ADS1248: PGA temperature effects and calibration questions

Part Number: ADS1248

My customer has the following:

The PGA (per the data sheet) has a tempco associated with supply voltage, data rate and specific gain.  For reference see Fig's 17-20: 5V and Fig's 21-24; 3.3V of the data sheet.  We set the gain to 16 and 32, use a 3V supply and 2.5V reference.  We have a operating temp of -20 to 55 C.  Our testing shows the tempco impacting our system performance.  Without going into circuit detail, we actually used this ADC to servo a TI DAC output.  Works well … at room temp.

 

ADC calibration:  However, our software engineer has read on the e2e forums, that ADC calibration can be painful to constantly tweek the gain cal constants.  Question: The curve shown on Fig 23: is that THE gain vs temp curve, or is that a typical curve?  If just typical, do we need to do a two point against the PGA, for each ADS1248 to fit and fix this gain skew?  Or, do you folks have some recommendations on calibrating out the gain & offset errors over temperature?

 

I don’t see how I can add a circuit to compensate.  This is in a thermocouple measurement circuit.  We do know the cold junction temperature.  There are also the built in temp sensing diodes.  I have checked “e2e” but nothing jumping out at me.

 

My associated questions:

1) PSRR is 100dB, min. Is this over temperature?

2) I believe one can do a self-cal (either at 25C or at Temp) – as outlined in section 9.4.5.3, page 40 of the datasheet by using

SELFOCAL. yes, there is a manual version by using SYSOCAL, but it sounds like we want to avoid this from a S/W perspective.

Gain error is calibrated with the SYSGCAL (and from what I can tell, this is a self-cal test). Correct?

 

Thanks!


Best Regards, Mark P

3 Replies

  • Mark,


    For the gain vs temperature curves in the Typical Characteristics section, I would call these typical curves for the gain, not exact. I'll skip to the two questions that you ask at the and then come back to the gain/offset calibration in more detail.

    1. The PSRR is listed in as 100dB as a minimum in the Electrical Characteristics table. This should be good for the temperature range. I would note there is some variance with PGA gain. This is noted in the graph in Figure 32.

    2. For most systems, we recommend a self offset calibration and no gain calibration. For SYSOCAL, this is most useful when there is some sort of front end outside of the ADC that contributes some large offset. The SYSOCAL would remove this offset. As for the gain calibration, the FSC register stores the gain calibration coefficient. That value is updated with a gain calibration. Generally, the values trimmed into the part will give good gain performance and are trimmed for each PGA gain.

    In the ADS1248, the input and reference are capacitively sampled into the ADC (the modulator of this delta-sigma type ADC). The FSC is for finely calibrating the difference between the input capacitors and reference capacitors.

    As for the temperature coefficient errors, it's hard for me to give recommendations without knowing the accuracy the customer is looking for. Also, I'd like to compare those numbers with the drift they are seeing in their system. What is the magnitude of the offset drift and gain error drift they get in their system? I would imagine some of the gain drift they see is the drift of the external 2.5V reference voltage. What external reference are they using?

    For most thermocouples, the accuracy is going to be worse than that of RTD measurements based on the sensor itself. For most thermocouples, I would have guessed that a 16-bit ADC would be enough for an accurate temperature measurement.


    Joseph Wu
  • In reply to Joseph Wu:

    Joseph,
    Here are the Customer responses:
    "I shall try my best to answer this with circuit basic description. A DAC8562 set’s a simulated thermocouple voltage. An analog diff channel on the ADS1248 reads that. Circuit servos based on the read. There is an op amp between the DAC and ADS to gain (actually divide) the DAC output based on 2.5V reference. At room temp this works extremely well. We observe a fairly linear response due to temp between -20C and 50C. I would say at the extremes, we are out 30uV – 40uV. We have a spec of 5uV. Maybe we are crazy. Especially when I compare to a competitors product.
    We have implemented an offset cal tied to reference cal. This did help a great deal between 23C and 50C. However, -20C show the 30uV to 40uV.
    No offense, but a competitor 2.5V reference is being used. However, I would expect that the reference being the same for both the DAC and ADS, these would ratiometrically wash? "

    For the 16-bit resolution observation: "Normally 16 bits, but wanting those uV of TC connections measurement – the ADS1248 looked like the correct part."
    I hope these responses help...

    Best Regards, Mark P

  • In reply to Mark Pearson:

    Mark,


    I had thought this was a simple thermocouple circuit, but it looks like there are a few more parts to their setup. I'd still like to know what the components are even if they aren't TI parts. It just helps to track down the drift characteristics of the all the components involved. This would also include the op-amp used as the buffer. It would help to have a full schematic to look at where the sources of error come from. If there are other components (series filter resistances, protection diodes that have leakage), there may be other errors to consider.

    There will be some ratiometric cancellation if the DAC and the ADC have the same reference, but then the DAC also has it's own gain drift which needs to be calibrated out. I'm not sure if the DAC or ADC has the larger drift both seem similar.

    To start, I think it would be best to have some precision measurements of both the ADC input (or DAC output) and the reference over temperature. This will give some idea if the the ADC measurement is really drifting, or if the DAC output has a larger drift.

    I would also note that most thermocouples have a fairly large tolerance error. The best thermocouples have an error of ±0.5°C plus a tolerance proportional to temperature. If a precision measurement is necessary, using an RTD is probably a better solution.


    Joseph Wu