LMH6612: High bandwidth Current sensing circuit

Allen Du

Part Number: LMH6612
Other Parts Discussed in Thread: ISO224, AMC3301

Hi Team,

I am building a circuit to detect current of a media high voltage circuit (400VDC, 0~8A), bandwidth need to be at least 20MHz.

As HV ground cannot be same as sensing circuit, I am trying instrumentation amplier, and did simulation with pspice.

I build a prototype, when I use arduino generates a PWM pluse and divide by 2 resistors, put the PWM pluse as input of the sensing circuit, the output looks ok, as the following picture.

But when i connect the sensing circuit to 400VDC, the output is not as expected. could anyone help advice how to improve it, thanks in advance.

over 2 years ago

0 Allen Du over 2 years ago

Prodigy 250 points

BTW, The op amp circuit is using transformer isolated power.

0 Michael Steffes over 2 years ago

Guru 48395 points

Have you looked at the common mode voltages on the input stages?

0 Allen Du over 2 years ago in reply to Michael Steffes

Prodigy 250 points

Hi Michael,

Thanks for the reply, yes I measured common mode voltages on the input stage, could you advice how to improve and reject athe common mode voltage?

0 kai klaas69 over 2 years ago in reply to Allen Du

Guru 140200 points

Hi Allen,

for a high common mode rejection the input circuitry must look perfectly symmetrical. So enhance the symmetry of your circuit.

Also, limit the bandwidth to the absolute minimum by adding a low pass filter to the output.

A way better aproach is to use a TI's current sense amplifier. These show awesome common mode rejections.

Kai

0 Allen Du over 2 years ago in reply to kai klaas69

Prodigy 250 points

Hi Kai,

Thanks for the suggestion, we need make the sensing circuit ground different with 400V ground, I already bought AMC3301 and ISO224 EVM board and tested them, but the bandwidth is lower than our application needed, could you advice any current sense amplifier has higher bandwidth and can measure current of high voltage circuit?

Looking forward to your reply.

0 kai klaas69 over 2 years ago in reply to Allen Du

Guru 140200 points

Hi Allen,

can you show a full schematic? Please also tell how the different grounds are connected to each other.

Kai

0 Allen Du over 2 years ago in reply to kai klaas69

Prodigy 250 points

Hi Kai,

I attached a picture shows the block diagram, the load is a heating coil, I don' have the internal schematic of the PSU, what I want to do is to measrue current on load, the sensing circuit can use 24V of control signal, there is isolation inside PSU, 24V ground is not same as 400V ground.

0 kai klaas69 over 2 years ago in reply to Allen Du

Guru 140200 points

Hi Allen,

I would do a low side current measurement. This would eliminate most -if not all- common mode noise issues. The big advantage is that you would not need to take a differential amplifier with its limited common mode rejection. A standard non-inverting OPAmp circuit would do.

Of course, this will mean that you have to connect signal ground of your circuit to the +400V circuitry. This will also mean that you have to power your circuitry by the help of a saftey mains transformer as discussed in this thread

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1088390/current-sensing-for-high-voltage-circuit

and to safety-isolate any signal you can come in contact to.

Kai

0 Allen Du over 2 years ago in reply to kai klaas69

Prodigy 250 points

Hi Kai,

Thanks, have a nice weekend:)

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LMH6612: High bandwidth Current sensing circuit