Separating Current Source Compliance from Output Resistance

A current source is described by both its usable output-voltage range and how much its current changes within that range. Compliance voltage identifies the headroom boundary. Output resistance describes the local slope of current versus load voltage before that boundary. A circuit can have ample voltage range and still produce unacceptable current variation.

Measure a curve rather than one load

Analog Devices discusses resistor matching and output impedance in Current-Output Circuit Techniques. For a board test, command a fixed current and sweep the load voltage using a suitable load or source arrangement. Measure voltage at the current-output terminals and current independently. Check instrument polarity and compliance so the test equipment does not impose the limit being attributed to the board.

As an example, suppose a nominal 1-mA source changes by 2 microamps over a 2-V load-voltage interval. The magnitude of its incremental output resistance is approximately 1 megohm over that interval. If current then collapses sharply near a rail, that is a separate headroom boundary. Do not use a fit through the collapsed region to describe normal output resistance.

Exercise the complete operating envelope

Repeat at minimum supply, maximum commanded current, and representative temperatures. In a transistor-assisted source, amplifier output swing and transistor operating voltage both matter. A Howland source additionally depends on resistor ratios and source impedance. A capacitive load introduces dynamic stability questions that a DC sweep does not answer.

• Define the allowed current error before declaring a compliance endpoint.
• Measure both sourcing and sinking when the circuit supports both.
• Check actual resistor dissipation at every test point.
• Observe settling and oscillation after a load-voltage step.
• Separate current-monitor error from output-current error.

Budget the system voltage

Add the receiver burden, cable drop, protection voltage, and required current-source headroom at the same operating condition. Increasing supply voltage may expand compliance but also increases dissipation at low load voltage. Tightening resistor matching may improve output resistance without materially changing compliance. These are different design adjustments.

Use receiver burden budgeting to build the external load and amplifier output-swing analysis to inspect the internal limit. Keep the measured current-versus-voltage plot with the board specification so later load changes can be assessed against real regulation performance.