AD5667BCPZ-R2 datasheet(27/32 Pages) AD

27 / 32 page

AD5627R/AD5647R/AD5667R, AD5627/AD5667

Rev. 0 | Page 27 of 32

APPLICATION INFORMATION

USING A REFERENCE AS A POWER SUPPLY FOR

THE AD56x7R/AD56x7

Because the supply current required by the AD56x7R/AD56x7 is

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

to supply the required voltage to the part (see Figure 67). This is

especially useful if the power supply is quite 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 AD56x7R/AD56x7. If the low dropout REF195 is

used, it must supply 450 μA of current to the AD56x7R/AD56x7

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

loaded, the REF195 also needs to supply the current to the load.

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

output) is

450 μA + (5 V/5 kΩ) = 1.45 mA

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

resulting in a 2.9 ppm (14.5 μV) error for the 1.45 mA current

drawn from it. This corresponds to a 0.191 LSB error.

2-WIRE

SERIAL

INTERFACE

SCL

SDA

VOUT = 0V TO 5V

VDD

GND

15V

REF195

AD5627R/

AD5647R/

AD5667R/

AD5627/

AD5667

Figure 67. REF195 as Power Supply to the AD56x7R/AD56x7

BIPOLAR OPERATION USING THE

AD56x7R/AD56x7

The AD56x7R/AD56x7 has been designed for single-supply

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

circuit in Figure 68. The circuit gives an output voltage range of

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

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 65535).

With 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.

2-WIRE

SERIAL

INTERFACE

R2 = 10kΩ

+5V

–5V

AD820/

OP295

AD5627R/

AD5647R/

AD5667R/

AD5627/

AD5667

VDD

VOUT

R1 = 10kΩ

±5V

0.1µF

10µF

+5V

SDA

SCL

GND

Figure 68. Bipolar Operation with the AD56x7R/AD56x7

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 AD56x7R/AD56x7

should have separate analog and digital sections, each having its

own area of the board. If the AD56x7R/AD56x7 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 AD56x7R/AD56x7.

The power supply to the AD56x7R/AD56x7 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 capacitor

ideally right up against the device. The 10 μF capacitor should be

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

have low effective series resistance (ESR) and 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 two-layer board.

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