ADA4941-1 introduces noise

Hello,

I am getting data from an AD7961 ADC with an ADA4941-1 in front as a single-ended-to-differential stage.

I am getting a surprising noise of +/-2mV when I short the ADA input to ground. This noise goes down to +/-0.2mV if I skip the ADA4941-1 stage and instead short both the differential inputs of the AD7961 to ground (the inputs still have their resitor-capacitor network as indicated in the part's datasheet).

I am working on home made prototyping PCB with the ICs mounted onto fine, commercial SMD-to-thru-hole adaptors. I know that this is not actually an ideal situation to get pure data in, but I can't figure out how the mV noise range is possible. I have used all the precautions, such as capacitors near the chips and precise voltage reference by ADR430. I am using this temporary solution to evaluate the circuit and move to a more professional PCB later on.

Any suggestion is appreciated.

Many thanks in advance,

Marco

  • Hi Marco,

    Thank you for using ADI products. Could you share your circuit schematic? Let's trace the possible noise sources from there.

    Best regards,

    Neil

  • Hi Neil,

    many thanks for your prompt answer. Please find attached the schematic.

    I need to acquire a +/-1V signal and for this reason I have setup the pair of resistor dividers respectively at the non-inverting (8) and REF (2) pins of the ADA4941-1. In this way when the INPUT range is +/-1V vs. GND, OUT+ ranges from 0 to +2V and the same does OUT- but phase-reversed.

    Best regards,

    Marco

    ADcircuit.pdf
  • Hi Marco,

    Thanks for the schematic. I think your resistors are responsible for the noise that you are seeing. Resistors contribute 4nV/rtHz times the square root of the resistor value in kilo ohms ( sqrt(R in kohms)*4nV/rtHz). When referred to output, these resistor noise will be multiplied by the gain of each gain block of your system, depending on its location. Based on your resistor values, I calculated at least 3.5mVp-p noise after the RC filter, which I think is close to the +/-2mV that you are seeing. There are 2 things you can consider to lower this noise. First is to lower your resistor values and second, lower the cutoff frequency of the RC filter at the ADC input.

    There are video tutorials by ADI available on YouTube that discusses noise on amplifiers. Here's the link.

    I hope this helps!

    Regards,

    Neil

    Message was edited by: Neil Kane Jacalan Fixed hyperlink.

  • You're welcome! Thanks again for using ADI products.

    Best regards,

    Neil

  • Hello Neil,

    many thanks for the suggestions. I have lowered by an order of amplitude the resistor values of the offset dividers with no gain in terms of noise. Instead I have changed the time constant of the input RC network of the AD converter, changing the 2.7nF caps with 100nF. This fixed the issue, I get clean signals both when I short the input to ground and when I put there a signal.

    Of course I know that 100nF could be too much depending on the required bandwidth, but this was just a test about your suggestions. One could then find a better RC network cutoff frequency and possibly raise a little bit  both the resistor and the capacitor instead of just putting there a big cap. This is a matter of fine tuning.

    Many thanks for your help, which was very appreciated.

    Best regards,

    Marco