opamp clamp

How's this?

ftp://66.117.156.8/Clamp.JPG

I need clean clipping, and can't overdrive some downstream mux's that run off +5 and ground.

John

Reply to
John Larkin
Loading thread data ...

Just guessing... but how about a TL431 across the Rf.

+----R1--+--R2-----+ | | | +-------> K--------+

Ikmin is 0.4mA. Iref current is 2uA

R1 and R2 become part of the op amp gain. D from BC

Reply to
D from BC

Or ...how about a rail to rail output op amp with +V at 5V.

D from BC

Reply to
D from BC

Your scheme "works", but how precise do you have to be, and how fast?

Being a preciseness nut I would tend to use a multi-op-amp scheme myself.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
 Click to see the full signature
Reply to
Jim Thompson

This schematics is no better then the classic diode clamping to +/- rails.

VLV

Reply to
Vladimir Vassilevsky

The clamp voltage, and indeed the signal, needn't be super precise. This is for a tachometer front-end, max bandwidth 100 KHz maybe. But we can have signals with a huge voltage range and need to clip clean. Downstream, it's convenient to use cheap mux's/amps/dacs/comparators that run 0 to +5, and some of them will freak if the signal gets past the rails.

The thing I did seems to be solid, and has low parts count. Of course, the common-base stage has high gain, so I'll have to make sure it's stable inside the opamp loop.

Oops, BAV99 is a series pair; I'll have to use the common-cathode version here.

John

Reply to
John Larkin

Maybe so, but we'd have to be sure it didn't wind up in saturation, and take a long time to recover. Some opamps can take milliseconds, or even seconds, to come out of overload. A closed-loop clamp avoids that particular hazard.

John

Reply to
John Larkin

Classic diodes where? To which rails?

Schematic?

John

Reply to
John Larkin

This is probably better: more tunable, and no added gain inside the loop.

ftp://66.117.156.8/Clamp2.JPG

John

Reply to
John Larkin

100KHz, and what is the loading? Where is the OA output going with the unclamped signal, what kind of impedance, R +C? And what kind of gain did you want out of that circuit?
Reply to
Fred Bloggs

Downstream, the load is pretty light, a bit more signal conditioning, some switching, and some comparators. But that stuff doesn't like being driven beyond the rails. If the customer grossly overloads it, like connects 120 volts RMS, we'd still like it to clip cleanly.

Gain=1 is about where it will run. Rf and Ri will be ballpark 50K, so we can't stand too much capacitance in the clamp circuit.

John

Reply to
John Larkin

how about one of these ?

formatting link

Jure Z.

Reply to
Jure Newsgroups

I've used that one, and the original HFA1130, which is somewhat faster. But it's overkill for my application, too fast, high bias currents, high power, and expensive. It needs Rf values in the hundreds of ohms, and I need 50K or so.

It is handy in other places, like driving fast adc's.

John

Reply to
John Larkin

Investigate some CMOS opamps. They tend not to "stick" by design.

Reply to
miso

That would work for the positive clamp level, but not zero volt clamp level; one would get about -600mV on the low end and violate the given conditions.

Reply to
Robert Baer

At least if the clamping is done as shown, the output can remain within the stated criteria.

Reply to
Robert Baer

I would suggest a silicon to -12V and a schottky to the NI of the op-amp; that would guarantee a positive and non-zero "low" level.

Reply to
Robert Baer

It's ikky but how about..

+----R1--+--R2-----+ | | | +------+-------> K--------+ | TL431 | | | Vin---R--|
Reply to
D from BC

If memory serves me correct, there are opamps with built-in clamps. The min and max voltages are applied to external pins.

--
Reply to nico@nctdevpuntnl (punt=.)
Bedrijven en winkels vindt U op www.adresboekje.nl
Reply to
Nico Coesel

On a sunny day (Mon, 24 Dec 2007 19:07:02 -0800) it happened John Larkin wrote in :

You need to limit BEFORE the gain stage to prevent opamp overdrive:

in ----------- R ------------------------- R --- CMOS opamp on 5 V supply

----------- |k \\ \\ Zener 5.1V +5.1 to -.7 V

+5 to 0 V |a ///
Reply to
Jan Panteltje

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.