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AD5933 receiver stage problem

TL;DR: I think the VDD/2 reference in the reciever stage of my AD5933 is messed up.

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I am working on an AD5933 for a bioimpedence application.

Following the paper https://www.mdpi.com/2079-9292/4/1/1 I have decouples the reciver-end of the AD5933 from the AFE and shorted out RFB and VIN. The intention is to make the first op amp at VIN into a voltage follower. The PGA is set to x1 amplification. I should be able to measure the fourier transform of a DC signal proportional to VDD/2.

Eksperimental setup:

Start Freq Register = 2^15 (32768=1kHz@16.776MHz)*
Freq Incr Register = 0
Settling Cyckles = 255

* The frequency corresponds to one whole DDS sinewave cycle in the DFT.

My problem is now:

1) When measuring the voltage signal at RFB (or VIN) with an oscilloscop during a scan I get a nice DC signal that creeps up. Starting from arround 3V at the beging of a scan ending at 55mV from the 5V rail.

2) This effect shows up in the DFT readouts; most prominent in the real register at this freqency

The imaginary register shows the same signal but at a lower level (and more noisy)

"What is going on with my chip"???

Teddy



Added TLDR note and tag
[edited by: tfsoft at 10:05 PM (GMT 0) on 15 May 2019]
  • What you observe is really unusual and it seems to be a hardware issue. When you short VIN and RFB pins the input OPAMP indeed becomes a follower that passes VDD/2 to the ADC input. The VDD/2 measured by the scope at the VIN/RFB short should be stable. Are you using the demo board or is the circuit something you built yourself? I would try to make sure that all pins are well soldered to the PCB.  If it is your design, does it use separate DGND and AGND, AVDD1,2 and DVDD? Could it be that AGND pin is not grounded? I would check if power supply voltage is stable and present at all the relevant pins of the chip and all grounds are tied together and to power supply zero.

    Once the hardware is functioning properly the Fourier transform of any DC signal should give you  both Re and Im values very close to zero when the sampled sequence encompasses integer number of excitation cycles.

    Good luck with the chip, hopefully is was not accidentally fried.

  • Thanks for your input.

    soldered the chip to a DIP converter and I use a breadbord and an arduino uno for power.

    All xVDD and xGND-pins are tied to the arduino 5V and GND.

    After the soldering I checked for continuity on the individual pins.

    I think the concusion must be that I need to order a new chip

  • All this is just speculation without seeing the actual circuit, but I am afraid it is likely the case. I would certainly try ordering some free samples.

    One rather remote possibility is that the chip or some part of it is somehow getting stuck in Standby mode. ADI documentation is pretty vague as to what specifically happens with the internal circuitry in Standby, so one thing to try would be to connect some resistor between RFB and VIN pins instead of the short, say 10-100k. If nothing else is connected to VIN the input OPAMP will still act as the follower. When the chip is out of Standby you may check with the scope if the voltage at RFB pin is VDD/2 and also see if the OPAMP is alive by poking at VIN with a screwdriver or something to inject some noise and see it it shows amplified at the RFB pin.