Precision Current Source Circuit Design

A current source delivers a defined current whatever the load does, within a range of output voltages. It is the natural way to drive a resistance sensor, a laser diode or an LED, and the accuracy of the current is what the whole measurement depends on.

The Basic Loop

A sense resistor is placed in series with the load, and the voltage across it is compared with a reference. The difference drives a pass transistor that adjusts the current until the two match.

That feedback loop is what makes the current independent of the load, the supply and the temperature of the pass device. What remains is the accuracy of the sense resistor, the accuracy of the reference and the offset of the amplifier.

the sense resistor is therefore the critical component. A metal foil or a precision wire wound part with a low temperature coefficient is used, and its value sets the current for a given reference voltage.

Sense Resistor and Its Errors

The voltage across the sense resistor is small in most designs, between fifty and two hundred millivolts, so that the power dissipated and the voltage lost are both acceptable. At that level the offset of the amplifier is a significant fraction of the signal.

A chopper stabilised amplifier with an offset in the microvolts keeps the error small. An ordinary amplifier with an offset of a millivolt against a hundred millivolt signal introduces a one percent current error before any other term.

Kelvin connections to the sense resistor are essential rather than optional. Two wire connection includes the solder joint and the trace in the measurement, and their temperature coefficient is far worse than that of the resistor itself.

compliance voltage and Headroom

compliance voltage is the range of output voltage over which the source can hold its current. At the top it is limited by the supply and the voltage the pass device needs, and at the bottom by the sense resistor and the minimum output of the loop.

A source that has to drive a resistance sensor through a long cable needs enough compliance to cover the cable drop as well as the sensor. The drop is proportional to the current, so it can be calculated rather than guessed.

The power dissipated in the pass device is the difference between the supply and the output voltage multiplied by the current. At the bottom of the compliance range that difference is largest, and the thermal design has to cover it.

temperature coefficient and Drift

The temperature coefficient of the source is the sum of the coefficients of the sense resistor, the reference and the amplifier offset. The resistor usually dominates, and a foil type with a coefficient of a few parts per million per degree is worth its cost.

Self heating of the sense resistor is part of the same problem. The power dissipated in it raises its temperature above the ambient, and the resulting change is a fixed error that varies with the current and with the airflow.

Where the best available components are not enough, the sense resistor is placed in a thermally stable area or in an oven. The improvement is real and the cost is a heater and the space for it.

load regulation and Output Impedance

load regulation is the change in output current when the load voltage changes, and it is expressed as a current per volt or as an equivalent output impedance. A perfect source has infinite impedance, and a real one is limited by the loop gain.

Increasing the loop gain improves the regulation, and the gain falls as the frequency rises. A source driving a load that changes quickly has to have enough bandwidth as well as enough gain.

Where the load is capacitive, as with a cable to a remote sensor, the loop can become unstable. A small series resistor or a compensation capacitor at the output restores the phase margin at the cost of some speed.

Topologies and Their Uses

A floating source with the sense resistor in the low side is the simplest and has the widest compliance range. Its disadvantage is that the load is not referenced to ground.

A high side source keeps the load grounded, which is what most sensors and loads require. It needs an amplifier that tolerates the full output voltage at its inputs, and the design is more involved.

A current mirror is cheap and imprecise, and it is used where the current only has to be roughly defined. A mirror with emitter degeneration resistors improves the matching and the regulation at the cost of a lower compliance range.

Layout and Grounding

Keep the sense resistor and the amplifier close together and the loop physically small. The feedback path is a low level signal, and a long trace picks up the switching noise of the rest of the board.

Route the sense pair as a differential pair from the inside of the pads to the amplifier inputs. Any shared return with the load current puts the load current into the measurement, which is the most common layout error in this circuit.

The pass device dissipates power and heats the area around it. Placing the sense resistor next to it introduces a thermal gradient that appears as a current error which changes with the load, and the effect is easy to miss because the schematic is correct.

Protection and Limits

The output has to survive a short circuit, which is the most common fault on a current source. The pass device is protected by its own current limit, and the loop has to recover cleanly when the short is removed.

An open circuit is the opposite case, and it drives the pass device to its maximum voltage. The compliance limit is what protects it, and the design should be checked with the output open and the current set to its maximum.

A reverse voltage across the output, which happens when a charged load is connected with the polarity reversed, destroys more of these circuits than any other event. A series diode or a clamp across the output is the standard protection and costs very little.

Verification and Faults

Verify the current by measuring the voltage across the sense resistor with a meter that has a high input impedance, and confirm the resistor value separately. Measuring the current with a meter in series adds its own burden voltage and changes the circuit.

Sweep the load voltage over the compliance range and record the current. The change over that range is the load regulation, and a change that is larger than the calculation predicts indicates a loop gain problem or a thermal effect.

A current that drifts upward over minutes without any change in the load is usually self heating of the sense resistor. The release checks that keep such a source consistent are collected in our PCB design release checklist, the assembly points in judging PCB quality, and the layout measures that keep a low level signal clean in our guide to mixed signal board design.

FAQ

What sense voltage should I use? Between fifty and two hundred millivolts. Below that the amplifier offset dominates, above it the power loss and the compliance requirement grow.

Why does my current change when the load changes? The loop gain is too low or the bandwidth is too small. Check the amplifier and the compensation.

Can I use a current mirror instead? For a rough current, yes. For a measurement, the matching and the regulation are not good enough.