Datasheet

 2004 Microchip Technology Inc. DS21897A-page 21
MCP4921/4922
6.4 Single-Supply Operation
The MCP492X is a rail-to-rail (R-R) input and output
DAC designed to operate with a V
DD
range of 2.7V to
5.5V. Its output amplifier is robust enough to drive com-
mon, small-signal loads directly, thus eliminating the
cost and size of an external buffer for most applications.
6.4.1 DC SET POINT OR CALIBRATION
A common application for a DAC with the MCP492X’s
performance is digitally-controlled set points and/or
calibration of variable parameters, such as sensor off-
set or slope. 12-bit resolution provides 4096 output
steps. If a 4.096V V
REF
is provided, an LSB would
represent 1 mV of resolution. If a smaller output step
size is desired, the output range would need to be
reduced.
6.4.1.1 Decreasing The Output Step Size
If the output range is reduced relative to AV
SS
, simply
reducing V
REF
will reduce the magnitude of each out-
put step. If the application is calibrating the threshold
of a diode, transistor or resistor tied to AV
SS
or V
REF
,
a theshold range of 0.8V may be desired to provide
200 µV resolution. Two common methods to achieve a
0.8V range is to either reduce V
REF
to 0.82V or use a
voltage divider on the DAC’s output. If a V
REF
is avail-
able with the desired output value, using that V
REF
is an
option. Occasionally, when using a low-voltage V
REF
,
the noise floor causes SNR error that is intolerable.
The voltage divider method provides some advantages
when V
REF
needs to be very low or when the desired
output voltage is not available. In this case, a larger
value V
REF
is used while two resistors scale the output
range down to the precise desired level. Using a com-
mon V
REF
output has availability and cost advantages.
Example 6-1 illustrates this concept. Note that the volt-
age divider can be connected to AV
SS
or V
REF
,
depending on the application’s requirements.
The MCP492X’s low, ±0.75 (max.) DNL performance
is critical to meeting calibration accuracy in production.
EXAMPLE 6-1: Set Point or Threshold Calibration.
V
REF
MCP492X
V
DD
SPI™
3
V
trip
R
1
R
2
0.1 uF
Comparator
G = Gain select (1x or 2x)
D = Digital value of DAC (0 – 4096)
V
OUT
V
REF
G
D
2
12
-------
=
V
CC
+
V
CC
–
V
OUT
V
trip
V
OUT
R
2
R
1
R
2
+
------------------


=
V
DD
R
sense