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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?

Thanks for your help!

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  • harry -

    Filters are centered around 100Hz - an audio application. For comparators, I've been looking at the ADCMP370, as it's got a very low Iq. (no bad pun here) For opamps, your suggestion of duals vs quads makes a lot of sense, in terms of space efficiency.

    So, please clarify - does it make sense to use a voltage reference for the active filter (at Vdd/2), and also use that same reference and divide that down for the comparator? Or, should I use the wizard's recommended VFB (substituted with a 4505 for power saving) and divide that down for the comparator? It's a 4 amp filter design. The ADA4505 has 40mA drive, that should be more than adequate. It does have a better Iq than the 8031.

    also - great read on the link, re: VFB and current drive and stability.

    thanks again!

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  • harry -

    Filters are centered around 100Hz - an audio application. For comparators, I've been looking at the ADCMP370, as it's got a very low Iq. (no bad pun here) For opamps, your suggestion of duals vs quads makes a lot of sense, in terms of space efficiency.

    So, please clarify - does it make sense to use a voltage reference for the active filter (at Vdd/2), and also use that same reference and divide that down for the comparator? Or, should I use the wizard's recommended VFB (substituted with a 4505 for power saving) and divide that down for the comparator? It's a 4 amp filter design. The ADA4505 has 40mA drive, that should be more than adequate. It does have a better Iq than the 8031.

    also - great read on the link, re: VFB and current drive and stability.

    thanks again!

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