Datasheet

MCP4801/4811/4821
DS22244B-page 26  2010 Microchip Technology Inc.
6.5 Single-Supply Operation
The MCP4801/4811/4821 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.
6.5.1 DC SET POINT OR CALIBRATION
A common application for the devices is a digitally-
controlled set point and/or calibration of variable
parameters, such as sensor offset or slope. For
example, the MCP4821 and MCP4822 provide
4096 output steps. If G = 1x is selected, the internal
2.048V V
REF
would produce 500 µV of resolution. If
G = 2x is selected, the internal 2.048 V
REF
would
produce 1 mV of resolution.
6.5.1.1 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 are to either
reduce V
REF
to 0.82V (using the MCP49XX family
device that uses external reference) 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 µF
Comparator
V
OUT
2.048 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
(a) Single Output DAC:
MCP4801
MCP4811
MCP4821
(b) Dual Output DAC:
MCP4802
MCP4812
MCP4822
G = Gain selection (1x or 2x)
D
n
= Digital value of DAC (0-255) for MCP4801/MCP4802
= Digital value of DAC (0-1023) for MCP4811/MCP4812
= Digital value of DAC (0-4095) for MCP4821/MCP4822
N = DAC bit resolution