Noise in Current Measurement

Hi Ben,

thanks for fast response, we'll order this other EVM. I work on the same project as Mehdi. the inductance is 20uH and we added an 100pF cap, which may not be ideal. The noise we're experiencing is picked up from the environment, it namely happens when we move the coil, So I wonder if we cannot improve immunity, or add an LPF, maybe reduce the length of the sensor wire....

Any suggestions there?

Is there any documentation related to current measurement using this part ?

olivier

Hello Ben,

Thank you for your response.
To add what Olivier said we have an other coil with an inductance about 13µH but it less sensitive than the 20µH.coil. It can not detect current less than 1.5A. 
Is there a specific inductance value with a good sensitivity which can correspond to a current measurement between 1 and 6A ?

Thank you for your help.

Hello Oliver,

I would have been more than happy to recommend register settings if this was a conventional LDC application but at this moment I do not have enough information to give you an accurate answer. Scope shots would be really helpful...

The LDC1000EVM coil has an inductance value of around 20uH, so you might be better situated for the 26uH inductor then for the 13uH one with the default settings but it really just depends on the toroid.

We have a few tricks to determine these settings, you can give them a shot.

The FAQ section in general is a good quick reference for the LDC1000.

Regards,

Varn Khanna,

Applications Engineer, Sensor Signal Path,

Silicon Valley Analog,
Texas Instruments,

Hello Varn,

Thank for your response and for sharing the Application Note

I have some questions about this :

• We can determine Rp with this equation :

But how is it possible ? Rp is in Ohm and L/(C*Rs) is in H/(F*Ohm). There are not the same unit.

• And here it is assumed that C is already known but we have to determine the right value for the capacitance.
  How can we determine C ?
• Why CFB pin should be 1Vpp if inductance value is maximum ? In the Application Note it recommends to have 4Vpp (page 5) 

I have scope shoot from INA and INB to GND with and without current :

• INA without current

• INB without current

• INA with 1.5A

• INB with 1.5A

I could notice that the frequency decreases a little when there is current but I don't how the scope can affect the programmation of Rpmin and Rpmax.

Thank you

The equation does not appear. It's the equation 1 in the Application Note (page 3)
Rp=L/(C*Rs)

Hello Mehdi,

The equation you are referring to for Rp is frequency dependent so you will need to resolve it in terms of impedance.

For choosing the capacitor, 

The LDC1000 operates between 5KHz and 5MHz, I would use the manufacturer specified inductance of the toroid and choose a capacitor that puts the frequency in this range. Please use the following equation to solve for C:

For the amplitude part, page 5 of the App Note is referring to the Sensor Signal Amplitude on INA and INB. Where as, when you are choosing the CFB cap you will be probing the signal on the CFB pin which should be 1Vpp at maximum inductance.

These subtle points aren't obvious in the App Note and I will provide this feedback to my team. I apologize for the confusion.

Thank you for posting the scope shots, please give me some time to provide you with some ball park register settings.

Regards,

Varn.

I had forgotten the result

Please find the attachement.

Resultat rogowski coil with shied.xlsx

Hello Mehdi,

I apologize for the delay in getting back to you. Could you confirm that you are using a COG cap with a low temp co in your measurement?

I would also be curious about if the toroid is self heating?

I will look at the data you posted and provide additional notes by end of today pacific time.

Regards,

Varn.

Hello Khanna,

I think i am using COG cap but I am not sure and I don't know if it is low temp. You can see in the picture the cap that I am using. This the only 100pF cap that we have currently.
Could you know if this cap correspond to the test ? If you have a ref for capacitance in radio spare could you send me the link ? I will order it.

You can see the reference of the toroidal coil with this link : Wurth Ferrite Toroidal Inductor
The size seems to be too large so that the toroid is self heating.

Could you tell why it is important to have a COG cap with low temp ? And how the cap can prevent the differents value of L for a same current ?

Thank you.

Mehdi

Hello Mehdi,

Using a cap with a high temp co will cause the frequency to shift with minor temperature variations which will affect your measurement.

For reference here is another e2e post talking about something similar.

In addition the cap needs to be as close to the toroid as possible.

I will try looking through Radio spare for an appropriate cap. But here is one from digikey, you just need to make sure the temperature coefficient of the capacitor is <+/-30PPM/C.

Regards,

Varn.

Hello Khanna,

I found a cap with roughly the same spec in RS here. I will order it and run a test.

But do you think the cap is the only reason of the diferent value of L for a given current ?

Because when I started the measure with 0A, I have increased the current until 3A. I could observe an inductance value which decreases slightly over time. I have continued to increase the current until 6A and when returned to 3A, the inductance was completely different than the first value. I can reiterate the operation and I will observe a third value of L.
Some times, the coil seems to be saturated when I returne to 0A,  the inductance value is very high and when I increase the current, the inductance fall.
You can see the video to illustrate this "saturation". The image in the video is reversed.

You can also see the result result below.

Result Rogowski Coil with LDC1612.xlsx

If you are any idea, please let me know.

Regards,

Mehdi

Hi Mehdi,

your use case is not one that the LDC has been designed to do or that we have previously explored. While it is certainly interesting research, I'm afraid we are not able to support it. Thank you for your understanding, and good luck with the evaluation.