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MCP4922T-EMS Datasheet(HTML) 21 Page - Microchip Technology

Part No. MCP4922T-EMS
Description  12-Bit DAC with SPI™ Interface
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Maker  MICROCHIP [Microchip Technology]
Homepage  http://www.microchip.com
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MCP4922T-EMS Datasheet(HTML) 21 Page - Microchip Technology

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 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 VDD 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 VREF 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 AVSS, simply
reducing VREF will reduce the magnitude of each out-
put step. If the application is calibrating the threshold
of a diode, transistor or resistor tied to AVSS or VREF,
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 VREF to 0.82V or use a
voltage divider on the DAC’s output. If a VREF is avail-
able with the desired output value, using that VREF is an
option. Occasionally, when using a low-voltage VREF,
the noise floor causes SNR error that is intolerable.
The voltage divider method provides some advantages
when VREF needs to be very low or when the desired
output voltage is not available. In this case, a larger
value VREF is used while two resistors scale the output
range down to the precise desired level. Using a com-
mon VREF output has availability and cost advantages.
Example 6-1 illustrates this concept. Note that the volt-
age divider can be connected to AVSS or VREF,
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.
VREF
MCP492X
VDD
SPI™
3
Vtrip
R1
R2
0.1 uF
Comparator
G = Gain select (1x or 2x)
D = Digital value of DAC (0 – 4096)
V
OUT
V
REFG
D
2
12
-------
=
VCC+
VCC
VOUT
V
trip
V
OUT
R
2
R
1
R
2
+
------------------


=
VDD
Rsense


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