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ADC with really "True differential input"

Hi collegues.

I need ADC with really differential input and single supply.

-IN connected to reference point (REF/2)

+IN connected to input signal (+\- 2.5v)

I tried AD7684 but DataSheet tells that +IN and -IN MUST be in opposite phase to reference point. I used MCP3301 which is pin2pin compatible with AD7684 but allow any signals on inputs.

Now we need 500kS ADC and MCP3301 couldn`t achieve this.

Could you advice ADC or solution for AD7693 to adapt it to described use case.

Thank you in advance.

  • Hi vlad8016,

             What Klaus mentioned are right. There are three most common types of ADC input structure a.) Single- Ended b.) Pseudo Differential c.) True Differential.  From your description above I can see that you are looking for ADC that has a pseudo-differential input structure, where the -IN can have a DC input and +IN will have an AC signal. Below is the list of ADIs Pseudo-Differential ADC and is divided base from their sampling rate. You may Consider this list and check them depending on your application. You can go to the ADI website to check the details. Hope this help.

    Type 100 kSPS 200 kSPS to 300 kSPS 400 kSPS to 1000
    1 MSPS to 5 MSPS
    16-Bit Pseudo Differential AD7651 AD7685 AD7652 AD7653
    AD7660 AD7664 AD7667
    AD7661 AD7981
    AD7680 AD7666 AD7980
    AD7683 AD7694 AD7686 AD7983
    AD7988-1 AD7988-5 AD7985



  • To get best performance out of an ADC with differential inputs you should be driving it with complimentary phases and common mode voltage of half the input range.

    Fixing one of the inputs to a DC level turns the converter into a single ended converter with detrimental effect on AC performance. Some parts, including the AD7684, have limitations on the common mode voltage range which are only a problem under certain circumstances.

    You have a few options here.

    - Use a ADC driver that converts your signal into a differential signal. This will give you the best performance.

    - Use a pseudo-differential ADC with the IN- attached to the ground of the signal source. This is very similar to what you are trying to do by hooking -IN to REF/2.

    - Use a single ended ADC which would work as above, but perform worse due to ground currents mixing with the signal return.

    - Find an ADC that has a wider common mode input range and use it as you were planning to. However performance won't be better than using the pseudo-differential ADC with -IN connected to the signal ground. It might be worse depending on how your REF/2 node tracks the common mode noise on your signal.


  • Thank you for replies.

    As I see in data sheet ADC with pseudo-differerential input couldn`t convert voltage below GND.

    For example AD7694. -IN should be connected to GND, and +IN should be between GND and VREF.

    So I can't convert negative voltage. If it is right I don`t understand real difference between pseudo-differential and single-ended. In pseudo-differential ADC -IN could be connected to GND+\- 0.1v and that`s all?

    May be I`m wrong?

  • Hi Vladislav,

    The guys are correct about the different input types but there is a form of pseudo-differential input that will allow you keep the IN- at Vref/2. We call it pseudo-bipolar, there are only a small number of parts that support it. See article link below that describes the different input types available with a nice table at the end with different parts and input types. To do exactly what you describe i would use the AD7682, it has 4 channels but you can just use one.

    Best Rgds,


  • Sorry just noticed you need 500ksps. Use one channel of the AD7699 instead.

  • Am I right that the best way is using AD8139 as differential converter in my situation?

  • The difference between a pseudo differential and single ended ADC is that the latter has no separate ground reference input. So the ground is shared with the power supply of the part. Any voltage drops DC or AC on the ground connection will be seen by the ADC as a shift in the input signal and therefore show up in your converted results. A pseudo-differential ADC provides you with a separate connection for the input signal return. Since it does not carry any other currents it won't add spurious signals to the input stage of the converter.

    Regarding the AD8139: That part should do nicely as long as you stay within its input common mode voltage range (1 to 4V on a single 5V supply).


  • Thank you for clarification. I`ve changeged my scheme, but there is another quection:

    Does ad8139 need additional OP-AMP on its input or it is possible to use it if input impedance about 50k? SCH is below.

  • No this won't work the way you have shown. You wiould need a low impedance source.

    Remember that both input terminals of the AD8139 will be driven to the output common mode voltage via feedback.

    For a high impedance source you would be better served using an instrumentation amplifier like an AD8231.

    It has high impedance inputs, programmable gain and an auxiliary opamp to convert the output to differential drive for your ADC. It also is a bit less expensive.


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    EZ Admin