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How to put a constant v-drop in a signal?

Hey there - I've now run into this problem twice in the last week. I want to be able to insert a constant voltage drop into a signal.

The first place where this would have been nice was that I had a comparator watching the drain and the source of an N-FET. I wanted the comparator to toggle the instant the drain went higher than the source (and the other way around, naturally). Unfortunately, the comparator that I was stuck using had fairly lousy input offset. It was OK for me to error more towards one state of the comparator, but not towards the other. So what I figured was that I'd drop one signal by as much as the worst case input offset (1.5mV). But I know of no way to do that. Instead, I just used a voltage divider that just barely dropped it by a constant amount, and since I knew the range of voltages that I'd be able to see I was able to make the minimum constant amount large enough to always be larger than the input offset.

Anyways - that worked - but dropping by a constant voltage would have better.

Now I have another application of a similar thing. I need to monitor the voltage of a signal that ranges from about 12-18V. It will be monitored by a singled ended ADC with a 3.0V max input. So I can just divide the signal by 6. However, that voltage will always range from

2-3V, so I'm losing two thirds of my ADC range. What a waste. What'd be awesome would be if I could put in a constant drop in that voltage so that I could use the full range of my ADC - either a 12V drop or a 2V drop would work, clearly. There are shunt references with voltages this large - so that'd definitely do it (but be fairly costly and large, as it seems most shunts are fairly large parts). Zeners are typically what I think about 5% or so, so that would be less than ideal.

Am I missing some better way of doing this? I suspect a 10V or 2V drop is a lot easier to achieve than a 1.5mV drop, but I've been wrong before!

Thanks!

-Michael

Reply to
Michael
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You can put a dropping element in series with the signal.

Zeners around 6.2v are pretty stable, or you could use an LMV431-type thing for an adjustable drop.

Another way is to insert a series resistor, then create the desired offset with a constant current sink.

Flying capacitors are another way--accurate & stable.

HTH, James Arthur

Reply to
James Arthur

Can't see the OP (please use something better than Google if that's the domain you used).

Anyhow, the OP might not gain much if the ADC is already 12bits or better. Zeners are horrible in precision and a LMV431 type thingie will be around 1%. So it's usually better to waste 2/3rd of the ADC range. More precise drops are possible but then the cost will be so painful that upping the ADC by two bits should cost less.

--
Regards, Joerg

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Reply to
Joerg

It all depends on the missing details.

Current sink + precision resistor could be quite accurate; flying cap stuff the same, and cheap.

If all he needs it monotonicity, anything works.

Cheers, James Arthur

Reply to
James Arthur

Search for an adder/subtractor circuit such as

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Use a stable voltage reference as the source of the offset you want and scale it up or down as needed.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

So, use an op amp and make an amplifier that has gain of

0.5 and DC offset of -12V. If you use the reference voltage from the ADC for the offset, it'll be as stable as your converter, and two calibration points will be the only setup requirement.

I've also seen (in very low-noise circuits) a mercury battery and a potentiometer in series with the signal wire.

Reply to
whit3rd

I did exactly that way back when in a product that needed custom offset and gain tailoring per channel.

I second the op-amp adder approaches, but they're kind of a pain: then you have gain and offsets to adjust, and common-mode errors to consider.

I'd thought the OP just wanted a simple offset, but I see he needs a scale factor too. Missed that.

Neat idea.

Cheers, James Arthur

Reply to
James Arthur

Those are frowned upon by the EPA and others. I had to modify my old Minolta camera to 1.55V since mercury cells have become unobtanium.

--
Regards, Joerg

http://www.analogconsultants.com/

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Reply to
Joerg

The Weston standard cell is a better voltage reference if you want to go that way, but the better semiconductor references are more stable (though the tolerance on the output voltage tends to be worse) and they are a lot more compact.

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-- Bill Sloman, Nijmegen

Reply to
bill.sloman

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