DRV421: DRV421

Manikandan,

The DRV421 will work with the core to generate an output based on the primary current but at high frequencies there are two thing that limit the bandwidth and the response.  First the core acts as a transformer and at higher frequencies the transformer effect dominates the current across the Rshunt.  This can be seen in our   DRV421 System Parameter Calculator on the bottom right graph.  This will be based on the core used and its properties.  The other dominating factor is the bandwidth of the Diff amplifier.  The bandwidth of the diff amp is 2MHz.  Can you give me a signature of what the 300Khz noise looks like?

Regards,

Javier Contreras

Hi Javier,

Thanks for your information.

The noise looks like a damped sinusoidal of 300kHz frequency, can you let know how to share the signature, because i'm not finding any share option.
From the system parameter xls sheet, please let me know difference between the transformer and DC path method.

Regards,
Manikandan.M

Hi Javier,

We are waiting for your information. In the graph of system parameter, there are two plots one is the blue color plot (transformer) and the other is red color plot (DC path). Please let know what are those two plots.

Regards,
Manikandan.M

Manikandan,

I apologize.  The DC path is the when the DRV421 is actively sampling the internal flux gate and driving the compensation coil through the Icomp pins to drive the flux to zero at the sensor.  This bandwidth is limited by a sampling rate and filter.  As you increase the frequency of your primary current your magnetic core begins to act as a transformer.  The Icomp pins will still provide the current but it is driven by the transformer effect (transformer path).  The Rshunt measures the compensation coil current in both conditions (DC and Transformer path) and the limitation comes from the Diff Amplifier.  The bandwidth of the diff amp is 2Mhz. 

Another issue you may actually be seeing is if you have a high frequency input the DRV421 sampling rate is about 250Khz.  You mentioned a noise source of 300KHz are you sure it is not 250Khz.

Regards,

Javier.

Hi Javier,

Thanks for your information. The 300kHz noise specified is measured at the primary side along with power frequency current (not measured at the output of DRV421).

Our application is to measure the power frequency current, but during system integration due to common mode noise current we are observing 300kHz noise current.

As you mentioned the sampling rate of flux gate IC is 250kHz, then we can not measure or compensate current below 125kHz, Please reply my understanding is correct. If that is the case, then the 300kHz can be read incorrectly and suppose say at first instance it sample at the peak of the 300kHz fault current, then it will sample some other value at the next instance and the output from DRV421 can be completely different. What is your opinion on this?

Regards,

Manikandan.M

Manikandan,

You can measure currents above 125kHz. The 250kHz sampling is for the DC path. The transformer properties of the magnetic core will dominate at higher frequencies and you can measure higher frequencies but you are limited to the 2MHz bandwidth of the diff amplifier.

At higher frequencies you may see some residual signal at 250kHz due to the sampling. This is due to the CMRR of the difference amplifier, let me explain. As the DRV421 is sampling it adjust the IComp value voltages to try and keep up with the change in magnetic field. The compensation current does not have an error but the common mode voltage does change as the DRV421 tries to compensate the fast changing signal. This creates a fast changing common mode voltage change on the input to the difference amplifier. If you are not seeing noise at 250Khz then it is not an issue.

Is your signal a DC value or a AC? If AC what frequency are you primarily using?

Do you have details of your core you can share?