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AD7794, port AIN5, contemporary use excitation currents and measure voltage input

Category: Hardware
Product Number: AD7794BRUZ
Software Version: 4F

I am using AD7794 in a custom design, the application is equivalent to evaluation board CN0359, so the component is used as a substitute for ADUCM360.

As shown in the first figure below, AIN5 (+/-)  are connected to a precision resistor (R35 in the picture) and I use AIN5(-) as an output to drive an excitation current into R35 (which is itself connected in series with a RTD).

While AIN5- is configured to drive the excitation current, I also use AIN5 port as differential input and acquire the (bipolar) voltage on channel AIN5. 

My questions are:

  • can AIN5 be used contemporary as IOUT and as an input channel?
  • is this a correct usage of the device, or should I modify the design to measure R35 from another channel (AIN3 or AIN4 are free)

From my tests, if I compute the ratio between:

  • the voltage that I read on channel AIN5 and
  • the excitation current configured in the ADC,

the resistance I obtain is very far from 1.5K, it is instead around 2K: when the excitation current circuit is enabled, is R35's end directly connected to the internal ADC's input (i. e. the same condition as in the second picture relative to ADUCM360)?

Thank you for your kind attention.

Top Replies

  • Hi  ,

    Sorry for the delayed response. You can use AIN5 simultaneously as IOUT and as an input channel. However, it is recommended to enable IOUT on a dedicated pin and use a separate input channel…

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