# How to measure current at output port

I have programmable current source in the board. The circuit blocks in the board is,

Microcontroller => DAC => Amplifier => Output port

I want to measure the current for range of DAC code to verify the circuit is functioning correctly. Its 16 bit DAC. So, i would like to measure current for DAC code 0x0000, 0x1000, 0x2000....0xFFFF. According to schematics the circuit can source up to 100mA.

How to use the multimeter to measure the current at a pin in output port.Should i have to connect load/resistor in series to multimeter?

Output Pin <=> Multimeter <=> Resistor <=> Ground(mounting hole in the board/power ground)

Any help is appreciated. Thanks

• 16bit is actually quite a nice DAC, and I'd rather trust your DAC than your measurement. What's the reason you're mistrusting your DAC? May 7, 2020 at 15:10
• Can your multimeter even measure with a resolution of 1.5uA? Or tens of uV (if you choose to measure voltage drop across the load resistor to determine current). May 7, 2020 at 15:23
• @MarcusMüller This is for testing the board. To make sure we can source up to 100mA. May 7, 2020 at 15:41
• @DKNguyen I am not trying to measure current for each dac code. 16 data points with step value of 1000 is good. 100mA/16 = 6.25mA. May 7, 2020 at 15:50
• Oh. I see...... May 7, 2020 at 15:56

Output Pin <=> Multimeter <=> Resistor <=> Ground(mounting hole in the board/power ground)

Yes, this is essentially correct.

If it's not too late, re-design your circuit to have two output terminals instead of one, with the second terminal being the ground or return terminal. This will save you having to connect your circuit to its load using a mounting hole.

Choose the resistor value depending on what compliance voltage (maximum output voltage) your current source is specified for. For example if the specification is 10 V compliance, then use a 100 ohm resistor. If the specification is 1 V compliance, use a 10 ohm resistor, etc.

• Thanks for suggestion. What would happen if i don't connect series resistor. Output Pin <=> Multimeter <=> Ground May 7, 2020 at 16:07
• @user12027865, if the current source is designed correctly, then it will supply the current it is designed to supply. Ideally you should test it both this way (assuming it's a linear design, 0 output load will require the most internal power dissipation for the supply), and with the maximum resistive load. May 7, 2020 at 16:17

When a circuit has a current output, there are two equally important specifications for it:

1. The current range (minimum and maximum current),
2. The compliance voltage range (minimum and maximum voltage).

You cannot expect to use a current output if both of those two numbers are not available. So the first step will be to know the compliance voltage range for your current source.

Recall that a current source has infinite impedance. So, there needs to be a voltage source (a resistor - could be 0 ohms i.e. a short, a battery, etc.) that determines the voltage. As long as the voltage is within the compliance range, the output will act like a current source, and will be able to provide currents within the specified range.

If the compliance voltage range is down to 0 volts or close to it (say <0.1 V), then you can connect a current meter straight across the output, and measure the current directly.

If the lower end of the compliance voltage range is higher, e.g. 1-5V, then a resistor won't help, since there's 0V drop across it when the output current is 0mA. Instead, you'll need a voltage source.

A lab power supply connected in series between the current source and the current meter will work fine for this purpose, but even simpler: a battery will work too.

If the compliance voltage is, say, 1V-5V, then an AA battery will do fine as a voltage source. It will be recharging and thus may fail prematurely - but for small currents this won't be a problem if the tests don't take long.

If you want to be testing such an output for a longer time (a day), then a discharged AA Ni-Cd or Ni-MH cell will be perfect: the voltage drop while charging will be above 1V, and the cell can easily pass the 100mA current, and a 1000mAh capacity cell will happily absorb 100mA for 10 hours.