Hello, I could not reply for your earlier email as we were busy testing the instrument and found some new observations which we didn't see earlier. We are checking our input circuit schematic as shown in my previous mail and checking the stability in display. While testing with mV source at input we found fluctuations happening in one of the instruments at higher input values while the other instrument was stable. As in my earlier mail I had said the instrument reads data from grounded thermocouple soldered on the body of the heater and after temperature goes above 500 degrees the temperature drops to ambient suddenly. The setting done for the instrument is as shown in the previous mail and I have therefore set the ADS1148 for internal reference and the resistor used at the REFP1 is for RTD which is not used here as IDAC is not turned here. While testing we found that the datasheet says that VREFCOM should be connected to (AVDD+AVSS)/2 for bipolar supplies but we have connected to AVss= -2.5V Does this cause problem for the problem that we are facing on field? i.e. the temperature measured by grounded thermocouple dropping to ambient after 500 degrees. Still the fluctuations obtained in 1 instrument didn't go. So we tested for different conditions.
1) We checked by displaying for single channel and avoidin g switching between channels and still the fluctuations were observed. The fluctuations were observed more at higher input values
2) We displayed values using system monitor function to display DVDD and here values were found to be constant.
Please suggest what could be the source for this problem. Also what should be the connection for VREFCOM as per my supply connections in the schematic.
Waiting for your reply
With regards,
Dhiren
