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Low noise signal clamping

On Thursday, November 28, 2013 6:00:29 AM UTC-8, Dirk Bruere at NeoPax wrot e:

damaging it. I suspect diodes are going to be too noisy, and am looking at using discrete transistors with their bases tied to the rails though a low value resistor. Is this viable? Does anyone have any suggestions?

I'd use an inverting op amp, powered on the same rails as your ADC, and diode-clamp at the pseudo-ground input of the op amp. It takes more parts, but you can use big diodes (the stray capacitance doesn't charg e).

It inverts your signal, of course.

A big Zener to ground works better than a diode to Vcc, which can blow the power supply's other client chips.

Reply to
whit3rd
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On Thursday, November 28, 2013 9:00:29 AM UTC-5, Dirk Bruere at NeoPax wrot e:

damaging it. I suspect diodes are going to be too noisy, and am looking at using discrete transistors with their bases tied to the rails though a low value resistor. Is this viable? Does anyone have any suggestions?

Are you worried about the leakage current of the reverse biased diode, or the noise of the series resistor.

If it's the diode current then you can use part of a small transistor inste ad of a diode. (less leakage.) I think it's the c-b junction. I've got a nice B.Pease paper at work that has typical numbers.

George H.

Reply to
George Herold

This circuit

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has a pot that makes either the output signal, or the upper limit on the output signal, depending on the switch setting. The other mux inputs do some other enable/shutdown logic.

This is, basically, using a mux as an analog lookup table or ROM. Stuff everything you know into the mux address bits, and connect the inputs to do the desired thing for each state.

Reply to
John Larkin

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