TIDA-00555: Reference design voltage measuring circuit

Hi Thilesh,

In Figure 10, R63, R65, and C55 form a low-pass filter on the input signal to the AMC1100. The values chosen, 12 ohms and 330pF, give a cutoff frequency around 20MHz. It appears that these values were taken from the AMC1100 datasheet's Typical Applications section (page 16). These values shouldn't have to change depending on input voltage - the same values appear in Figure 9 - Circuit Diagram of U Current Input.

Since the AMC1100 is designed to accept a ±250mV input, a separate resistor divider is required to measure higher voltages. In TIDA-00555, this divider consists of the series combination of FB9, R67, R68, R69, and R70 along with R60. That configuration results in an attenuation of 1/1627, allowing voltages up to ~406V to be measured.

Page 16 of the TIDA-00555 design guide describes the design procedure for this voltage divider based on the input voltage range and required input impedance of the measurement circuit.

Hi Thilesh,
The typical applications section in the AMC1301 doesn't appear to include that low pass filter. That being said, it may be a good idea to include a place-holder for it (replace the 12 ohm resistors with 0 ohm, and leave the capacitor in the schematic, but don't install it on your PCB). If you run into issues with noise in your final circuit, you can always install components there to low pass filter the input into the AMC1301.