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AD7768 Common Mode Voltage Causes Conversion Error

Question asked by AraAdamian on Mar 27, 2018
Latest reply on Mar 29, 2018 by NiallM

Hi, 

 

We are using an AD7768 in our design and are observing some noise on the digitized data only when we have a common mode voltage greater than 0V on our input. For example, if I put in a sine wave centered around the CM voltage (1.024V), the sine wave is digitized perfectly. If I have a -100mV or +100mV DC-offset from the CM, I start seeing impulses on the digitized data the somehow relates to the sampling rate. The "impulses" are roughly ~175 Hz in fast mode (sample rate of ~16MHz), and ~22Hz in eco mode (sample rate of ~2MHz). Any idea whats going? The magnitude of the impulse varies with the DC offset. If it is 100mV, the impulse is ~15mVpp. If it is 500mV, the impulse is ~30mVpp. I have verified these impulses are not part of the analog input waveform or the reference voltage.

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