Datasheet

16 Digital Potentiometer Design Guide
Digital Potentiometer Solutions
Voltage Windowing
Resistance Options and Resolutions
Microchip offers Digital Potentiometer devices with typical
RAB resistances of 2.1 K, 5 K, 10 K, 50 K and 100
K. The devices offer either 6-, 7- or 8-bits of resolution.
The step resistance (RS) is the RAB resistances divided by
the number of RS resistors in the RAB ladder.
The step resistance is important to understand when
you are using the device in a rheostat mode, or the
potentiometer is being windowed by resistors on the
Terminal A and/or on the Terminal B.
Voltage Windowing
Terminal A and Terminal B may for example be any
voltage within the device specification limits. Lets call the
voltages at these nodes VA and VB. So the voltage across
the resistor RAB (VAB) is | VA - VB |. The VAB voltage is
determined by the values of the R1, R2 and RAB resistors.
As the VAB voltage becomes smaller relative to the voltage
range, the effective resolution of the device increase,
though the resolution is limited to between the VA and VB
voltages.
This allows a less precise device to be used for more
precise circuit tuning over a narrower range. When
replacing a mechanical potentiometer, this configuration
can be used and R1 and R2 may be any resistance
(including 0).
Resistance Options
RAB 6-bit (63) 7-bit (127) 7-bit (128) 8-bit (256)
2100 33.33333333 – – –
5000 79.36507937 39.37007874 39.0625 19.53125
10000 158.7301587 78.74015748 78.125 39.0625
50000 793.6507937 393.7007874 390.625 195.3125
100000 – 787.4015748 781.25 390.625
How the VAB Voltage Effects the Effective Resolution
VAB
mV per Step Effective VAB Resolution vs. Fixed VDD
Comment
6-bit (63) 7-bit (127) 7-bit (128) 8-bit (256) 6-bit (63) 7-bit (127) 7-bit (128) 8-bit (256)
5.00 79.4E 39.4E 39.1E 19.5E 6-bit 7-bit 7-bit 8-bit VAB = VDD
2.50 39.7E 19.7E 19.5E 9.8E 7-bit 8-bit 8-bit 9-bit VDD = 5.0V
1.25 19.8E 9.8E 9.8E 4.9E 8-bit 9-bit 9-bit 10-bit VDD = 5.0V