[SOLVED] current measurement on the output of a step down dc/dc converter

Hi,

I had been trying to measure the current drain of my project on the 1.5V step down dc/dc converter output line to a processor.
I am using 34401A mutimeter connecting in a series on the pcb trace out.
The problem is when I power up my DUT, the current drain is around 60-70+ mA but my DUT is not turning ON.

I try short a wire out and measured using a current probe with oscilloscope, and managed to measured a around 100mA after calibrate. My DUT
was able to turn ON. However consistency in the data is a concern to me. Variation can sometimes be 60mA - 110mA using
this method.

what is the problem with measuring using multimeter? does it have to do with high resistance or a "loading" in the multimeter causing
the whole DUT fail to power up properly. I know that supply line is critical cause it is supplying to the main processor.
how can I do to use the multimeter to measure to compare the data with respect to the Oscilloscope + current probe?

Thanks!

Can you please draw your circuit ?

It could be that the resistance of the DMM is high enough that the voltage drop it causes is enough to mess with the DUT. If possible you can modify the converter to take feedback from the load side of the DMM, thus correcting for the voltage drop. If that's not possibly then you should get a current sense resistor with a low enough resistance and measure its voltage drop with a precise voltmeter.

On specification sheets of 34401A, drop voltage of the DMM is
Range Drop voltage
10mA < 0.1V
100mA < 0.6V
1A < 1V
3A < 2V
In facat, drop voltage is smaller than the spec, but it's too large for 1.5V.

Your DMM have 5 Ohm shunt resistor at mA range and 0.1 Ohm in A range of current measurement! V
So 0.5V drop at 100mA .....

thank you Yas_Hashi for taking the trouble to go through the 34401 manual. Agreed with you on the loss is too high for my DUT. I never thought of checking the
spec...:idea:
Js, I think I got the idea now. sensitive lines voltage drop causes the DUT to fail to power up.

thanks again everyone

I have many multimeters whose shunt resistance is too high to be useful
at low VOUT, high VIN.

My test lashup for current sharing and load-step consists of a pair of Hall
sensors (Digi-Key; forget the mfr) and a 0.01 ohm precision resistor for
calibration of the Hall transfer function (volts per amp) in series. The Hall
sensors are trivial series resistance but have a fairly rough accuracy.