AD7302

–14–

REV. 0

Programmable Current Source

Figure 35 shows the AD7302 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. Transistors suitable

to place in the feedback loop of the amplifier include the BC107

or the 2N3904, which enable the current source to operate from

the operating characteristics of the of the transistor. Suitable

amplifiers include the AD820 and the OP295 both having rail-

to-rail operation on their outputs. The current for any digital

input code can be calculated as follows:

I = 2

× V

AD7302

10µF

0.1µF

AGND

AD780/REF192

REF IN

GND

0.1µF

EXT REF

4.7k

Ω

470

Ω

+5V

LOAD

AD820/

OP295

DGND

Figure 35. Programmable Current Source

Coarse and Fine Adjustment Using the AD7302

The DACs on the AD7302 can be paired together to form a

coarse and fine adjustment function as shown in Figure 36. In

this circuit DAC A is used to provide the coarse function while

DAC B 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

DAC B has a resolution of 148

µV giving a fine tune range of

reference of 2.5 V. The amplifiers shown allow a rail-to-rail

output voltage to be achieved on the output. A typical applica-

tion for such a circuit would be in a setpoint controller.

AD7302

10µF

0.1µF

AGND

AD780/REF192

OR

REF IN

GND

0.1µF

EXT REF

R2

51.2k

Ω

R4

390

Ω

R1

390

Ω

+5V

DGND

AD820/

OP295

R3

51.2k

Ω

Figure 36. Coarse/Fine Adjust Circuit

Power Supply Bypassing and Grounding

In any circuit where accuracy is important, careful consideration

of the power supply and ground return layout helps to ensure

the rated performance. The printed circuit board on which the

AD7302 is mounted should be designed so the analog and

digital sections are separated and confined to certain areas of the

board. If the AD7302 is in a system where multiple devices

require an AGND to DGND connection, the connection should

be made at one point only, a star ground point that should be

established as closely as possible to the AD7302. The AD7302

should have ample supply bypassing of 10

µF in parallel with

0.1

µF on the supply located as close to the package as possible,

ideally right up against the device. The 10

µF capacitors are the

tantalum bead type. The 0.1

µF capacitor should have low

Effective Series Resistance (ESR) and Effective Series Induc-

tance (ESI), such as the common ceramic types, which provide

a low impedance path to ground at high frequencies to handle

transient currents due to internal logic switching.

The power supply lines of the AD7302 should use as large a

trace as possible to provide low impedance paths and reduce the

effects of glitches on the power supply line. Fast switching sig-

nals like clocks should be shielded with digital ground to avoid

radiating noise to other parts of the board and should never be

run near the reference inputs. Avoid crossover of digital and

analog signals. Traces on opposite sides of the board should run

at right angles to each other. This reduces the effects of feed-

through through the board. A microstrip technique is by far the

best, but not always possible with a double-sided board. In this

technique, the component side of the board is dedicated to

ground plane while signal traces are placed on the solder side.