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why do we have to discard the first few readings? - AD7793

Question asked by Invensys on Jul 17, 2012
Latest reply on Jul 22, 2012 by harryh



We are using AD7793 to digitalize the input signal transmitted by the sensor. There is a large RC low pass filter at the input; R is 200K and C is 0.33uF. The buffer in the ADC is turned on and the mode is set to be continuous. If we run it at the slow output rate 4.17Hz, the first few readings always come in low, especially the first reading, which can be 2mV off out of 0.5V input signal. The rest of the readings are within uVs from the ideal reading. If we set the output rate 470Hz, hundreds readings are just expontentially approaching a value. My question is what caused this drop in the input even with the input buffer on? The reference does not seem to be the problem. We tried both internal and external references, it made no difference.  We took out the C, and the problem goes away. However, in our application, the large filtering is needed. Do we have to put a buffer between the filter and the input of the ADC?