND and VCC. What are those modes intended to be used for?

I think they can probably used only for AC signals like microphones or other audio signals because the auto-calibration (GIO/VIO switch) doesn't work for them. I fear that the offset could be larger than the input range (at least for the 31.6x and 100x modes) what would make them completely useless for DC signals. Or can we make assumptions on the ratios of the internal resistors so that it's possible to calibrate at least the offset in 1x mode and then switch to higher gain?

I wonder if this could be used to measure low voltage sensor signals like thermocouples directly.

Comments

  • evanhevanh Posts: 8,960
    edited 2020-03-26 - 19:43:46
    Yeah, you've summed it up, not intended for DC.

    Interestingly though, the revC silicon change allows measuring of the PinA VIO/2 centre point, or zero-bias point as Chip referred to it recently. So that feature may be effective for handling DC when they come available.

  • As the adc's are based on current input, not voltage, a current mirror can easily shift the input signal to the input range of the ADC
  • evanhevanh Posts: 8,960
    edited 2020-03-26 - 20:09:56
    Ah, no, I think there's buffer op-amp in between. The external inputs are voltage. I could be wrong.
  • jmgjmg Posts: 14,278
    evanh wrote: »
    Ah, no, I think there's buffer op-amp in between. The external inputs are voltage. I could be wrong.

    The PINA connects via a choice of resistors to a virtual earth 'MID' biased Opamp.
    evanh wrote: »
    Interestingly though, the revC silicon change allows measuring of the PinA VIO/2 centre point, or zero-bias point as Chip referred to it recently. So that feature may be effective for handling DC when they come available.
    That may help a little.

    ManAtWork wrote: »
    I think they can probably used only for AC signals like microphones or other audio signals because the auto-calibration (GIO/VIO switch) doesn't work for them.
    Yes
    ManAtWork wrote: »
    Or can we make assumptions on the ratios of the internal resistors so that it's possible to calibrate at least the offset in 1x mode and then switch to higher gain?
    That would work, to a point.
    50% of the 1 cal Vcc/Gnd would be close to MID and as evanh says, the rev C NC allows closer MID checks
    ManAtWork wrote: »
    I wonder if this could be used to measure low voltage sensor signals like thermocouples directly.
    Even with an external chopper (get get AC signal), the noise floor of the P2 is likely to be poor. Best to use an external amplifier or higher performance ADC for thermocouples.
  • evanh wrote: »
    revC silicon change allows measuring of the PinA VIO/2 centre point, or zero-bias point

    Yes, but the result is the digital bit pattern of the AD-converted bias point. You still don't know the actual voltage at the bias node. To measure a termocouple you have to connect one wire of the thermocouple to the ADC pin and the other to a constant voltage with the exact voltage level of the internal bias point. You could trim the external bias voltage with a DAC output and an external analogue MUX to match the internal bias point.

    ErNa wrote: »
    As the adc's are based on current input, not voltage, a current mirror can easily shift the input signal to the input range of the ADC

    Yes, if you had a perfect current mirror. Practical (affordable) current mirrors I've seen (BCV-something) have offset voltages in the 5mV range and gain tolerances of +/-20..50%. Temperature drift makes manual trimming hard. I fear they are pretty useless for high precision applications.

    A voltage driven current source built with a chopper stabilized precision OP-amp should work, though. Not much cheaper than an external ADC but simpler.

    An external chopper (analogue MUX) to generate an AC signal might also work. You have to be careful because cheap analogue switches have unbalanced charge injections which is seen as a DC offset. But you could not only chopper the polarity but also the frequency making it possible to calculate and compensate the error.
  • evanhevanh Posts: 8,960
    edited 2020-03-27 - 09:29:39
    ManAtWork wrote: »
    To measure a termocouple you have to connect one wire of the thermocouple to the ADC pin and the other to a constant voltage with the exact voltage level of the internal bias point.
    You only need to know the temperature and voltage at the electrical terminals of the two thermocouple wires. The rest is all relatively simple maths.

    Biggest problem with thermocouple measuring is noise removal, particularly common-mode DC. Electrical isolation is pretty much a necessity.

    The nature of the thermo-electric effect means it affects all wires but normally this effect is cancelled by the fact that the same metal is used along all wires in the circuit. The remaining non-thermocouple wiring is self-cancelling. This is why just knowing the temperature at the terminals works so well.


    As for the maths, it's as simple as knowing that when probe temperature is same as terminal temperature then you always have 0.0 Volt measurement. So probe temperature = measured volts * calibrated scale + terminal temperature.

    Well, it's a little more complicated by the fact that thermocouple behaviour isn't a linear scale. A lookup table is often used for the voltage to calibrated scale translation.

  • Noise is no problem because the signal changes very slowly. You can apply a really low bandwidth low pass filter averaging many, many samples.

    In this thread I focused on the ADC and how to trim offset errors. Other issues with thermocouples like cold junction compensation and linearization have already been discussed in this old P1 thread.
  • evanhevanh Posts: 8,960
    edited 2020-03-27 - 10:08:08
    My point is you don't need any added voltage. What you need is to know the temperature of the terminals.

    And there isn't a low pass filter that can remove the DC drifts you get from common mode interference.

  • evanhevanh Posts: 8,960
    edited 2020-03-27 - 11:35:25
    Oh, the internal bias point, apologies, that certainly didn't sink in what you meant. It won't be a steady target I don't think. You'll be adding more drift trying to match it with an external voltage.

    Use either an external voltage amp to bring the voltage up to x1 gain on the internal ADC, or better still an external ADC with isolation.

  • jmgjmg Posts: 14,278
    edited 2020-03-27 - 23:49:24
    ManAtWork wrote: »
    An external chopper (analogue MUX) to generate an AC signal might also work. You have to be careful because cheap analogue switches have unbalanced charge injections which is seen as a DC offset. But you could not only chopper the polarity but also the frequency making it possible to calculate and compensate the error.

    That's probably the best 'low cost helper' approach.
    You could also use wider MUX to allow a zero and full scale (mV) calibrate from a divider, because P2's ADC-gain is also not precise.
    That would be 2 digital control lines, plus an Analog In, and (maybe) a SetCal output.
    The low charge injection MUXs do move up in price, and something like NAU7802 etc may be good enough (i2c 24b ADC with PGA)
  • Could there be some solution along the lines of using a sample-and-hold to capture the ADC's bias point, and then connecting the thermocouple across the output of the sample-and-hold and the ADC lines? Would it require too many analog switches?
  • jmgjmg Posts: 14,278
    Could there be some solution along the lines of using a sample-and-hold to capture the ADC's bias point, and then connecting the thermocouple across the output of the sample-and-hold and the ADC lines? Would it require too many analog switches?

    A cap on the ADC pin should capture the average bias point, and you could toggle a thermocouple on top of that with an analog switch, or you could equally toggle it on the GND leg of the cap, to allow one side of the thermocouple to be GND. The p-p AC change is the thermocouple mV, but you still need a gain calibrate pass.
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