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.
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.
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.
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)