# VFB as a voltage reference

Hi -

I am using an ADA4505 VFB as a voltage reference to obtain Vdd/2 (Vdd = 3.3V) as a midpoint reference in an active filter circuit, using a pair of 1% 10K resistors at the NI input. the 4505 is being supplied by an LDO, which is preceded by a switcher supply.

My question is, how stable would this VFB output be for a comparator? The comparator requires a threshold of around 200mV, so I would need to take the VFB output and divide it down to feed Vref of the non-inverting comparator.

My thinking is, perhaps the instability of the 3 different devices (switcher, LDO, VFB) plus the resistor divider may contribute a lot of instability so my Vref at comparator could vary by some conjugate of those numbers that could end up too low or high. I can tolerate around 50mV on the threshold.

So...bad idea? Should I use a dedicated voltage reference?

Parents
• Groger,

Stability is really a function of the output impedance and the capacitive load.

In general, the lower the power/supply current, the wimpier the output, so 50 pF

can cause oscillation.  Unfortunately, there is no standardized parameter and test

conditions, so there is nothing in the search table for you to choose.  But a lot of

data sheets have a graph of overshoot vs. CLoad, so that gives you a clue.

You don't need a RRI/O op amp, and I would suggest using more supply current.

This will give you lower output impedance.

What is the max supply current you can allocate?

Is this battery powered?

What LDO??

Note that ripple or noise on the 3.3V will be divided in half by the 10k/10k, and go straight

through the voltage follower, so you need a cap at the midpoint of the two resistors to ground.

Say 1u-47uF.

Depending on the input impedance of the comparator, and whether or not it changes, you might

need a cap on it's input. (inverting or non-inverting config??;  what part number?)

I hope you are not using an op amp as a comparator;  bad!  See:

and the references at the end of the article.

If you need some capacitance on the "output" of the op amp, don't put it on directly;

Harry

• Groger,

Stability is really a function of the output impedance and the capacitive load.

In general, the lower the power/supply current, the wimpier the output, so 50 pF

can cause oscillation.  Unfortunately, there is no standardized parameter and test

conditions, so there is nothing in the search table for you to choose.  But a lot of

data sheets have a graph of overshoot vs. CLoad, so that gives you a clue.

You don't need a RRI/O op amp, and I would suggest using more supply current.

This will give you lower output impedance.

What is the max supply current you can allocate?

Is this battery powered?

What LDO??

Note that ripple or noise on the 3.3V will be divided in half by the 10k/10k, and go straight

through the voltage follower, so you need a cap at the midpoint of the two resistors to ground.

Say 1u-47uF.

Depending on the input impedance of the comparator, and whether or not it changes, you might

need a cap on it's input. (inverting or non-inverting config??;  what part number?)

I hope you are not using an op amp as a comparator;  bad!  See:

and the references at the end of the article.

If you need some capacitance on the "output" of the op amp, don't put it on directly;

Harry

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