DAC8562

REV. A

–9–

APPLICATIONS SECTION

Power Supplies, Bypassing, and Grounding

All precision converter products require careful application of

good grounding practices to maintain full-rated performance.

Because the DAC8562 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 preva-

lent; therefore, special care must be taken to assure that its

inherent precision is maintained. This means that particularly

good engineering judgment should be exercised when address-

ing the power supply, grounding, and bypassing issues using the

DAC8562.

The power supply used for the DAC8562 should be well filtered

and regulated. The device has been completely characterized for

a +5 V supply with a tolerance of

±5%. Since a +5 V logic sup-

ply is almost universally available, it is not recommended to

connect the DAC directly to an unfiltered logic supply without

careful filtering. Because it is convenient, a designer might be

inclined to tap a logic circuit s supply for the DAC’s supply.

Unfortunately, this is not wise because fast logic with nanosec-

ond transition edges induces high current pulses. The high tran-

sient current pulses can generate glitches hundreds of millivolts

in amplitude due to wiring resistances and inductances. This

high frequency noise will corrupt the analog circuits internal to

the DAC and cause errors. Even though their spike noise is

lower in amplitude, directly tapping the output of a +5 V system

supplies can cause errors because these supplies are of the

switching regulator type that can and do generate a great deal of

high frequency noise. Therefore, the DAC and any associated

analog circuitry should be powered directly from the system

power supply outputs using appropriate filtering. Figure 28

illustrates how a clean, analog-grade supply can be generated

from a +5 V logic supply using a differential LC filter with sepa-

rate power supply and return lines. With the values shown, this

filter can easily handle 100 mA of load current without saturat-

ing the ferrite cores. Higher current capacity can be achieved

with larger ferrite cores. For lowest noise, all electrolytic capaci-

tors should be low ESR (Equivalent Series Resistance) type.

100µF

ELECT.

10-22µF

TANT.

0.1µF

CER.

TTL/CMOS

LOGIC

CIRCUITS

+5V

POWER SUPPLY

+5V

+5V

RETURN

FERRITE BEADS:

2 TURNS, FAIR-RITE

#2677006301

Figure 28. Properly Filtering a +5 V Logic Supply

Can Yield a High Quality Analog Supply

The DAC8562 includes two ground connections in order to

minimize system accuracy degradation arising from grounding

errors. The two ground pins are designated DGND (Pin 10)

and AGND (Pin 12). The DGND pin is the return for the digi-

tal circuit sections of the DAC and serves as their input thresh-

old reference point. Thus DGND should be connected to the

same ground as the circuitry that drives the digital inputs.

Pin 12, AGND, serves as the supply rail for the internal voltage

reference and the output amplifier. This pin should also serve as

the reference point for all analog circuitry associated with the

DAC8562. Therefore, to minimize any errors, it is recom-

mended that the AGND connection of the DAC8562 be con-

nected to a high quality analog ground. If the system contains

any analog signal path carrying a significant amount of current,

then that path should have its own return connection to Pin 12.

It is often advisable to maintain separate analog and digital

grounds throughout a complete system, tying them common to

one place only. If the common tie point is remote and an acci-

dental disconnection of that one common tie point were to

occur due to card removal with power on, a large differential

voltage between the two commons could develop. To protect

devices that interface to both digital and analog parts of the sys-

tem, such as the DAC8562, it is recommended that the com-

mon ground tie points be provided at each such device. If only

one system ground can be connected directly to the DAC8562,

it recommended that the analog common be used. If the

system’s AGND has suitably low impedance, then the digital

signal currents flowing in it should not seriously affect the

ground noise. The amount of digital noise introduced by con-

necting the two grounds together at the device will not adversely

affect system performance due to loss of digital noise immunity.

Generous bypassing of the DAC’s supply goes a long way in re-

ducing supply line-induced errors. Local supply bypassing con-

sisting of a 10

µF tantalum electrolytic in parallel with a 0.1 µF

ceramic is recommended. The decoupling capacitors should be

connected between the DAC’s supply pin (Pin 20) and the ana-

log ground (Pin 12). Figure 29 shows how the DGND, AGND,

and bypass connections should be made to the DAC8562.

15

16

DGND

AGND

DATA

13

DAC-8562

12

10µF

0.1µF

TO OTHER

ANALOG CIRCUITS

20

+5V

10

TO POWER GROUND

CE

CLR

Figure 29. Recommended Grounding and Bypassing

Scheme for the DAC-8562