Analog Input Impedance
It is the load the driver's analog reference input presents to the source. If the input impedance is close to the source impedance, the reference voltage is divided down and the speed setting comes out lower than expected. As a rule, the input impedance should be at least ten times, preferably a hundred times, the source impedance.
When you connect a potentiometer the source impedance is not constant; it takes its highest value with the pot in mid position and is approximately equal to a quarter of the pot value. On a 10 kΩ pot that means about 2.5 kΩ.
- Division error: if the input impedance is 25 kΩ, the reference at mid position falls by about 9%; this is felt as a sag in the middle of the speed curve. When the input impedance is 250 kΩ, the error drops below 1%.
- Long cable: a high input impedance is open to noise; that is why the input stage is often designed together with a high-impedance buffer, using a small RC filter at the input.
- Measurement effect: when you measure the reference with a multimeter, the instrument's own 10 MΩ impedance comes into play; the value you read may differ slightly from what the driver sees.
If the expected speed is not reached when the speed pot is fully turned, lowering the pot value (for example 5 kΩ instead of 47 kΩ) most often solves the problem.
Context where this term is used: DC Motor Driver Selection Guide