Datasheet

© 2011-2012 Microchip Technology Inc. DS22272C-page 65
MCP4706/4716/4726
8.3 Application Examples
The MCP47X6 devices are rail-to-rail output DACs
designed to operate with a V
DD
range of 2.7V to 5.5V.
The internal output op amplifier is robust enough to
drive common, small-signal loads directly, thus
eliminating the cost and size of external buffers for
most applications. The user can use gain of 1 or 2 of
the output op amplifier by setting the Configuration
register bits. Also, the user can use internal V
DD
as the
reference or use external reference. Various user
options and easy-to-use features make the devices
suitable for various modern DAC applications.
Application examples include:
• Decreasing Output Step Size
• Building a “Window” DAC
• Bipolar Operation
• Selectable Gain and Offset Bipolar Voltage Output
• Designing a Double-Precision DAC
• Building Programmable Current Source
• Serial Interface Communication Times
• Software I2C Interface Reset Sequence
• Power Supply Considerations
• Layout Considerations
8.3.1 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 MCP4726 provides 4096 output
steps. If voltage reference is 4.096V, the LSb size is
1 mV. If a smaller output step size is desired, a lower
external voltage reference is needed.
8.3.1.1 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 small step size are using
lower V
REF
pin voltage or using a voltage divider on the
DAC’s output.
Using an external voltage reference (V
REF
) is an
option, if the external reference is available with the
desired output voltage range. However, occasionally,
when using a low-voltage reference voltage, the noise
floor causes a SNR error that is intolerable. Using a
voltage divider method is another option, and provides
some advantages when external voltage reference
needs to be very low, or when the desired output
voltage is not available. In this case, a larger value
reference voltage is used, while two resistors scale the
output range down to the precise desired level.
Figure 8-3 illustrates this concept. A 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.
FIGURE 8-3: Example Circuit Of Set Point
or Threshold Calibration.
EQUATION 8-1: V
OUT
AND V
TRIP
CALCULATIONS
R
1
V
CC
+
V
CC
–
V
OUT
I
2
C™
2-wire
V
REF
Optional
MCP47X6
V
DD
V
O
R
2
C
1
R
SENSE
Comp.
V
DD
V
TRIP
V
trip
V
OUT
R
2
R
1
R
2
+
--------------------
⎝⎠
⎜⎟
⎛⎞
=
V
OUT
= V
REF
• G •
DAC Register Value
2
N