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Using Differenti​al Probe TA043 with Rigol 1204B Scope

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Using Differenti​al Probe TA043 with Rigol 1204B Scope

Postby wzy75 » Mon Dec 15, 2014 6:17 am

I bought a TA043 differential probe a while ago. I wanted to use it to measure the differential voltage across a current sensing resistor that is in series with a H-bridge load and use this voltage and the resistance value to calculate load current.

The current to be measured has a frequency of 0.1 - 5.0 MHz and a max magnitude of about 10 A peak-to-peak. The current sensing resistor is 0.01 Ohm. Therefore, the max magnitude of the voltage at the input of the differential probe is about 0.1 V peak-to-peak. As an additional piece of information, the current is the load current generated by an H-bridge having MOSFETs as switches. The load, a coil with an inductance of about 0.7 uH, is in series with the 0.01 Ohm resistor and they are connected to the output nodes of the H-bridge.

We were using a current transformer with a toroid ferrite (through which one of the load leads passes) as the reference for current measurements. We found that the waveform by TA043 is significantly different than that of the current transformer. The scope we used for displaying both signals is Rigol 1204B 200 MHz scope.

Please see attached picture for details.

1) The pink curve is the signal from the current transformer (voltage across its burden resistor);

2) The blue curve is the voltage signal from TA043 (voltage across 0.01 Ohm resistor in series with load);

3) Frequency is 1 MHz;

4) The load is a coil having an inductance of about 0.7 uH.

We are reasonably sure that the bandwidth of the current transformer is OK for the frequency range that we use. So we though the problem is likely with the differential probe measurements method but we do not know what is wrong.

Any help on possible causes of this discrepancy in the current waveform would be greatly appreciated.
Using Differential Probe TA043 with Rigol 1204B 200MHz to Measure Current at 1 MHz
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Re: Using Differenti​al Probe TA043 with Rigol 1204B Scope

Postby Gerry » Tue Dec 16, 2014 5:24 pm

Hi Zhiyong,

The current sensing transformer although it may have quite a high frequency response is unlikely to be able to react to almost instantaneous changing current (it can't accurately reproduce a pulse as the core limits the frequency response to less than 8 harmonics).

What you're measuring is the output from circuitry containing switching components that, depending upon how they are set up to operate can change the excitation almost instantaneously (which can be seen as a spike as the effective voltage becomes additive). The differential probe is faithfully reproducing the transients in the waveform, while the current transformer is effectively integrating them.

However, the differential probe could also be picking up other switching transients if you have the connecting lead between probe and oscilloscope too close to the switching components (as this part of the probe is not differential). If the scope end of the probe is too close to the H bridge, you can be more confident of a true representation of the waveform by adding probe tips to the differential probe to increase the distance (the short tips should just pull-off exposing banana connectors to snap on standard meter probes - which could also be fused, giving you additional CAT III measurement capability).
I hope this clarifies things for you.


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