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ADS1255 Reference

Other Parts Discussed in Thread: OPA350, REF5025, ADS1255, ADS1256

The schematic for the ADS1256EVM (SBAU090C) shows an OPA350 buffering the REF5025 2.5V reference.  Is this necessary? The REF5025 has more than enough current capability to drive the reference input of the ADS1255/6.  Doesn't using the opamp just add more noise and drift? 

  • Barry,


    The REF5025 is probably capable of driving the reference input on it's own, but there are a couple of factors. I'll describe the problem and why the buffer may or may not be necessary.

    The input to the reference has apparent resistance of 18.5kOhms, which is really 135uA with a 2.5V reference. However, really the reference pins are sampling the reference with capacitors and discharging them into the modulator into the ADC. So really this is a dynamic charge transfer and not really a resistance. This charge transfer occurs each modulator cycle. In the ADS1256, this happens every 520ns to 260ns depending on the PGA setting.

    So each time the reference is sampled, the reference must fill the reference sampling caps and have the value settle to the reference voltage within 520ns to 260ns. If there is an external reference capacitance, the amount of settling is less. This is because the reference capacitance (external to the device) can help ballast the amount of charge needed to fill the reference sampling cap in the ADC. Less external capacitance will require more settling time because there is less charge available to fill the sampling cap.

    If the reference doesn't settle within the sampling time of the modulator, this appears as a gain error. It's a bit hard to predict how much gain error you'd see because it can depend on the settling time and the dynamics of the capacitance on the reference pins.

    For the EVM, the original board was designed with a different reference, but the OPA350 is still a good buffer because of it's low offset and offset drift. It also has a high bandwidth so ideally the reference would settle quickly. I suspect that the gain error without the reference buffer would be similar to with the reference buffer. However, I've never run any tests to check this.

    I think the safe thing to do would be to keep the reference buffer. It has already been tested on the ADS1256EVM and the settling will be fast enough to keep the gain error low.


    Joseph Wu