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Hello everyone, My question is about the use of AD7705 in differential mode configuration. Is it possible to have two simultaneous input signals at the device? In other words, the input signal1+ and signal1- at Ain1+ and Ain1-, respectively. While the inpu

Category: Hardware
Product Number: AD7705

AD7705/AD7706 datasheet describe that the AD7705 is 2 fully differential input channel ADCs. But this procedure is not clear how to proceed in the schematic.

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  • The AD7705 is fully-differential. It subtracts the voltage at AIN1(+) from the voltage

    at AIN1(-), and that difference value is what is digitized. If I parse your question

    literally ... AIN1(+) and AIN1(-) must be applied "simultaneously". But I'm guessing

    that's not what you're really asking. I'm guessing what you're asking is, "I want

    to digitize the absolute value of the signal applied to AIN1(+) and the signal

    applied to AIN1(-), applied simultaneously to the two pins, rather than the

    difference between them."  If that's your question, the answer, I believe, would

    be "no", 


    How to proceed depends upon your application,  If all you need from the part

    is to digitize 2 (or even 3) voltages simultaneously against a fixed value, then

    you want to use the AD7706, which is the pseudo-differential variant, which

    has three inputs measured against the COMMON pin.  If you have a more

    complicated application where you sometimes you want to measure 

    differences between two pins and other times you want to measure the

    absolute value of those pins, you'd be better off looking at a more complex

    part that has internal multiplexers that changes the "personality" of the pins

    under digital control. Here's an example of a part like that (ADI has many,

    you would need to pick the one that's the best match for your application):


    https://www.analog.com/media/en/technical-documentation/data-sheets/ad4130-8.pdf


    Finally, if you're not familiar with "pseudo-differential" vs "fully differential", the

    two links below are different explanations of the concepts:


    https://www.analog.com/media/en/technical-documentation/product-selector-card/2pb_sarinputtypesfb.pdf


    https://www.analog.com/media/en/technical-documentation/tech-articles/overview-on-adc-converter-inputs--maxim-integrated.pdf


    Hope this helps !

  • Thank you, John, for the explanation.

    However, my question was incomplete.

    I have two pairs of pins (A-B) and (C-D) of a sensor.

    The conections are the following: A in AIN1(+); B in AIN(-); C in AIN2(+) and D in AIN(-).

    The CHN1 subtracts the voltage at AIN1(+) from the voltage at AIN1(-), which corresponds to the difference value between A and B that is digitized. Simultaneously, the CHN2 subtracts the voltage at AIN2(+) from that at AIN2(-), returning the difference between C and D pins.

    Is this configuration possible using AD7705?

  • No, in that case you're looking for a "simultaneous ADC", that captures differential signals on N channels simultaneously and then digitizes the multi-channel snapshot. The product selection table here:


    https://www.analog.com/en/parametricsearch/10994#/


    lets you browse the parts that do this function. If you limit the search to 2 channel parts, there are 48. The key things to filter on are your sampling rate requirements and your desired accuracy. Good luck,

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