Current sensing circuit VS Application Note

[denes44]

Hello,

I'm wondering why is the current sensing circuit is so different in this project compared to the recommended circuit in the application note.

This project:

Application note:

The biggest difference is the R4/5 value (100 Ohm vs. 1 kOhm), and the lack of R1/2.
But the capacitor values is also very different (330 nF vs 18 nF).

Do you have some insights why these values worked better for you?

[denes44]

Ohh I see, that that circuit and the different numbers might come from the ATM90E26's Application Note:

However the 100 Ohm for the R4/5, and the lack of R1/2 is for the shunt circuit, not the current transformer circuit.

Now I'm even more confused...
Is it possible that the circuit just wasn't updated when changing from the ATM90E26 to the ATM90E32?

[CircuitSetup]

When the meter was originally put together, I believe there was some noise getting through from the CT inputs, so R4/R5 was lowered to modify the frequency of the low-pass filter.

So, the 6 channel meter low pass filter is: 1/(2π∗100Ω∗0.33µF) = 4.8kHz
Application note: 1/(2π∗1kΩ∗18nF) = 8.8kHz

FYI, you wouldn't want R1/R2 AND R6 for burden resistors (total of 26.8Ω) unless you plan on using smaller CTs exclusively with a secondary output around 25mA. The ATM90E32 supports a max of 720mV for each input.

[denes44]

Ahh, okay, so the R4/C6 (and R5/C7) doesn't affect the measurement, it's just an RC low pass filter before the ADC input.
I was wondering whether I can get away with 22nF (JLCPCB doesn't have 18 nF as a basic part). But it seems like it won't affect things very much.

And if I understand it right, in the application note R1/2 are the burden resistors, they are not needed in addition to a separate burden resistor (R6).
Any idea why they are using two resistors with the middle connected to ground instead of one like in your circuit?