AD7801
–12–
REV. 0
AD7801 as a Digitally Programmable Indicator
A digitally programmable upper limit detector using the DAC is
shown in Figure 34. The upper limit for the test is loaded to the
DAC, which in turn sets the limit for the CMP04. If a signal at
the V
IN
input is not below the programmed value, an LED will
indicate the Fail condition.
1/4
CMP-04
AD7801
V
DD
REFIN
DGND AGND
D7
D0
V
OUT
V
IN
PASS/
1/6
74HC05
1kΩ
PASS
1kΩ
FAIL
10 F
0.1
F
+5V
DV
DD
Figure 34. Digitally Programmable Indicator
Programmable Current Source
Figure 35 shows the AD7801 used as the control element of a
programmable current source. In this circuit the full-scale
current is set to 1 mA. The output voltage from the DAC is
applied across the current setting resistor of 4.7 kΩ in series with
the full-scale setting resistor of 470 Ω. Suitable transistors to
place in the feedback loop of the amplifier include the BC107
and the 2N3904, which enable the current source to operate
from a minimum V
SOURCE
of 6 V. The operating range is
determined by the operating characteristics of the transistor.
Suitable amplifiers include the AD820 and the OP295, both of
which have rail-to-rail operation on their outputs. The current
for any digital input code can be calculated as follows:
I =
2V
REF
D
()
256 (5 kΩ)
()
AD7801
V
OUT
REF IN
V
DD
= 5V
V
DD
AGND DGND
10µF0.1µF
0.1µF
EXT REF
V
OUT
V
IN
GND
AD780/ REF192
WITH V
DD
= 5V
4.7kΩ
470Ω
+5V
AD820/
OP295
2N3904/
BC107
V
SOURCE
LOAD
Figure 35. Programmable Current Source
Coarse and Fine Adjustment using two AD7801s
The two DACs can be paired together to form a coarse and fine
adjustment function for a setpoint as shown in Figure 36. In this
circuit, the first DAC is used to provide the coarse adjustment
and the second DAC is used to provide the fine adjustment.
Varying the ratio of R1 and R2 will vary the relative effect of the
coarse and fine tune elements in the circuit. For the resistor
values shown, the second DAC has a resolution of 148 µV
giving a fine tune range of 38 mV (approximately 2 LSB) for
operation with a V
DD
of 5 V and a reference of 2.5 V. The
amplifier shown allows a rail-to-rail output voltage to be
achieved on the output. A typical application for the circuit
would be in a setpoint controller.
AD7801
V
OUT
REF IN
V
DD
= 5V
V
DD
AGND DGND
10µF0.1µF
0.1µF
EXT REF
V
OUT
V
IN
GND
AD780/ REF192
WITH V
DD
= 5V
OR
AD589 WITH V
DD
= 3V
+5V
AD820/
OP295
R1
390Ω
AD7801
V
OUT
REF IN
V
DD
AGND DGND
R2
51.2kΩ
V
O
R3
51.2kΩ
R4
390Ω
0.1µF
Figure 36. Coarse and Fine Adjustment
