REV. 0

AD7398/AD7399

–12–

CIRCUIT OPERATION

The AD7398 and AD7399 contain four, 12-bit and 10-bit,

voltage-output, digital-to-analog converters respectively. Each

DAC has its own independent multiplying reference input. Both

AD7398/AD7399 use 3-wire SPI-compatible serial data interface,

with an asynchronous

RS pin for zero-scale reset. In addition, a

LDAC strobe enables four channel simultaneous updates for

hardware synchronized output voltage changes.

D/A Converter Section

GND

R

AD7398/AD7399

R

Figure 5. Simplified DAC Channel

DAC OPERATION

The internal R-2R ladder of the AD7398 and AD7399 operate

in the voltage switching mode maintaining an output voltage

that is the same polarity as the input reference voltage. A propri-

etary scaling technique is used to attenuate the input reference

voltage in the DAC. The output buffer amplifies the internal

The nominal DAC output voltage is determined by the exter-

VV

D

OUT

REF

=×

4096

(For AD7398)

(1)

VV

D

OUT

REF

=×

1024

(For AD7399)

(2)

Where D is the 12-bit or 10-bit decimal equivalent of the data

In order to maintain good analog performance, bypass power

supplies with 0.01

µF ceramic capacitors (mount them close to

the supply pins) and 1

µF–10 µF Tantalum capacitors in paral-

lel. In additions, clean power supplies with low ripple voltage

capability should be used. Switching power supplies may be

used for this application but beware of its higher ripple voltage

and PSS frequency-dependent characteristics. It is also best to

supply the AD7398/AD7399’s power from the system’s analog

supply voltages. (Don’t use the digital 5 V supply).

The reference input resistance is code dependent exhibiting

worst case 35 k

Ω for AD7398 when the DAC is loaded with

alternating codes 010101010101. Similarly, the reference input

resistance is 40 k

Ω for AD7399 when the DAC is loaded with

0101010101.

The AD7398/AD7399 is designed to approach the full output

to the same power supply. This power supply should be low

noise and low ripple, preferably operated by a suitable reference

voltage source such as ADR292 and REF02. The output swing

CLK

GND

CS

SDI

DAC

REGISTER

POWER

ON RESET

DAC

REGISTER

DAC

REGISTER

DAC

REGISTER

A

C D

LDAC

RS

12/10

B

DAC A

DAC B

DAC C

DAC D

ADDRESS

DECODE

SERIAL

REGISTER

INPUT

REGISTER

INPUT

REGISTER

INPUT

REGISTER

INPUT

REGISTER

4

AD7398/AD7399

Figure 4. Simplified Block Diagram