The power of ad7989-5 must be 2.5V? i use 3.3V, and then i use single mode directly ,the result is that if ain+ is under 2.0v, the adc out is wrong, how to solve the problem on pcb board?, 3.3V or single mode？
AD7989 is a true differential ADC. Input signal must be antiphase around Vref/2. In your case the other analog input is connected to GND which violate the restriction. This affect the results you are getting, specially putting one analog signal to GND in a differential ADC. I would suggest to change to a Pseudo- Diff part like we recently released the AD4000 or to use a fully differential amplifier like the ADA4932 to do the SE to Diff conversion before the AD7989. If you want a 2MSPS 18bit part you can also look at AD4003 newer part recommended for a new design. For the VDD we can guarantee the part operating above Absolute Maximum Rating(AMR).
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