I will use ADR4525 as a reference source to an ADC, and would like to know whether I need to put an output buffer or not. What should I consider?
Thanks in advance.
the ADC is a 24 bit Sigma Delta.
About the reference input current the data sheet states that typical input resistance is 6 kohm, hence I think that input current would be 417 uA.
About reference cap size, the data sheet says the external caps should be chosen such that the bandwidth is limited to less than 10 Hz. They also suggest an external reference circuit, but, as it is not an ADI ADC, the obviously suggest their own reference part...
The answer will depend on the following :
If you are using a SAR I recommend to read the following article:
The ADR4525 can source this up to 10mA of current and so an average current of 417uA input current will not be a problem. It is likely that the input current is dynamic (glitchy) which can cause some ringing and introduce dynamic errors. The level of error will be determined by the size of glitches and the reference load regulation / output impedance. The glitches can be suppressed by adding a capacitor between the reference and the ADC - I would suggest 10uF. Using a higher cap load can cause instability issues for the reference output. If the glitches are small and below the level of error which is acceptable with a 10uF in place then you will not need a buffer.
If you decide to use an RC filter on the output of the reference to limit the bandwidth then you will make sure that the offset error generated by the input current flowing through the resistor is below the acceptable error level you want. Keeping the resistor small to achieve whilst keeping the capacitor value low enough for the circuit to be stable will mean there is a practical limit to how much you can limit the bandwidth with an RC filter. Whether you require a filter will be decided based on the noise of the ADC compared to the reference and what output data rate you are using. If the digital filter on the ADC is set to a lower -3dB frequency than the RC -3dB frequency then there is no reason to use the external RC filter.
If you find that you need to limit the bandwidth in order to meet the noise performance requirements then using a buffer after the RC filter will prevent minimise any offset errors as the input current of the opamp will be much lower than the ADC.