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Methods to measure floating voltage

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onemilimeter:
"Differential voltage probe" vs "LEM LV25-P voltage transducer"

[1] LV25-P provides isolation voltage measurement.

[2] I assume "(High-voltage) Differential voltage probe" (e.g. Tektronix product) does NOT provide isolation voltage measurement. Please correct me if I'm wrong.

[3] If "Differential voltage probe" is safe for high-voltage floating voltage measurement, then why do we need "isolation voltage" measurement like LV25-P?

Thanks.

alm:

--- Quote from: onemilimeter on September 16, 2010, 08:03:10 pm ---"Differential voltage probe" vs "LEM LV25-P voltage transducer"

--- End quote ---
These are very different products, the LEM transducer is for mounting on a PCB for inclusion into a product, the Tek probe is for attaching to a scope.


--- Quote from: onemilimeter on September 16, 2010, 08:03:10 pm ---[1] LV25-P provides isolation voltage measurement.

--- End quote ---
Isolation voltage sounds like a Hipot test, neither of the products will do that. I assume you mean isolated voltage measurement (isolating the measurement device from the voltage under test)?


--- Quote from: onemilimeter on September 16, 2010, 08:03:10 pm ---[2] I assume "(High-voltage) Differential voltage probe" (e.g. Tektronix product) does NOT provide isolation voltage measurement. Please correct me if I'm wrong.

--- End quote ---
Both of the probes (banana leads) of the differential probes are isolated up to the rated voltage. Neither of them is grounded (hence differential), and can be connected to any potential within the specified limits.


--- Quote from: onemilimeter on September 16, 2010, 08:03:10 pm ---[3] If "Differential voltage probe" is safe for high-voltage floating voltage measurement, then why do we need "isolation voltage" measurement like LV25-P?

--- End quote ---
You can't really integrate a Tek probe into your product, they're probably much more expensive and much larger. But a PCB mounted device isn't very convenient for measurements on power electronics. The bandwidth of the LEM device is probably lower, and so is the rated voltage I think (can't find a spec of differential mode voltage).

onemilimeter:
Hi alm...

Thanks. I'm aware that the "differential voltage probe" and "LEM LV25-P voltage transducer" are two different things.

Here's my application. I got a 3-phase AC motor. I wish to measure the phase voltages and these phase voltages will be fed to a controller. The motor is controlled using SVPWM (space vector PWM at 20kHz) method. At first I think of using isolated voltage measurement method like LEM LV25-P. However, LEM LV25-P has limited bandwidth, i.e. less than 10kHz, and hence it can not capture correctly the 20kHz switching voltage waveform. We have a "differential voltage probe" and it can capture the high frequency switching voltage waveform correctly. I do not know much about the circuit insides the "differential voltage probe". Does it using the same "isolated" technology like the LEM LV25-P? If it's not how does it provide "isolated" or "floating" voltage measurement?

Thanks.

onemilimeter:
Found this IC:


--- Quote ---The AD629 can replace costly isolation amplifiers in applications that do not require galvanic isolation.
--- End quote ---

If a difference amplifier can do the job, what do we need a "galvanic isolation"? Please share if you know an application that need "galvanic isolation" and a difference amplifier can not be used?

Or, the CMRR of a difference amplifier is not high enough for some applications?

Thanks.

onemilimeter:
Please refer to the specifications of a differential voltage probe given below. What does "Common Mode Voltage DC + pk AC: 6500V" mean?

I'm interested to learn how to achieve such design specifications (e.g. 6500V, DC-50MHz). I found several difference amplifiers, e.g. INA117 and AD629, but their bandwidths are only 200kHz and 500kHz, respectively, and they are designed to use in application with the presence of common-mode signals up to ±270VDC only.

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