AD7656A return only two values

Dear community,

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,

Sylvain

  • Thanks for your reply.

    Here is my (very minimalist) schematic. As you can see, both AGND and DGND pins are tied together.

  • Hello Harry,

    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.

    Sylvain

  • Thank you for your help.

    I checked potentials at RefcapX pins, it was  0. And when I link Refcap to RefOut, it start working ! You're right, it's a reference problem. But my pin 1 (RefBuf En) is tied to ground, this reference buffers are supposed to be enabled.

    I will check every connections/soldering, it's probably a miswire somewhere.

    Sylvain

  • Hi Sylvain,

    Are you supplying the AD7656 with a reset pulse after powering up? After power up the device must receive a reset pulse. It could be that the device on power up is expecting an external reference.

    I also noted from your schematic the REFCAPX pins and REFIN/REFOUT pin show only one capacitor at the pin. Please note for optimum performance that a 10uF and 100nF capacitor is required on each of these pins. (See the Typical Connection Diagram in the datasheet)

    Regards

    Niall