How to wire NI 9205 COM terminal for measurement of isolated DC-DC converter?

[らくだこうろぎ]

Hi everyone,

I want to measure the performance of an isolated DC-DC converter.

Voltage at the input and the output (Vin+, Vin-, Vout+, Vout-) of the converter are sensed by NI9205.

Current at the input and the output (Iin+, Iin-, Iout+, Iout-) are also measured by NI9205

using 2 current sensors (1 milli Ohm resistors).

The each measurement are performed by "dfferential mode"

In this case, do I have to connect the com terminal of NI9205 to all of [Vin+, Vin-, Vout+, Vout-, Iin+, Iin-, Iout+, Iout-]

using bias resistors (~10 kOhm) ?

If so, it is too much trouble!

If you have more simple idea, please tell me......

Thank you

[Henrik_Volkers]

Well, each input can output a bias current of up to 100pA.

Asuming a real floating DC/DC converter (on one side, say output 😉 )  with a inherent capacity to COM. Now the bias currents of the four inputs (400pA worst case) will charge this capacitor ... an the inputs will drift to one of the boundaries (abs(common mode + signal) < 10.4V )

you can try higher bias path resistors like 1M , or with a detailed shematic of the DC/DC converter, provide a current path on any other point ....

(never tried, but maybe you can provide a bias resistor , and just open it for a short time of measurement)

If you run into EMC problems,  just look at the current, it will flow! Not only the bias current, also these nasty current spikes, that transforms into fields and jump around 😉  but they all are laisy, you 'just'  have to provide an 'easy' path if you can't avoid them.

If you want more detailed tips, you need to provide more detailed information about your setup. (every cable / connection! )