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AM263P4-Q1: ADC External Vref questions

Steven Hansen
Intellectual 686 points
Part Number: AM263P4-Q1
Other Parts Discussed in Thread: OPA4350, , AM263P4, REF35

Tool/software:

Hi All,

We are considering using an external ADC voltage reference with AM263P4-Q1. I've looked at the schematic for the AM263P4 control card that uses a pair of OPA4350 opamps to buffer the external reference. But I still have a few questions below:

1. Is the capacitance recommended for the Vref pins in the datasheet (4.7uF + 0.1uF + 0.1uF) still recommended when using an external reference, or can we go much lower? The reason to reduce capacitance is to help with stability of the external buffer. Even a "High Cload" opamp like OPA4350 only plots stability in the datasheet up to 1uF, and it gets fairly peak-y even there. Can we reduce the capacitance with an external buffer? Can we reduce it significantly (<10nF) and allow the external buffer to provide stability?

2. On the AM263P4 control card, separate opamps buffer the ADC references for VREF G0/G1 and VREF G2 respectively. The AM263P4-Q1 datasheet mentions these reference inputs must be separated when using the internal references since they don't have enough current drive to be shorted together. But when using an external reference like REF35180 that has significant current drive, can I short G1/G2/G3 together and drive them with one external reference? Or are there ADC performance reasons not to do this?

  • 0
    Intellectual 686 points

    Checking back in.

    Also wanted to add that REF35 can drive up to 10uF of external capacitance, so if it's allowed to short VREF G0/G1 to VREF G2 (when using an external reference), it seems like I could drive all three using REF35 while keeping the recommended capacitance on each pin.

  • 0
    TI__Mastermind 24593 points

    Hello Steven,

    The recommendations are given with the internal references in mind. If you are providing an external reference source such as the REF35180 then the reference input circuitry should designed for optimal performance of the given IC. It may be possible to drive all three reference voltages with the single source, but signal integrity is a critical factor here as noise on this rail will couple into all of the ADC's. Providing a dedicated op-amp buffer per pin would help to isolate each ADC grouping and ensure proper bandwidth and current drive to the VREF pins.

    Best Regards,

    Zackary Fleenor

  • 0 in reply to Fleenor
    Intellectual 686 points

    Hi Zack,

    That makes sense, but I've been unable to find an op-amp that can provide a stable response with the recommended amount of capacitance on each VREF. Even a "high Cload" opamp like OPA4350 starts to become severely underdamped (60% peaking) at only 1µF, and the datasheet suggests we use five times more (4.7µF) per VREF.

    It seems there are three options:

    1. Reduce the capacitance so we can use a separate opamp to buffer each VREF without instability

    2. Use a device like REF35 that can drive that much capacitance but short all VREF pins together

    3. Live with slightly unstable opamp buffers on each VREF

    Any idea which is preferred here?

  • +1 in reply to Steven Hansen
    TI__Mastermind 24593 points

    Hey Steven,

    I consulted with our analog design team about this. We would recommend option #2 in this case. However, you will still need distribute the capacitors at the pins, the same way you would if using the internal reference.

    Best Regards,

    Zackary Fleenor