Datasheet

16
Signal Chain Design Guide
Setting the DC Set Point for Sensor Circuit
A common DAC application is digitally controlling the set
point and/or calibration of parameters in a signal chain.
The figure below shows controlling the DC set point of a
light detector sensor using the MCP4728 12-bit quad DAC
device. The DAC provides 4096 output steps. If G = 1 and
internal reference voltage options are selected, then the
internal 2.048 Vref would produce 500 µV of resolution.
If G = 2 is selected, the internal 2.048 V
ref would
produce 1 mV of resolution. If a smaller output step size
is desired, the output range would need to be reduced.
So, using gain of 1 is a better choice than using gain of 2
configuration option for smaller step size, but its full-scale
range is one half of that of the gain of 2. Using a voltage
divider at the DAC output is another method for obtaining
a smaller step size.
Sensor Calibration/Compensation
Setting the DC Set Point
LDAC
8
10
9
7
6
VDD
3
1
2
4
5
V SS
V OUT A
V
OUT D
V
OUT C
V
OUT B
V
DD
SCL
SDA
RDY/BSY
R 6
R 3
R 5
R 4
0.1 µF
Analog Outputs
10 µF
To MCU
R 1
R 2
Comparator 1
Light
VDD
V
TRIP1
0.1 µF
R 1
R 2
Comparator 2
Light
VDD
VTRIP2
0.1 µF
R 1
R 2
Comparator 3
Light
VDD
VTRIP3
0.1 µF
R 1
R 2
Comparator 4
Light
VDD
VTRIP4
0.1 µF



V OUT = V REF x
Dn
G X
V TRIP = V OUT
x
R
1 + R 2
Where Dn = Input Code (0 to 4095)
G
X = Gain Selection (x1 or x2)
4096
R
2
Quad DAC
R
SENSE
MCP6544(1/4)
RSENSE
MCP6544(2/4)
RSENSE
MCP6544(3/4)
RSENSE
MCP6544(4/4)
–
+
MCP4728
–
+
–
+
–
+