AD7801 Analog Devices, AD7801 Datasheet - Page 12

• Manufacturer Part Number: AD7801
• Description: +2.7 V to +5.5 V, Parallel Input, Voltage Output 8-Bit DAC
• Manufacturer: Analog Devices

Specifications of AD7801

• Resolution (bits): 8bit
• Dac Update Rate: 833kSPS
• Dac Settling Time: 1.2µs
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Voltage Out
• Dac Input Format: Par

AD7801
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
indicate the Fail condition.
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 k in series with
the full-scale setting resistor of 470 . 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
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:
DV
DD
IN
+5V
input is not below the programmed value, an LED will
Figure 34. Digitally Programmable Indicator
0.1 F
D7
D0
DGND
V
DD
AD7801
SOURCE
10 F
REFIN
AGND
I
of 6 V. The operating range is
V
OUT
256 (5 k )
2 V
REF
V
IN
D
CMP-04
1/4
74HC05
1k
FAIL
PASS/
1/6
1k
PASS
–12–
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 V
giving a fine tune range of 38 mV (approximately 2 LSB) for
operation with a V
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.
EXT REF
AD780/ REF192
AD589 WITH V
WITH V
AD780/ REF192
WITH V
EXT REF
GND
V
IN
DD
GND
V
OR
Figure 35. Programmable Current Source
= 5V
IN
DD
V
Figure 36. Coarse and Fine Adjustment
OUT
= 5V
DD
V
0.1µF
OUT
= 3V
0.1µF
0.1µF
DD
0.1µF
10µF
of 5 V and a reference of 2.5 V. The
REF IN
0.1µF
10µF
AGND DGND
REF IN
REF IN
AD7801
V
DD
V
AGND DGND
AGND DGND
AD7801
AD7801
DD
= 5V
V
DD
V
V
DD
DD
V
= 5V
OUT
V
V
OUT
OUT
51.2k
R3
51.2k
390
AD820/
OP295
R1
R2
+5V
AD820/
OP295
LOAD
390
R4
V
SOURCE
+5V
REV. 0
4.7k
470
2N3904/
BC107
V
O