Datasheet

MCP4901/4911/4921
DS22248A-page 28  2010 Microchip Technology Inc.
6.4 Single-Supply Operation
The MCP4901/4911/4921 devices are rail-to-rail volt-
age 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 an external output buffer for most
applications.
6.4.1 DC SET POINT OR CALIBRATION
A common application for DAC devices is
digitally-controlled set points and/or calibration of
variable parameters, such as sensor offset or slope.
For example, the MCP4921 and MCP4922 provide
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 an 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