Suppressing Noise from unknown Source when measuring AC Current

[Amr Wael]

Hello Everybody ,
I am testing a Digital Multimeter PCB designed based on the IC (FS9922-DMM4) , All measurements are working fine except for the AC current measurement.
As you can see in the following photo this is how the current is measured , For the ampere range the current pass through the 5 mOhm resistor R19 generating a millivolt that enters the IC through the SA measurement point that is then amplified by a gain of 10.

Theoretcally the millivolt signal should reach the SA point without any problems since it's a high input impedance in this direction however as you can see in the following two photos ,
the first photo shows the millivolt signal measured on the shunt resistor and it's just the same before R23 then after R23 at the SA measurement point the signal transforms completely to be the signal in the second photo. My questions are as follows , first is this undesired transformation of the signal is some form of noise ? second question how to suppress this effect to obtain a clear signal on the SA measurement point ?

 

[KlausST]

Hi,

it may be a measurement artefact. Maybe caused by scope EARTH connection.
If you want us to verify this we need to see the whole wiring in one phot, (plus some detail pictures)
* Measurement tool power supply
* measurement tool power connection (for current measurement)
* load connection
* scope signal connection
* scope power connection
also exact type of scope

***

You should focus on the frequency range of interest --> a low pass filter will be useful

Klaus

 

[Amr Wael]

I will send you the photos in a while ,
my desired range is super low , like few tenths of Hertz where do you suggest placing the Low pass filter? I don't have full access to the opamps output since th opamp is inside the IC , or do you suggest placing it before the opamp?

 

[KlausST]

Hi,

I recommend a (foil) capacitor at SA pin to GND.

PCB layout may have impact on noise but more impact on current measurement accuracy.
If you want us to verify it you may upload..

I´ve just took a quick view into the datasheet.
* there may be internal filters.
* for AC you need a rectification circuit

Klaus

 

[Amr Wael]

Hi,

I recommend a (foil) capacitor at SA pin to GND.

PCB layout may have impact on noise but more impact on current measurement accuracy.
If you want us to verify it you may upload..

I´ve just took a quick view into the datasheet.
* there may be internal filters.
* for AC you need a rectification circuit

Klaus

Yes sir , I am already using the average rectification circuit that is in the data sheet page 21 , diagram 18

 

[D.A.(Tony)Stewart]

Looks all like layout noise, lack of decoupling and excessive probe and ground lengths.

OPIN should identical to OPIP except 10x lower impedance and negative feedback to probe injected noise. If not the same and actually 10x or more better, that proves my point.

 

[Amr Wael]

Yes sir , I am already using the average rectification circuit that is in the data sheet page 21 , diagram 18

The problem is solved when I connected the COM pin (one terminal of the shunt resistor R19) to the body (Shield) of the power supply , it's even solved when connecting it to the case of the pc or any other tool that's supplied from the same AC Line. I am happy that the problem is solved but I literally don't understand what was the problem or why was it solved this way.

Hi,

I recommend a (foil) capacitor at SA pin to GND.

PCB layout may have impact on noise but more impact on current measurement accuracy.
If you want us to verify it you may upload..

I´ve just took a quick view into the datasheet.
* there may be internal filters.
* for AC you need a rectification circuit

Klaus

 

[KlausST]

Hi,

Often the noise dissapears when you disconnect something.
Thus I expect a problem else where.

but I literally don't understand what was the problem or why was it solved this way.

...the requested pictures will give some insight...

Klaus

 

[Amr Wael]

@KlausST Yes I am sorry for not providing the Pictures , It's currently weekend in my country will provide them when I return to the workplace.

 

[D.A.(Tony)Stewart]

The problem is solved when I connected the COM pin (one terminal of the shunt resistor R19) to the body (Shield) of the power supply , it's even solved when connecting it to the case of the pc or any other tool that's supplied from the same AC Line. I am happy that the problem is solved but I literally don't understand what was the problem or why was it solved this way.

It's usually caused by high common mode, CM , noise levels into an unbalanced differential amp producing a differential noise. Shunting one side bypasses the CM noise source. But an INA with shielded connections might also work.

 

[Taihung]

Noise makes more Noise
You just add small 100pF parallel with R8 is fine.
I designed similar system with non-isolated, using LM324 opam and MCU STM32F411.