MCP4822 If the application is calibrating the threshold of a diode, Application Circuit

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MCP4822 circuit
MCP4821 MCP4822
6.5
Single-Supply Operation
6.5.1.1
Decreasing The Output Step Size
The MCP482X devices are Rail-to-Rail (R-R) input and
output DACs designed to operate with a V
DD
range of
2.7V to 5.5V. Its output amplifier is robust enough to
drive common, small-signal loads directly, thus eliminat-
ing the cost and size of an external Buffer for most
applications.
If the application is calibrating the threshold of a Diode
Transistor or Resistor tied to AV
SS
, a threshold 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 (would require MCP492X device
and external voltage reference) or use a voltage divider
on the DAC’s output. Typically, 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. Using two
Resistors to scale the output range down to the precise
desired level is a simple, low-cost method to achieve
very small step sizes. 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.
The MCP482X family’s low ±0.75 (max.) DNL
performance is critical to meeting calibration accuracy
in production.
6.5.1
DC SET POINT OR CALIBRATION
A common application for a DAC with the MCP482X
family’s performance is a digitally-controlled set point
and/or calibration of variable parameters, such as sen-
sor offset or slope. 12-bit resolution provides 4096 out-
put steps. If G = 1 is selected, then the internal 2.048
V
REF
would produce 500 µV of resolution. If G = 2 is
selected, the internal 2.048 V
REF
would produce 1 mV
of resolution.
V
DD
R
SENSE
V
DD
V
OUT
R
1
V
CC
+
V
TRIP
V
CC
MCP482X
R
2
SPI™
3
D
-
V OUT = 2.048
G --------
12
2
R2
V trip = VOUT
--------------------
R 1 + R 2
0.1 uF
G = Gain select (1x or 2x)
D = Digital value of DAC (0 – 4096)
EXAMPLE 6-1:
Set Point or Threshold Calibration.
DS21953A-page 20
©
2005 Microchip Technology Inc.