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Old May 9th 2018, 06:32 AM   #1
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Red face Help Signal Attenuation

We have a question we cannot understand…

We are university students working on a pet project, we are trying to pick up an electrical current passing through a conductor via a capacitive sensor (i.e. attached to a charge amplifier with filters) that is not in direct contact with the circuit.

The signal we detect seems to be larger when the conductor is in contact with one electrode only.

As soon as the circuit is completed (we connect the conductor at both ends) the signal we detect on our sensor is either attenuated or disappears altogether…

The signal source is a battery powered generator with no direct connection to the sensing circuit. We are simply trying to detect an electric field.

The signal is about 2 volts over 0.45ms

Does anyone know why this is or what is happening?

Any help really appreciated...
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Old May 9th 2018, 07:05 AM   #2
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This is common.

What do you see on an oscilloscope with a trailing length of wire connected to the input?

Nowearth the other end of the wire.

Again what do you see?

In the abscence of a resistive load the scope input will pick up random RF signals, perhaps even tens of volts since the everage electric field in a room may be several volts/metre.

You need a resistor across the input of your measuring device to 'settle' the input.
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Old May 10th 2018, 09:39 AM   #3
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Am I correct in surmising that you are simply placing a wire across the terminals of the signal generator?

Is your signal generator "happy" to work with a dead short across it's output terminals?
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