Switch to reset charge amplifier

One cute trick for this is to invert the power supplies to the op amp--with suitable current limiting resistors, of course--and let the ESD protection diodes form a diode bridge that connects the output to both the inputs. (I forget where I heard of that one--I certainly didn't invent it.)

Op amps obviously aren't specified for this, so you'll have to test yours or ask the apps folks. (They probably won't know either, but it's worth a try.)

Also the gigohm resistor probably doesn't help you at 120C--50 pA of bias current puts you in the shot noise limit.

Cheers

Phil Hobbs

Doesn't your amp have a reset input that can be controlled by a logic level? Oh, come to think of it, our Kistler amps need a switch closure. And I think some of the newer hideous ones omit this feature...

I'd seriously consider a reed relay to avoid any leakage or charge injection weirdness from using a MOSFET. Then again, a MOSFET might work. It'd take some poring over specs, then going home to dinner and wondering what they mean, then having the lightbulb go in the shower the next morning. You know what I mean?

If I go to a job interview, how am I going to explain the way I do design?

Am 03.04.2012 16:53, schrieb Geoffrey:

a couple of years ago I have seen a charge amplifier for X-Ray detectors which used the FET in the input stage to reset the integrator in a tricky way. The lid of the FET's metal can (I think it was a 2N4416) was was opened. To reset the integrator the crystal of the FET was lit by a pulsed LED until Vout was zero.

Cheers, Alexander

"If I go to a job interview, how am I going to explain the way I do design?"

No problem just be honest. "Well I think this is the way I would solve that problem, but it usually takes me a day or two to process things. Can I drop you an email if I have a better idea in the shower tomorrow morning?"

George H.

Oh I see I omitted a word.

What if what I meant to say was: "then having the lightbulb go *out* in the shower the next morning?"

Heh heh.

Sure, but wait till you get out of the shower first.

That's a neat idea! If the signal is unipolar the OP could put an LED across it and and reset it by shinning a second LED into the first.

George H.

Many CT systems use current to frequency converters, the X-ray photodetecto= r arrays have similar large C issues. This way the I to v converter is alw= ays draining the charge a bit. The IFC had a range of picoamps to 4 ma, re= sulting in 5 Hz to 10 Mhz, from the oscillator. This gets rid of dumping th= e integrator. Picker/Phillips had the patent.

Steve=20

tor arrays have similar large C issues. =A0This way the I to v converter is= always draining the charge a bit. =A0The IFC had a range of picoamps to 4 = ma, resulting in 5 Hz to 10 Mhz, from the oscillator. This gets rid of dump= ing the integrator.

My two cents for the design would be an op-amplifier with a high fedility gain for the feedback. A second stage may be included as a limiter, including the diodes in a back to back configuration for reset.

eeL

As a test engineer at an earlier time i am troubled by the conditions you are subjecting your charge amplifier to. What constraints do you have that you cannot put your charge amplifier in a benign location, away from the heat and vibration?

?-)

The amplifier will be placed in the Vee of a motor racing engine, the piezo sensor being used to measure the pressure in the combustion chamber. It is possible that a solution involving longer cables could be found, but there are high levels of EMI due to ignition systems and other power electronics. The temperature is a bit of a bother - few charge amps are specified at 125 degrees due to the high leakage and bias currents at that temperature.

I should think that you'd get a huge signal from that setup. Why not run well-shielded coax to a remote charge amp in a friendlier location?

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I was wondering. There is no need to subject the charge amplifier to = that environment. It belongs in the driver compartment. Far less heat, and a bit less vibration. Of course you are paying about US$18/ft for the special shielded cable for either placement. (Shake and bake piezoelectric accelerometer cable from Endevco[, and perhaps Ling or Umhholz-Dickie].)

?-)

What's special about piezo cable? Any teflon coax, or semi-hardline for super shielding, would work. He'll probably have lots of signal.

Teflon is not a good idea, due to microphonics.

Depends on how much signal he has. I'd expect an engine explosion to make lots of signal.

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That also depends on how much gets to the sensor crystal. It can't take direct exposure to the combustion chamber gasses, for heat and chemical issues. Thus attenuation, maybe a lot of it, before the sensor crystal "sees" it.

?-)

Our engine in-cylinder pressure sensors are flush with the head, so see the combustion chamber environment directly. I think all they have is a thin metal foil over the sensor material surface.

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We may never know unless someone sections one.

?-)