QA451 and DC Power

@matt, when using the QA451 are there any gotchas if not using the dc power to the dut?

Hi @Moto, there shouldn’t be. The use case here is you want to power a module (DUT) and some steps of the test will require measuring the supply current. Note the switch is one-way. That is, current is expected to flow from S+ to S-. If you try to flow current from S- to S+ (eg S- is at higher potential than S+), then the switch is always appear closed (it’s a single MOSFET switch) and you can’t control it.

The switch is smart in that when you turn it on, it will first energize one mosfet that switches in a 10 ohm resistor. This allows you to charge any big caps on your DUT (without this, the current would only be limited by the ESR of the DUT caps). After 1 second, a second mosfet switches on that is the true switch (S+ connected direction to s+ via a 10 mohms or so). See page 13 for more info here.

If you are testing a DUT with massive bulk capacitance and larger rail voltages, it 10 ohms pre-charge R might not allow the DUT caps to completely charge and an overcurrent situation might result when the main switch turns on.

The current measurement resolution is around 10 mA, so on a 0.5W amp it might not be enough. But on a large class D it will let you make efficiency measurements.

Thx Matt. For the situations where we are not making measurements that require knowing the input current to the dut , the switch is irrelevant then, correct?
Say measuring an amp that is powered by 120vac and always on for the test.