RC Signal Filter (RC Low-Pass Filter)
The simplest circuit for filtering out high-frequency noise, made of a series resistor and a capacitor to chassis. The cut-off frequency equals the inverse of 6.28 times the product of the resistance and the capacitance; 10 kΩ and 100 nF give roughly 160 Hz. It is used to separate slow signals such as the speed reference from PWM noise.
Switching noise on the motor cable is at 20 kHz and above; the speed reference, on the other hand, is a slow signal that changes a few times per second. That frequency gap makes it possible to separate the two with a simple filter.
- Choosing values: The cut-off frequency must sit above the fastest change of the real signal and below the noise. For a speed reference the 50–500 Hz band works well in practice.
- Resistance limit: The series resistor forms a divider with the input impedance; going above 10 kΩ can cause a measurable drop in the reference.
- Delay: A filter also means a slower response. A 160 Hz cut-off adds roughly 1 ms of delay to a command change; negligible for a speed reference, but a value to watch in current protection.
Place the capacitor as close as possible to the input and to the chassis connection point; long leads reduce the filter's effectiveness at high frequency.
Where this term is used: DC Motor Driver Selection Guide