I have problems with AD7656A. It is connected in hardware mode, with internal reference, VDD/Vss +/-6V, input +/-5.0V max., input channels V1-V6, no STANDBY, CONVSTA+CONVSTB+CONSTSTC tight together, SPI interface, no daisy chaining.
My problem is that it return only two different values when I apply a voltage on input channels :
- -32768d if input voltage is under 0V (every bits low on DOUTA)
- +32767d if input voltage is more than 0V (every bits at 1 on DOUTA)
Same result with all channels.
Ok, my board is not very clean (build on a lqfp to dip board adapter), decoupling is not very good... I suppose it could affect precision, noise, but it's probably not the root of problem. Do you have an idea ?
Thanks for your help,
Thanks for your reply.
Here is my (very minimalist) schematic. As you can see, both AGND and DGND pins are tied together.
Can you post a schematic? What is the relationship between the ground return on your signal source
and AGND and DGND pins on the ADC?
What are the pin numbers in the lower left corner?
It's acting like you have the internal ref disabled.
Tell me about pins 51, 61, 62, 63
Sorry, my schematic is barely legible.
Pin 51 (RefIn/out) is connected to a 10uF tantalum capacitor
Pin 61 (Ser/Parsel) to 5V
Pin 62 (H/SSEL) to ground
Pin 63 (WR/RefEn) to 5V
The potential at pin 51 is 2.5V.
Thanks again for your help.
The only things I can think of is a bad part or a miswire, i.e., your board doesn't match your
schematic. I'll email the product line apps engineer to take a look at this thread.