Differential input for ADAU1701 ADC

Hello to all ADAU1701 fans,

I would like to share a very simple circuit for expanding the standard audio ADC input

to a fully differential input path. It works very well and rises the SNR (in my hardware)

to 102dBA.

You have to select a low noise OpAmp with RRO for best performance. PLS note that

the full scale input is R1 * 100uA (eff). The buildtin ADCX-resistor must not be taken

into account.

Best regards

Ulli

Parents
  • Hello Alexei,

    at first yes, the circuit can work with single supply of 3.3V as the adau1701 itself.

    I constructed it for a +4dBu differential input. The advantages are as follows:

    1. self biasing trough the ADC input, no extra DC path required.

    2. full scale input voltage range can be easily modified by changing R1 (and R2)

    3. lowers noise generated by the adc input opamp (as Bob already pointed out)

    For best performance R1 and R2 have to be well matched. The same applies to matching of R3 and R4

    The input may be connected fully differential or pseudo differential (one input grounded, signal on the other)

    R3 = R4 = 6,2kOhms keep the opamp output swing well above GND and below 3.3V even in a worst case

    scenario with +AINX connected to GND and signal applied to -AINX.

    In my design the opamp (dual) is OPA2377 (sorry Analog Devices), but Aanalog Devices has

    for sure comparable parts..

    Hopefully I could help to enlight the circuit a little bit more ;-)

    Best regards

    Ulli

Reply
  • Hello Alexei,

    at first yes, the circuit can work with single supply of 3.3V as the adau1701 itself.

    I constructed it for a +4dBu differential input. The advantages are as follows:

    1. self biasing trough the ADC input, no extra DC path required.

    2. full scale input voltage range can be easily modified by changing R1 (and R2)

    3. lowers noise generated by the adc input opamp (as Bob already pointed out)

    For best performance R1 and R2 have to be well matched. The same applies to matching of R3 and R4

    The input may be connected fully differential or pseudo differential (one input grounded, signal on the other)

    R3 = R4 = 6,2kOhms keep the opamp output swing well above GND and below 3.3V even in a worst case

    scenario with +AINX connected to GND and signal applied to -AINX.

    In my design the opamp (dual) is OPA2377 (sorry Analog Devices), but Aanalog Devices has

    for sure comparable parts..

    Hopefully I could help to enlight the circuit a little bit more ;-)

    Best regards

    Ulli

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