How to clamp a signal?

Question

There is a DAQ system where one of the input is measuring pulses. As in the figure the pulse signals are coming through a BNC cable entering into very small box called "8V clamp" and leaving this little box with a BNC and going to one of the DAQ channels to measured. I noticed that if the pulse amplitudes are greater than 8V this little box is stepping them all down to 8V. But if the pulse amplitude is less than 8V pulses passes through without any change. I think it is somehow protecting the input of DAQ kind of limiting to 8V.

My question is what can this circuitry be? A comparator or? And it has only BNC connections which carry pulse signals. If it has opamp in it how can it be powered since BNC only is carrying the signal and there is no other wiring to this little box?

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

It could very easily just be this. A shunt 8V zener diode. I don't know your frequency or voltage parameters. Is there a power/voltage rating on this little box? Is it custom or is there a mfg. part number?

Edit: There is going to be a limiting resistor in there somewhere. If you measure the resistance from your IN to your OUT port, you might actually measure the current limiting if it's in series. Or, you could apply a known voltage and measure the current draw to complete your schematic.