Datasheet

MCP4902/4912/4922
DS22250A-page 28  2010 Microchip Technology Inc.
6.4 Single-Supply Operation
The MCP4902/4912/4922 family of devices are rail-to-
rail voltage output DAC devices designed to operate
with a V
DD
range of 2.7V to 5.5V. Its output amplifier is
robust enough to drive small-signal loads directly.
Therefore, it does not require any external output buffer
for most applications.
6.4.1 DC SET POINT OR CALIBRATION
A common application for the DAC devices is
digitally-controlled set points and/or calibration of
variable parameters, such as sensor offset or slope.
For example, the MCP4922 provides 4096 output
steps. If the external voltage reference (V
REF
) is
4.096V, the LSb size is 1 mV. If a smaller output step
size is desired, a lower external voltage reference is
needed.
6.4.1.1 Decreasing Output Step Size
If the application is calibrating the bias voltage of a
diode or transistor, a bias voltage range of 0.8V may be
desired with about 200 µV resolution per step. 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.
Using a V
REF
is an option if the V
REF
is available with
the desired output voltage range. However,
occasionally, when using a low-voltage V
REF
, the noise
floor causes SNR error that is intolerable. Using a
voltage divider method is another option and 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.
Example 6-1 illustrates this concept. Note that the
bypass capacitor on the output of the voltage divider
plays a critical function in attenuating the output noise
of the DAC and the induced noise from the
environment.
EXAMPLE 6-1: EXAMPLE CIRCUIT OF SET POINT OR THRESHOLD CALIBRATION
V
DD
SPI
3-wire
V
TRIP
R
1
R
2
0.1 uF
Comparator
G = Gain selection (1x or 2x)
D
n
= Digital value of DAC (0-255) for MCP4901/MCP4902
V
OUT
V
REF
G
D
n
2
N
------  =
V
CC
+
V
CC
–
V
OUT
V
trip
V
OUT
R
2
R
1
R
2
+
--------------------



=
V
DD
R
SENSE
DAC
= Digital value of DAC (0-1023) for MCP4911/MCP4912
= Digital value of DAC (0-4095) for MCP4921/MCP4922
N = DAC Bit Resolution
V
REF
V
O
MCP4901
MCP4911
MCP4921
(a) Single Output DAC:
(b) Dual Output DAC:
MCP4902
MCP4912
MCP4922