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

indicate the Fail condition.

1/4

CMP-04

AD7801

REFIN

DGND

AGND

D7

D0

PASS/

1/6

74HC05

1k

Ω

PASS

1k

Ω

FAIL

10 F

0.1 F

+5V

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 k

Ω 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

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

=

()

256 (5 k

Ω)

()

AD7801

REF IN

AGND DGND

10µF

0.1µF

0.1µF

OUT

GND

AD780/ REF192

4.7k

Ω

470

Ω

+5V

AD820/

OP295

2N3904/

BC107

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

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

REF IN

AGND DGND

10µF

0.1µF

0.1µF

OUT

GND

AD780/ REF192

OR

+5V

AD820/

OP295

R1

390

Ω

AD7801

REF IN

AGND DGND

R2

51.2k

Ω

R3

51.2k

Ω

R4

390

Ω

0.1µF

Figure 36. Coarse and Fine Adjustment