14 mV Voltage clamp???

[conmanxx]

Hello

I need to clamp a millivolt signal from the output of a detector to 14mV.
The signal cannot exceed 14mV or it will trigger an alarm

Hoping to use a network of passive components perhaps.
Usually I'd use a clamping diode but that would be for something in the volts range. The signal max is so small I don't think it's possible.

Any ideas, I'm all out?!

 

[KlausST]

Hi,

I don´t think there is an easy soultion for it.

You say "or it will trigger an alarm". Maybe thats exactly what it is supposed to do.
* Why do you want to avoid the alarm?
* Why not simply disconnect/disable the alarm?

Klaus

 

[conmanxx]

Hi,

I don´t think there is an easy soultion for it.

You say "or it will trigger an alarm". Maybe thats exactly what it is supposed to do.
* Why do you want to avoid the alarm?
* Why not simply disconnect/disable the alarm?

Klaus

Yes, in normal circumstances it would trigger an alarm, correct.

However we have this "special" application where we need to prevent it from doing so.
An yes disabling the alarm is the other option. This would require firmware modification which I don't have control off (and it could be costly!).

I think to do it I'd need an op amp. Gain up the signal so 14 mV input gives a 2 V output then use a 2V clamp diode.

 

[KlausST]

Hi,

It depends on expected precision, noise, signal source impedance, load impedance, signal voltage range, signal speed and so on.

I´m trying a high quality solution:
What you need:
* analog switch SPDT
* percision comparator
* 14V reference circuit

Circuit:
* signal coming (from sensor?) to the NC pin of an analog switch.
* 14mV reference to the NO pin of analog switch.
* connect the COM of the analog switch to the output ... in direction of alarm system
* input signal (from sensor) to IN+ of comparator
* 14mV reference to IN- of comparator
* OUT of comparator to the control pin of the analog switch

Working principle:
* the analog switch usually routes the sensor signal to the alarm system.
* when the input_signal >14mV then the comparator makes the analog switch to change state and feed the 14mV to the alarm system.

Thus it acts like an active signal limiter. It makes the initial signal (up to 14mV) to distort as low as possible.

Proper decoupling of the 14mV reference signal is urgent not to generate oscillations. Maybe a small hysteresis is useful.

Klaus

 

[asdf44]

Normally a signal like that would have gain applied and then a limit applied after the gain. I.E. multiply it by 100 and then apply a limit of 1.4V. In that case you'd apply the limit to the 1.4V signal which would be much easier.

If that's not how your system works or you have no access to the larger signal then yes, you're going to need to construct an opamp circuit with a precision limit.

I'm sure there are lots of ways to do it: One way is an open drain regulator 'trying' to regulate a node to 14mV. By nature of being open drain it can only pull down, thus it will do nothing when the signal is below 14mV. Details like windup and saturation are tricky in instances like this however.

 

[BradtheRad]

An ordinary diode can be a voltage reference, just like a zener or led. Run sufficient current through it so it is stable. Connect a resistive divider (potentiometer) across it. You can tap it for any value from 0-600mV.

Example:

 

[conmanxx]

Hi,

It depends on expected precision, noise, signal source impedance, load impedance, signal voltage range, signal speed and so on.

I´m trying a high quality solution:
What you need:
* analog switch SPDT
* percision comparator
* 14V reference circuit

Circuit:
* signal coming (from sensor?) to the NC pin of an analog switch.
* 14mV reference to the NO pin of analog switch.
* connect the COM of the analog switch to the output ... in direction of alarm system
* input signal (from sensor) to IN+ of comparator
* 14mV reference to IN- of comparator
* OUT of comparator to the control pin of the analog switch

Working principle:
* the analog switch usually routes the sensor signal to the alarm system.
* when the input_signal >14mV then the comparator makes the analog switch to change state and feed the 14mV to the alarm system.

Thus it acts like an active signal limiter. It makes the initial signal (up to 14mV) to distort as low as possible.

Proper decoupling of the 14mV reference signal is urgent not to generate oscillations. Maybe a small hysteresis is useful.

Klaus

Thank you for the reply. I like the suggestion, I could see it possibly working!