AD5313 datasheet(18/20 Pages) AD

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REV. 0

AD5330/AD5331/AD5340/AD5341

–18–

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 k

Ω 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 VSOURCE of 6 V. The operating range is deter-

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:

REF

=×

()

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 k

Ω

AD5330/AD5331/

AD5340/AD5341

GND

VDD = 5V

EXT

REF

VOUT

AD780/REF192

WITH VDD = 5V

GND

VIN

VOUT

VREF

VDD

4.7k

0.1 F

10 F

470

LOAD

VSOURCE

AD820/

OP295

Figure 38. Programmable Current Source

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.

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