Hello I am Designing Thermocouple temperature Display Using MSP430AFE233. MSP430AFE has 24 bit ADC. It also has on chip PGA which can be used to amplify signal. But one drawback of it is that it has low input impedance nearly 200K. Does 200 K input impedance is sufficient to avoid overloading of thermocouple . also I have 100 Ohm resistor in series with 24 bit ADC which is used to design the Antialiasing filter. Does 200 k input impedance is sufficient, or it will cause to change the input voltages. What is the power level of thermocouple signals, Does thermocouple can easily drive the 200 K load without Loading. And also I don't want use any opamp at input because it will cause following problem
1} I have to generate Negative rail power supply , Because Thermocouple signal within 55mV. This voltage is near the rail in single power supply.
2) I have to correct the offset error due to OPAMP in the circuit. (ADC offset can be easily compensated in software due to internal short terminal input channel measurement ) . But OPAMP offset error is hard to remove .
3) also adding another opamp increases cost. And My aim to achieve greater accuracy with low cost. ( that is why I chose MSP430 AFE).
Does any one can Solve my Problem? 200K input impedance is satisfactory for thermocouple measurement with the thermocouple sensitivity 40uV/c (K type). Does it will affect full scale accuracy And if yes then what will be total error in full scale?