The circuit below is from a TI reference design, but my question is generic. I'm generating a voltage reference to add a DC bias to an AC signal for ADC purpose. Could I use the output of the voltage reference IC, in this case LM4041, directly instead of making it passing through an op amp buffer? How do I decide that?
Electrical – When to add an op amp buffer to a voltage reference IC
bufferoperational-amplifiervoltage-reference
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Best Answer
Not if you want low noise and some decent degree of accuracy. For low noise note the RC on the output of the op-amp - it has a low pass frequency of about 16 kHz so, any noise above that coming from the op-amp or voltage reference is progressively attenuated. There's no reason why the 100 nF couldn't be 10 uF (if the op-amp can handle the RC without oscillating) and get a 3 dB noise point of 160 Hz (even better for noise).
So, given that the LM4041 produces circa 20 uV RMS of wide-band noise it seems sensible to use an RC circuit.
I'm not a big fan of op-amp buffers because they can bring all sorts of errors to the party but the RC is a must in my book because it simultaneously keeps noise down and offers a low impedance to the ADC at frequencies above where the raw output from the 4041 can't deliver low impedance (look at figures 7 and 8 in the TI data sheet).