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Discontinuity in AD7352 ADC Output using internal reference

Hi guys,

I am trying to test AD7352 using AD7352 evaluation board provided by ADI, I am using the internal reference of 2.048 V.

I am facing issue when i am feeding a sine wave of 10KHz and 4.4Vpp. I am feeding the signal using a signal generator and reading the ADC output in Xilinx SDK. In the design, i have a provision to store ADC O/P data in FIFO and DSP can read the O/P data.

Following are my observations:

1. The max digital word corresponds to 2.048V (4095) and min digital word corresponds to -2.04V (0), this looks correct.

2. When the reconstructed (ADC output plotted) sine wave crosses 0V ( mid-range), there is abrupt change in the waveform(refer attachment), it goes to lowest value and then comes back to 0V and follows the Sine pattern therafter. This discontinuity

repeats in every cycle.

I tried feeding square wave of same frequency and amplitude(4.4Vpp). I reconstructed the waveform(refer attachment) and it looks fine. 

I also fed a sine wave of 10KHz frequency and amplitude of 3Vpp. The reconstructed waveform(refer attachment) has a peak voltage corresponding to digital word of 0.77V and the discontinuity repeats in every cycle. 

Please help us resolve the issue when I feed a wave of Vpp < 4.096V and when the sine wave crosses 0V.

I feel that the eval board is designed in which input voltage (peak to peak) should always be greater than or equal to 2* Vref,  (due to fixed Vcom= Vref/2) am I correct?

Please answer.


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