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 18.104.22.168, 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?
Best Regards, Mark P
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In reply to Joseph Wu:
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
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