Other Parts Discussed in Thread: DAC8562, LMT85
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!