AD4022 Issues

Hi,

I am trying to read the differential voltage from a pair of hall sensors using the AD4022. The AD4022 seems to read fine until a certain positive or negative voltage is read. It doesn't seem to be a data unpacking issue, as I have two separate boards, and the problem happens at slightly different voltages between the two. In the figures below, the data read by the AD4022 is shown in the top plot, and in the bottom plot I have the voltage recorded by a "ground truth", which is an AD7609 that I have verified works. 

It seems to me that the AD4022 is mixing bits after a certain point, and so the voltage goes up in "steps", which is shown best by this first figure.

   

Again, it doesn't seem to be a data unpacking issue, as when the voltage spikes and then goes down in the second figure from 1.85 to 1.15 V, the first byte of the data from the AD4022 goes from 0x47 to 0x31, as verified on the Oscilloscope. However, the ground truth voltage is steadily increasing in that entire time, so the first byte should not decrease.

   

This third figure shows that this happens for negative voltages as well, around -0.8 Volts.

 

The AD4022 is on a custom PCB as shown in the schematic below.

I am communicating with the device as specified on page 31 of the datasheet. I have copied page 31 below, as well as a scope probe showing my communication.

Please let me know if there is any other information that would be useful in figuring this out.

Thanks,

Lukas

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  • +1
    •  Analog Employees 
    on Nov 17, 2020 10:49 PM

    Hi Lukas,

    Thanks for your query.

    Please note the "Common Mode Input Range" requirement listed in Table 1 of the AD4022 data sheet. Basically this specification means that the AD4022 expects the average of the IN+ and IN- voltages to be equal to VREF/2:

    (VIN+ + VIN-)/2 = VREF/2

    Looking at the circuit with the hall sensors + AD4022, it doesn't look to me like this requirement is being met. Operating the AD4022 out of this common mode input range can result in the nature of errors you are seeing.

    For your circuit which is using a VREF of +4.8V, the average/common mode voltage on the IN+/IN- pins must be (nominally) +2.4V.

    Basically, for any amount of voltage that the IN+ pin moves away from VREF/2, the IN- pin must move an equal amount but negative amount. For example, if VREF/2 = 2.4V and VIN+ = 3.4V, VIN- must be 1.4V. This would correspond to a differential input voltage of 2V with a common mode voltage of 2.4V.

    Regards,

    Tyler

  • Hi Tyler,

    Thank you for the reply. I remember seeing that inputs must be 180 degrees out of phase in the datasheet, but I didn't really understand what that meant or why it was needed, and thus promptly forgot about it. Looks like I should read the datasheet a bit more closely next time haha. I appreciate your explanation of the issue.

    Best,

    Lukas

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