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 kΩ 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
a min V
SOURCE
of 6 V. The operating range is determined by
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
REF
× D/(5E +3 × 256) mA
AD7302
V
OUT
A
10µF0.1µF
V
DD
= 5V
V
DD
AGND
AD780/REF192
WITH V
DD
= 5V
REF IN
GND
V
OUT
V
IN
0.1µF
EXT REF
4.7kΩ
470Ω
+5V
LOAD
V
SOURCE
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
approximately 2 LSBs for operation with a V
DD
of 5 V and a
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
V
OUT
A
10µF
0.1µF
V
DD
= 5V
V
DD
AGND
AD780/REF192
WITH V
DD
= 5V
OR
AD589 WITH V
DD
= 3V
REF IN
GND
V
OUT
V
IN
0.1µF
EXT REF
R2
51.2kΩ
R4
390Ω
R1
390Ω
+5V
V
OUT
DGND
AD820/
OP295
V
OUT
B
R3
51.2kΩ
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.
