Some comparator questions

Following up my posting with one more question:

- Is there any better way to add hyteresis to comparators? The "ADCMP609" from Analog Devices looks like it lets you program the current with a resistor which is perfect, but it's unreleased.

I have not found many others like it, that also mention < 100 ns switching time, which is what I desire.

Thanks again analog gurus. John.

Adding comparator hysteresis externally has potential hazards. Both edges of a fast spike can make it through to the output (obviously!) before the positive feedback can make it back around, so you *can* wind up with low-level spikes propagating through. Any added delay in the feedback loop - stray C in the resistor feedback parts, or your added gate - make it worse.

It can work if you prefilter the input so its edges are relatively slow compared to comparator prop delay, but that can be a real waste in some situations.

Some people do make comparators with r-r outputs.

What works pretty well is a dual comparator with different thresholds, driving an R-S flipflop. Or a comparator with internal hysteresis.

John

Hi jjlarkin,

Thanks for your suggesti>

I don't know if this "requirement" eases my situation any, but I'd ideally like 40mV of hysteresis, symmetrically on-top of the reference. So if Vref = 2V, ideal trip points are 2.02V and 1.98V . Most comparators I've seen only offer ~4mV of hysteresis.

I guess the dual-comparators you're suggesting work in a "window" configuration?

I suppose I could use an op-amp to produce add/subtract 20mV to my reference and bus each one to the dual-comparator driving the R-S flip- flop. But that's a lot of external components for a "simple" task :/ .

Thanks, John.

A somewhat nice comparator can be made using the standard RS485 driver chips, MAX485 etc. Still fixed hysterisis, but around 50 mV, much bigger than the typical comparator values. Also selectable output slew, depending on whether you choose the slew rate limited devices or not.

But there are thousands of samples available, and I don't think it's that far away from release. I work in the group that developed it. Drop me a private email and I'll give you a contact name.

Steve

If the comparator input terminals aren't drawing significant current, you could get a 20mV shifted reference for the second comparator with a pair of resistors, and maybe a capacitor.

-- john

The lowish cost, 5 pin LMV7219, offers rail to rail output and built in hysteresis, with an extremely clean switching action.

The simplest comparator you may use on this situation is a cheap (but with less than 1mV offset) operational amplifier with dual supply (+ and -). This will asure you a simetrical hysteresis. The problem would be probably the dual supply itself.

Vasile