Reducing noise on a voltage reference

Question

I need a stable, low noise voltage reference that I can adjust between 1.4V and 1.8V. The output should source or sink up to 0.25mA @150Hz max. The output will provide a +1.65V bias for a bipolar signal driven by another op amp into the ADC on a microcontroller. (I need some adjustability because the bipolar signal has got some d.c. offset error I want to null out).

A similar question was posted here Are all voltage reference ICs able to sink as well as source current? and I wanted to develop that with a proposed design (below).

My question is: where would I put a capacitor to decouple noise in the reference? Could I assume a 100nF between the 2.5V reference diode's anode and cathode would suffice or would I ned further filtering on the input?

(By the way, I'm changing from the TLE2141 to a TL071AN in my circuit because the TLE2141 has too large a supply current.)

Answer 1

Most of the noise (perhaps 1-2mVp-p) will be due to the reference, so putting a capacitor on the op-amp non-inverting input (with a series resistor in the case of the TL071) makes some sense. 100nF with a 20K series resistor (about 22-23K equivalent total) will give you a cutoff frequency of around 71Hz if I did the math right. Similar to @WhatRoughBeast's comment but with added series resistance.

On the other hand if you use a lower noise reference such as an LM4040-2.048 or -2.5 which has something like 250uVp-p noise typically, the capacitor and resistor may not be necessary, since the amplifier is contributing significantly.

Answer 2

Consider this

^{simulate this circuit – Schematic created using CircuitLab}

Your output noise density will be defined by THREE contributors

(1) noise density of your voltage reference

(2) noise density of your opamp internal KT noise and its VDD (power supply) deterministic and randome noise,

and

(3) the 100 ohm resistor from opamp output to the 1,000uF capacitor, at about 1.3 nanoVolt/rtHz. The output bandwidth will be [100 ohm, 1mF = 0.1 second or 1.6Hz] scaled up by PI/2 from the Integration DC to infinity of ArcTangent, or 2.6Hz bandwidth.

The total integrated noise due to Rout (opamp output pin, to 1,000uF) will be 1.3nV * sqrt(1.5) ===1.5 nanoVolts RMS

Its your job to manage (1) and (2)

And you might ask: is the 10Kohm feedback resistor not also a noise contributor? Its inside the loop.