ad5331bru-reel7 Analog Devices, Inc., ad5331bru-reel7 Datasheet - Page 18

• Manufacturer Part Number: ad5331bru-reel7
• Description: 2.5 V To 5.5 V, 115 Ua, Parallel Interface Single Voltage-output 8-/10-/12-bit Dacs
• Manufacturer: Analog Devices, Inc.

AD5330/AD5331/AD5340/AD5341
Programmable Current Source
Figure 38 shows the AD5330/AD5331/AD5340/AD5341 used
as the control element of a programmable current source. In this
example, the full-scale current is set to 1 mA. The output volt-
age from the DAC is applied across the current setting resistor
of 4.7 kΩ in series with the 470 Ω adjustment potentiometer,
which gives an adjustment of about ± 5%. 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
mined by the operating characteristics 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 and resistor value can be calculated as follows:
Where:
G is the gain of the buffer amplifier (1 or 2)
D is the digital equivalent of the digital input code
N is the DAC resolution (8, 10, or 12 bits)
R is the sum of the resistor plus adjustment potentiometer in kΩ
EXT
REF
AD780/REF192
WITH V
GND
V
IN
DD
V
= 5V
OUT
I
=
0.1 F
0.1 F
SOURCE
G
×
V
V
AD5330/AD5331/
of 6 V. The operating range is deter-
AD5340/AD5341
REF
REF
10 F
V
DD
GND
V
×
DD
= 5V
(
2
N
V
OUT
D
×
R
)
AD820/
OP295
mA
5V
V
SOURCE
LOAD
4.7k
470
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
AD5330/AD5331/AD5340/AD5341 is mounted should be
designed so that the analog and digital sections are separated,
and confined to certain areas of the board. If the device is in a
system where multiple devices require an AGND-to-DGND
connection, the connection should be made at one point only.
The star ground point should be established as closely as pos-
sible to the device. The AD5330/AD5331/AD5340/AD5341
should have ample supply bypassing of 10 µF in parallel with
0.1 µF on the supply located as close to the package as pos-
sible, 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), like the common ceramic types that provide a low
impedance path to ground at high frequencies to handle tran-
sient currents due to internal logic switching.
The power supply lines of the device 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 signals such
as clocks should be shielded with digital ground to avoid radiat-
ing noise to other parts of the board, and should never be run
near the reference inputs. Avoid crossover of digital and ana-
log 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.