AD5626 datasheet(12/20 Pages) AD

12 / 20 page

AD5626

Rev. 0 | Page 12 of 20

APPLICATIONS INFORMATION

POWER SUPPLIES, BYPASSING, AND GROUNDING

All precision converter products require careful application of

good grounding practices to maintain full rated performance.

Because the AD5626 has been designed for 5 V applications, it

is ideal for those applications under microprocessor or micro-

computer control. In these applications, digital noise is prevalent;

therefore, special care must be taken to ensure that its inherent

precision is maintained by exercising particularly good engineering

judgment when addressing the power supply, grounding, and

bypassing issues using the AD5626.

Use a well-filtered and regulated power supply for the AD5626.

The device has been completely characterized for a 5 V supply

with a tolerance of ±5%. Because a 5 V logic supply is almost

universally available, it is not recommended to connect the

DAC directly to an unfiltered logic supply without careful

filtering. Tapping a logic circuit supply for the DAC supply

is unwise because fast logic with nanosecond transition edges

induce high current pulses. The high transient current pulses

can generate glitches hundreds of millivolts in amplitude due

to wiring resistances and inductances. This high frequency

noise corrupts the analog circuits internal to the DAC and

causes errors.

Even though their spike noise is lower in amplitude, directly

tapping the output of a 5 V system supply can cause errors

because these supplies are of the switching regulator type that

can and do generate a great deal of high frequency noise. There-

fore, power the DAC and any associated analog circuitry directly

from the system power supply outputs using appropriate filtering.

Figure 25 illustrates how a clean, analog-grade supply can be

generated from a 5 V logic supply using a differential LC filter

with separate power supply and return lines. With the values

shown, this filter can easily handle 100 mA of load current

without saturating the ferrite cores. Higher current capacity

can be achieved with larger ferrite cores. For lowest noise, all

electrolytic capacitors should be of the low equivalent series

resistance (ESR) type.

FERRITE BEADS:

2 TURNS

+5V

RETURN

TTL/CMOS

LOGIC

CIRCUITS

POWER SUPPLY

100µF

ELECT.

10-20µF

TANT.

0.1µF

CER.

Figure 25. Properly Filtering a 5 V Logic Supply Yields a High Quality

Analog Supply

To fit the AD5626 in an 8-lead package, only one ground

connection to the device is accommodated. The ground

connection of the DAC serves as the return path for supply

currents as well as the reference point for the digital input

thresholds. The ground connection also serves as the supply rail

for the internal voltage reference and the output amplifier.

Therefore, to minimize errors, connect the ground connection

of the AD5626 to a high quality analog ground, such as the one

previously described. Generous bypassing of the DACs supply

effectively reduces supply line induced errors. Local supply

bypassing consisting of a 10 μF tantalum electrolytic capacitor

in parallel with a 0.1 μF ceramic capacitor is recommended.

Connect the decoupling capacitors between the DAC supply

pin (Pin 1) and the analog ground (Pin 7).

Figure 26 shows how the ground and bypass connections

should be made to the AD5626.

CLR

SCLK

LDAC

SDIN

VOUT

VDD

GND

AD5626

10µF

0.1µF

TO ANALOG GROUND

Figure 26. Recommended Grounding and Bypassing Scheme for the AD5626

UNIPOLAR OUTPUT OPERATION

This is the basic mode of operation for the AD5626. As shown

in Figure 27, the AD5626 is designed to drive loads as low as

2 kΩ in parallel with 500 pF. The code table for this operation is

provided in Table 6.

CLR

SCLK

LDAC

SDIN

VOUT

VDD

GND

AD5626

10µF

2kΩ

500pF

0.1µF

0V ≤ VOUT ≤ 4.095V

Figure 27. Unipolar Output Operation

Table 6. Unipolar Code Table

Hexadecimal Number

in DAC Register

Decimal Number

in DAC Register

Analog Output

Voltage (V)

FFF

4095

4.095

801

2049

2.049

800

2048

2.048

7FF

2047

2.047

000

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