March
2005, Issue 176
Joystick
Technology
HIGHER
RESOLUTION
Most
digital potentiometer manufacturers don't produce devices
with more than 256 taps. If you are using a digital
potentiometer's output as a voltage divider rather than
its resistive properties, then you can substitute a
serial DAC for it.
Serial
DACs such as the MCP4922 have a voltage output. They
are available in single and double devices similar to
digital potentiometers. In fact, the same SPI routine
can be used as the one communicating with the digital
potentiometer. Both formats require 16 bits of data
to be sent to control a single device. Packages with
dual devices require two 16-bit writes.
Figure
5 shows the communication differences between the digital
potentiometer and the DAC. The DAC has independent reference
inputs meant for 4.096- or 2.048-V references. A 2×
gain is available through the *GA configuration bit.
This will allow a 2.049-V reference to actually produce
a 0- to 4.096-V output. It provides an easy conversion
between the data byte value and voltage output because
1 bit equals 1 mV.
|
(Click
here to enlarge)
|
Figure
5—Although both devices use a 16-bit SPI format,
the commands within the data format are different.
Of the 16 bits, the DAC has 12 bits of data while
the digital potentiometer uses only 8 data bits. |
