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.

Thanks,

Gaurav

attachments.zip
  • Hi Gaurav,   

       This looks weird. You are using an sinewave generator for the analog input source. May I know if the source has a 50 ohm termination. The AD7352 evaluation board was design and ready for a source that has 50ohm source resistance, in which the default s19 was connected. If the source has no 50 ohm, then you can disconnect the s19.

    Regards,

    Jonathan

  • Hi Jonathan,

    Thanks for the reply. Yes, the source has 50 ohm termination.  I am using Agilent 33521A waveform generator. Let me know if you need any more input.

    Regards,

    Gaurav

  • Hi Gaurav,

       Thanks for the feedback. In the evaluation board when using the internal reference of 2.048, that means the common mode voltage is 1.024V ( U8-A output voltage divide by R10//R11). The Max input voltage range in table 2 of the datasheet is Vcm+/-Vref/2. Can you lower the input voltage of the sinewave. Base from the settings, the max input voltage to the AD7352 would be 2.048V. You can refer to the Figure 20 of the datasheet as this much very similar to the evaluation board.

    Regards,

    Jonathan

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