Electrical – Voltage limitation circuit for analog signal

analogvoltage

I want to design a circuit which takes two analog signals with a DC-offset via koax inputs, and forwards them.

In between the voltages should be capped at a user-defined level (between 200 mV and 500 mV), if the input voltage is higher, then it should be capped at the cutoff level. The reaction time of the circuit should be as fast as possible. The input signals have a frequency up to ~2-3 kHz. What is the best way to achieve that?

Bonus: Amplification of the signals in the circuit is also possible.

Best Answer

Seeing that you have to deal with very low-frequency signals (<3 kHz), the easiest approach here would be something like abusing the voltage clipping properties of a real Opamp:

By doing something like

--> Amp1 (gain = 10) --> Zener-based voltage limiter --> attenuator (gain = 1/10) -->

you could have something that works like that without disturbing the signal all too much.

To explain:

to limit voltages, and, for example, generate a stable reference voltage or cut away voltage spikes in power supplies, you'd simply do

schematic

simulate this circuit – Schematic created using CircuitLab

The Zener diode used is a peculiar semiconductor: It works pretty well as a diode, i.e. it inhibits flowing of current from the positive side to the negative side, up to a breakdown voltage, from which on it becomes very conductive (i.e. low resistance).

Now, since this is not adjustable, you can't use it in your "user-defined level" scenario.

Hence, you'll need adjustable gain on the input, and the inverse gain at the output:

schematic

simulate this circuit

Note that you get adjustable overall gain as "bonus".

This is the rare case where it makes sense to chose a high gain for the opamp stage – the higher the gain here, the higher breakdown Zener voltage you can select, and the more "exact" and less distorted your output becomes. However, if you select a Zener diode (or diode cascade) with a high voltage, you will also need to make sure your Opamp has a supply voltage that can generate that at all.

By the way, you might want to read up on Opamp circuits and active and passive filters – it would make potentially a lot of sense to integrate a low-pass filter in the output stage (which really isn't more than adding a capacitor), since the clipping operation probably has much higher bandwidth than your 3 kHz signal, and I don't know what that does to your signal chain; but that's something that I can't consider without knowing what you want to do with the signal in the end.