Electrical – How to make a low pass filter of current, instead of voltage

currentcurrent-sourcefilterlow pass

Is there a way to low-pass filter current, rather than voltage?

Let's suppose there is a circuit consisting of a current source and a load. The current source may have a little bit of current noise at high frequencies. I'd like to figure out what can be inserted in between the source and load in this case to suppress that.

(Note: the load may not be resistive, and may have strong nonlinearities)

Best Answer

A shunt capacitor will filter it.

It will of course lower the high frequency impedance from the infinity of a current source.

So putting a series L after the cap will restore it.

schematic

simulate this circuit – Schematic created using CircuitLab

Aside from that, you need to make a low noise current source. LM317's would not be a good place to start.

You are not giving any idea of what noise , voltage or current levels, and how constant-current it needs to be, so hard to make any suggestions, but a simple emitter follower can be very quiet, close to the thermal noise from R2.

schematic

simulate this circuit

In the emitter follower R2 sets the current, ie the current rises until the voltage across R2 is too high, and base current can't flow.

A simple emitter follower has poor current regulation, but good high frequency performance as there is no feedback involved.

schematic

simulate this circuit

Here is an arrangement that gives excellent regulation, low noise at low and high frequencies, and also allows you to switch the LED at very high frequencies, without compromising the noise.

  • lots of caps to filter noise, both big electros and small ceramics
  • the voltage ref has its noise filtered to give a quiet reference to the opamp.
  • the opamp gives an accurate constant current
  • we use an emitter follower, as it has intrinsic constant current at high frequencies without the opamp doing anything
  • instead of turning it on and off, we steer the current between the test led, and a second matched led. This means that the currents and voltages stay constant during switching. The constant current circuit can have noise filtering, yet still be switched at speed.
  • We filter the supply to the leds to reduce the noise input from the power supply.

This is the circuit I used in optical instruments for turbidity, transmission and flourescence.

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