DAC8512

–9–

REV. A

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 DAC8512 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

DAC8512.

The power supply used for the DAC8512 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 induce 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

supply can cause errors because these supplies are of the switch-

ing 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 sup-

ply outputs using appropriate filtering. Figure 23 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 low

ESR (Equivalent Series Resistance) type.

100

µF

ELECT

.

10-22

µF

TANT.

0.1

µF

CER.

TTL/CMOS

LOGIC

CIRCUITS

+5V

POWER SUPPLY

+5V

+5V

RETURN

FERRITE BEADS:

2 TURNS, FAIR-RITE

#2677006301

Figure 23. Properly Filtering a +5 V Logic Supply Can Yield

a High Quality Analog Supply

In order to fit the DAC8512 in an 8-pin package, it was neces-

sary to use only one ground connection to the device. The

ground connection of the DAC serves as the return path for

supply currents as well as the reference point for the digital in-

put thresholds. The ground connection also serves as the supply

rail for the internal voltage reference and the output amplifier.

Therefore, to minimize any errors, it is recommended that

the ground connection of the DAC8512 be connected to a high

quality analog ground, such as the one described above. Gener-

ous bypassing of the DAC’s supply goes a long way in reducing

supply line-induced errors. Local supply bypassing consisting 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 1) and the analog ground

(Pin 7). Figure 24 shows how the ground and bypass connec-

tions should be made to the DAC8512.

6

2

GND

V

DD

8

DAC8512

10

µF

0.1

µF

V

OUT

1

+5V

TO ANALOG GROUND

CS

CLR

5

3

4

LD

SCLK

SDI

V

OUT

7

Figure 24. Recommended Grounding and Bypassing

Scheme for the DAC8512

Unipolar Output Operation

This is the basic mode of operation for the DAC8512. As shown

in Figure 24, the DAC8512 has been designed to drive loads as

low as 2 k

Ω in parallel with 500 pF. The code table for this op-

eration is shown in Table II.

10

µF

0.1

µF

0V

≤ V

+5V

2k

Ω

500pF

6

2

V

DD

8

DAC8512

1

CS

CLR

5

3

4

LD

SCLK

SDI

7

GND

V

OUT

Figure 25. Unipolar Output Operation

Table II. Unipolar Code Table

Hexadecimal Number

Decimal Number

Analog Output

in DAC Register

in DAC Register

Voltage (V)

FFF

4095

+4.095

801

2049

+2.049

800

2048

+2.048

7FF

2047

+2.047

000

0

0

Typical Performance Characteristics—