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There is an article called” EXPLORING DIFFERENT SAR ADC ANALOG INPUT ARCHITECTURES” on analog devices website.

About Pseudo Differential, it says that If a pseudo differential device with limited IN voltage range, such as the AD7980, needs to reject undesired signals greater than the absolute input voltage range, the signal chain designer may need to consider an instrumentation amplifier to remove these larger common modes before presenting the signal to the ADC. For differential application, as with the pseudo differential devices, if large common modes are present in the system, an instrumentation amp should be used to condition the bulk of the common mode. Would you please show me an reference circuit that explains this approach?


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[edited by: tschmitt at 2:03 PM (GMT 0) on 20 Apr 2020]
  • Hello,

    Hopefully the sketch below clears this up. Basically the scenario described is if you have some sensor, etc. that has a large common-mode output voltage. In the figure provided, this is represented by VCM.

    For many ADCs, connecting such a sensor directly up to the ADC's inputs would violate their absolute input voltage range specifications. In the article, the example of the AD7980 is given, which has an IN- input range of only +/-100mV relative to its GND pin. So if VCM is >0.1V then this specification would be violated.

    Instrumentation amplifiers are exceptional at removing common-mode signals by design, hence why they are recommended for this purpose in that article.

    There are several reference designs which show a signal chain utilizing an instrumentation amplifier with pseudo-differential and differential ADCs. For pseudo-differential ADCs, CN-0393 shows the AD8251 PGIA interfacing with the ADAQ7988. For differential ADCs, CN-0345 and CN-0385 show the AD8251 PGIA driving the AD7982 and AD4003, respectively. The ADA4254 PGIA data sheet also shows an example of it interfacing with the AD4007.