Other Parts Discussed in Thread: ADS122U04EVM, ADS122U04, ADS1247, ADS124S08EVM, ADS124S08, ADS124S06, ADS1120, TIDA-00018, TIDA-00189, ADS1118, ADS1118EVM, MUX36D04, ADS114S08
I am trying to combine the designs in page 49 and 52 in ADS1220 Datasheet, to make one module to handle 2 Thermocouples or 1 RTD on its inputs.
The design specs are:
1. Unipolar 3.3 Volts supply.
2. Internal reference used for thermocouple and external 1 KOhm resistor used as Rref used for RTD.
3. Fc-Diff = 51 Hz and Fc cm = 1085 Hz
The two thermocouple inputs are TC1(Ain0,Ain1), TC2(Ain2,Ain3) or 1 RTD input.
The thermocouple input needs to be biased and filtered in this case I will have 4 filters on all inputs and bias resistors, the problem is when using the module as 1 RTD input,
the bias resistors and filters will remain in the signal path, I am not sure what this will cause! I don't want to use analog switches to avoid making the design more complicated and adding some extra noise.
I am thinking of having a bipolar supply to take the bias resistors off the design then, only RC filter will remain in current path when using RTD. Will this filter badly degrade the RTD measurement?
I could not see any reference design from TI using bipolar supply, is there any disadvantage of using bipolar supply?
One last point, if the used thermocouple was grounded to the machine. will the unipolar supply with bias resistors work as it should?