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# AMC1200: input RC filter

Part Number: AMC1200

Hi,

in AMC1200 datasheet Figure 32 typical application diagram,

we have R2=12ohm, R3=12ohm,

My customer is using AMC1200 to measure AC phase current of a 3 phase rectifer just similar to the typical application.

But we find the error will be smaller if we make R2=24ohm, R3=0ohm.

Why do we suggest R2, R3 both 12ohm? What's the internal input stage circuit of AMC1200 that makes we suggest put 2 same ohm resistor to do the filter?

• Hi Howard,

These resistors are part of a differential input RC filter to help with anti-aliasing. When using R2=24ohm, R3=0ohm, the negative input will not be protected from high frequency noise.

Is the customer accounting for the voltage drop across these resistors? The differential input impedance is typically 28kohm, they should see a voltage drop of approximately 24/28024 (2*R_filter/R_total) - not much but it should be accounted for.

Figure 31. shows the Equivalent Input Circuit.

• Alex,

according to the link, it's for anti aliasing filter, then I wonder what's the sampling frequency of AMC1200? I couldn't find it in the datasheet.

• Hi Howard,

Yes it is for an anti-aliasing filter.

The sampling frequency is equal to the fclk frequency = 10MHz; as shown in figure 31.