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what will be the total error in thermocouple measurement due ADC input Imedance.

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?