AD5665RBCPZ-R2 datasheet(32/36 Pages) AD

32 / 36 page

AD5625R/AD5645R/AD5665R, AD5625/AD5665

Data Sheet

Rev. C | Page 32 of 36

APPLICATIONS INFORMATION

USING A REFERENCE AS A POWER SUPPLY FOR

THE AD56x5R/AD56x5

Because the supply current required by the AD56x5R/AD56x5 is

extremely low, an alternative option is to use a voltage reference

to supply the required voltage to the part (see Figure 73). This is

especially useful if the power supply is noisy or if the system

supply voltages are at some value other than 5 V or 3 V, for

example, 15 V. The voltage reference outputs a steady supply

voltage for the AD56x5R/AD56x5. If the low dropout REF195 is

used, it must supply 450 µA of current to the AD56x5R/AD56x5

with no load on the output of the DAC. When the DAC output

is loaded, the REF195 also must supply the current to the load.

The total current required (with a 5 kΩ load on the DAC

output) is

1 mA + (5 V/5 kΩ) = 2 mA

The load regulation of the REF195 is typically 2 ppm/mA,

resulting in a 4 ppm (20 µV) error for the 2 mA current drawn

from it. This corresponds to a 0.263 LSB error.

Figure 73. REF195 as Power Supply to the AD56x5R/AD56x5

BIPOLAR OPERATION USING THE

AD56x5R/AD56x5

The AD56x5R/AD56x5 have been designed for single-supply

operation, but a bipolar output range is also possible using the

circuit shown in Figure 74. The circuit gives an output voltage

range of ±5 V. Rail-to-rail operation at the amplifier output is

achievable using an AD820 or an OP295 as the output amplifier.

The output voltage for any input code can be calculated as follows:

























−

























536

where D represents the input code in decimal (0 to 65,535).

If VDD = 5 V, R1 = R2 = 10 kΩ,

536

−













This is an output voltage range of ±5 V, with 0x0000 corre-

sponding to a −5 V output and 0xFFFF corresponding to a

+5 V output.

Figure 74. Bipolar Operation with the AD56x5R/AD56x5

POWER SUPPLY BYPASSING AND GROUNDING

When accuracy is important in a circuit, it is helpful to carefully

consider the power supply and ground return layout on the board.

The printed circuit board containing the AD56x5R/AD56x5

should have separate analog and digital sections, each having its

own area of the board. If the AD56x5R/AD56x5 are in a system

where other devices require an AGND-to-DGND connection,

the connection should be made at one point only. This ground

point should be as close as possible to the AD56x5R/AD56x5.

The power supply to the AD56x5R/AD56x5 should be bypassed

with 10 µF and 0.1 µF capacitors. The capacitors should be

located as close as possible to the device, with the 0.1 µF capaci-

tor ideally right up against the device. The 10 µF capacitor is

the tantalum bead type. It is important that the 0.1 µF capacitor

have low effective series resistance (ESR) and low effective

series inductance (ESI), for example, common ceramic types of

capacitors. This 0.1 µF capacitor provides a low impedance path

to ground for high frequencies caused by transient currents due

to internal logic switching.

The power supply line itself should have as large a trace as

possible to provide a low impedance path and to reduce glitch

effects on the supply line. Clocks and other fast switching

digital signals should be shielded from other parts of the board

by digital ground. Avoid crossover of digital and analog signals

if possible. When traces cross on opposite sides of the board,

ensure that they run at right angles to each other to reduce

feedthrough effects through the board. The best board layout

technique is the microstrip technique where the component

side of the board is dedicated to the ground plane only, and the

signal traces are placed on the solder side. However, this is not

always possible with a 2-layer board.

2-WIRE

SERIAL

INTERFACE

SCL

SDA

VOUT = 0V TO 5V

VDD

GND

15V

REF195

AD5625R/

AD5645R/

AD5665R/

AD5625/

AD5665

2-WIRE

SERIAL

INTERFACE

R2 = 10kΩ

+5V

–5V

AD820/

OP295

AD5625R/

AD5645R/

AD5665R/

AD5625/

AD5665

VDD

VOUT

R1 = 10kΩ

±5V

0.1µF

10µF

+5V

SDA

SCL

GND

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