PXIe-4139: incorrect current measurements at freqs abobe 100k

[Alex_Lelekov]

Hello all!

We need to measure Li-Ion battery impedance. The single-frequency oscillations injection method was used, for every point in a required frequency range:

1) Setting the DC operating point of the DUT

2) Insertion of sinusoidal oscillations to the voltage of the DUT by the shape setting of SMU PXIe-4139 output signal.

3) Making current and voltage signals measurement.

4) Selection of the main harmonic amplitude of measured current and voltage, the impedance calculations for a given frequency point.

But, while measurements of the high frequency spectrum (10kHz-400kHz) SMU cannot provide proper sygnal because limited Fs = 100k. So we try to use external generator, desired sinusoid was injected by a current transformer with a variable transformation ratio. The rest of the measurement sequence is the same. Scheme attached.

 

And, I observe impedance overration on test DUTs, from the frequency of 100 kHz with a slope of +20 dB/decade, like as inductive component added. I try to make parallel current and voltage measuring with the second measuring system (multifunctional DAQ module PXIe-6366). The current sensor was a metallized heat-resistant lacquer resistors (russian MLT type), which do not have a reactive components up to frequencies of the 6 MHz.

The SMU configured to One-point voltage stabilization mode. After the source operation completes, the measurements of current and voltage channels were made. The AC power supply frequency was filtered by second order filter. The SMU PXIe-4139 and DAQ PXIe-6366 measurements were synchronized.

The results revealed that the voltage channel measurement of SMU and DAQ are identical up to noise amplitude. But current measurements with SMU showed, that sine amplitude was decreases from 100 kHz with the slope of -20 dB/decade, compared with DAQ current measurements, see Bode plot (measured ratio of signal from the internal current sensor of the SMU to the external current sensor).

 

A detailed SMU documentation study did not give any information on the detected effect. Current ADC work independently of control loop. Why SMU incorrectly measure current at high freqs?

[Alex_Lelekov]

Perhaps, first post was not clear. I expected, that current sensor have flat frequency characteristic, but it actually have -20dB/decade from 100kHz. Is it true? Or what i'm doing wrong?

[ChrisR.]

I'm not sure I understand 100% what you're doing, but there are a couple of things that could affect bandwidth.  One is the current range that you're using.  The lower current ranges (e.g. 1 µA) will have lower bandwidth than higher current ranges, due to capacitance in the measurement path.  Secondly, the limited overall voltage loop bandwidth can make the SMU output appear inductive.  (Search for "virtual inductance" in your NI-DCPower help.)  This may reduce the apparent current bandwidth depending on the driving impedance.

I suggest trying a higher current range and experimenting with custom transient response settings.