AD7718_ Thermocouple Application

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Question One customer used AD7718 to sample the thermocouple's output. The signal chain 
 is: Thermocouple + RC Filter + ADC.
 
 She tested AD7718, configuring the input as differential input, when she 
 connected the two input to the GND, the output is OK,the output noise is 
 several uV. Once she used the K Type thermocouple as the input, the output 
 become worse,the noise can be 100 uV maximum. Also she tested 
 cs5523+10k+0.1uF(Cutoff Frequency is 1kHz),CS5523 is better than AD7718. 
 
 So she felt CS5523 is better than AD7718. 
 
 Whether you have ever seen similar application? Maybe you can share some points 
 with us?
 Answer When the AD7718 is configured for differential inputs, the analog inputs are 
 buffered. The voltage on each analog input pin needs to be between GND + 100mV 
 and AVDD - 100mV. The customer has the thermocouple biased about GND. With both 
 inputs shorted to GND, the customer is actually outside spec (even though they 
 are measuring good noise). The thermocouple voltage should be biased up so that 
 the voltage on AIN+ or AIN- is above 100mV. The customer could use the 
 reference voltage to bias up the thermocouple voltage.
 
 The AD7793 and AD7794 have a bias voltage generator on chip which simplifies 
 the design. So, another option is to select the AD7793/4 rather than the AD7718.

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AD7718: FAQ