AD7690 Readings / Output code

Hello,

Hopefully someone may be able to help me out...

I am testing out an AD7690 18-bit ADC for an application. Currently I can get a reading from the ADC, though it is not what I expect.

Vref is set to 2.048 V and there is 1.45 V onto IN+, IN- is connected to ground.

I assume that I should still get a code between 0x00000 and 0x1FFFFF for IN+, so I would expect to see roughly 0x16A80 (192800 decimal) for the 1.45 V, what I am getting however is around 0x1284B (75851 decimal). Working with the calculation of 15.625 uV per bit, (2.048*2 / 2^18).

My question really is given that I have IN- connected to ground, can I still expect to be able to read a code as above? Or does it not work that way? If I am expecting the correct code then it may well be the communications that is the issue.

Best Regards

Anthony

  • 0
    •  Analog Employees 
    on Sep 2, 2014 5:31 PM

    Hi Anthony,

    For a differential converter like this it is best to use it in that fashion. We have plenty of pseudo-diff/SE parts like the AD7686 or AD7988-5 that can be used as you describe and are pin compatible. Generally the differential ADCs like the AD7690 require the common mode voltage to be maintained at Vref/2 or 1.024V in you case. Your common mode in the above described scenario is much lower (1.45V)/2 = 0.725V. I would try to keep the common mode within 100mV of Vref/2. Hope this helps.

    Best Rgds,

    Alan

  • Hi Alan,

    Thanks for the response.

    Is it not OK to use the AD7690 as single ended? Just taking IN- to GND like on the AD7686? We went for the AD7690 because of the 18-bits.

    In this case would the common mode voltage matter if I am only using IN+ ?

    Best Regards

    Anthony

  • 0
    •  Analog Employees 
    on Sep 2, 2014 7:21 PM

    Hi Anthony,

    I'm not sure why you would want to use the AD7690 single-ended as you lose half the range effectively making it a 17 bit converter? The AD7686 is pseudo-differential input which means it is designed to have the IN- connected to ground. This is not the case with the differential AD7690. In order to maximize the performance of the AD7690 IN+/IN- should swing 0-Vref 180deg out of phase around a VCM of Vref/2, see below.

    Best Rgds,

    Alan

  • Hi Alan,

    Thanks for  the explanation. The 17-bits is fine for us, the higher the better really. As far as I can see you only do up to 16-bit single ended ADC's, that's why we went for the AD7690.

    I'm sure it should work as we expect, but with everything being untested I'm trying to get something going to start.

    We do have the 10 Lead Pulsar Eval board (Rev.C) which has the AD7686 on board, we were hoping to swap this out for the AD7690 and use the software you guys provide to test out our circuit... but from what I gather this is not possible due to the EEPROM chip on the board, is this correct that I would need a new EVAL board to use the software you provide for the AD7690? It seems a shame I can't just swap the chip out as they are pin compatible.

    Best Regards

    Anthony

  • 0
    •  Analog Employees 
    on Sep 3, 2014 5:47 PM

    Hi Anthony,

    I will send you an email on this. Again this is not the recommended usage for the AD7690 so you may run into performance issues.

    Best Rgds,

    Alan