ADE7758 Current Measurement Problem?

[naz$]

I am using ADE7758 Energy meter Ic. In IC datasheet they given Typical Application Circuit Diagram. i followed that correctly.

In current channel section, single Rb resistor connected between the Input and GND. but somehow it cause common mode noise problem and current readings are different then expected. so i have added resistors on both inputs and grounded from common point. Now i wondering are the given values of resistors correct or not?

 

[FvM]

The data sheet suggests single ended, grounded shunt with differential low pass filter. I can't recognize any advantage of using two shunts instead. If you have common mode noise problems, it's probably due to inappropriate layout.

 

[D.A.(Tony)Stewart]

The OA's may have great CMRR but cables are notoriously bad for balanced CMRR.

I suggest you choose a CM choke of appropriate L values with a Pi filter configuration using only STP wire grounded at source only.

 

[Easy peasy]

the switch mode supplying the local power is likely the root cause of the CM issues ......

 

[KlausST]

Hi,

The given information is not useful information about the problem.

If your circuit does not work as good as the datasheet specifies, there basically are two sources
* either the datasheet specification is wrong
* or your application is not designed according datasheet recommendations.
Indeed there may be datasheet errors, but don't expect it.
Thus focus on your application. Showing a datasheet schematic does not help to find the problem.
==> Show us exactly your schematic, your PCB layout, your power supply, your wiring, your measurement values, your expectations.
Your given schematic does not show the problem ... it shows how you want to cure the problem.

but somehow it cause common mode noise problem and current readings are different then expected.

* common mode noise
The ADC has differential inputs. Common mode noise should be cancelled out.
Thus I rather think your problem is not "common mode noise"

* current readings
I don't know your expectation, nor do I see your reading values. So I'm not able to find out whether your expectation is wrong, and whether the readings are within or outside specified range.

From the given information I can not find out whether there is a DC problem, a low frequency, problem, a high frequency problem, an accuracy problem....

Klaus

 

[naz$]

Attached my project schematic, have changed from the one given in datasheet. wonder if the value of R25, R24, R23, R26, R27, R28 or capacitors C23, C20, C26, C25 are correct.

 

[KlausST]

Hi,

First there needs to be a problem.
Then you may find a solution.

Still the problem is not clear.
Thus I can´t tell whether your filter can be a solution or not. Nor I can tell whenter the values are good or not.

We see unknown L value
We see a resistor .... but don´t know what you expect it to do
And we see a two stage RC low pass filter.
We don´t see the signal source = what´s behind JP1. What sensor, how is it wired, what cables, what length...

I assume it won´t be a problem for the ADC.
But I can´t say if it can help you either.


Klaus