AD7398 查看數據表（PDF） - Analog Devices

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AD7398

Quad, Serial-Input 12-Bit/10-Bit DACs

Analog Devices 

AD7398/AD7399

THEORY OF OPERATION

VDD

VREFA VREFB VREFC VREFD

AD7398/AD7399

CS

CLK

SDI

4

ADDRESS

DECODE

SERIAL

REGISTER

12/10

POWER

ON RESET

INPUT

DAC

REGISTER REGISTER

DAC A

VOUTA

INPUT

DAC

REGISTER REGISTER

DAC B

VOUTB

INPUT

DAC

REGISTER REGISTER

DAC C

VOUTC

INPUT

DAC

REGISTER REGISTER

DAC D

VOUTD

RS

GND LDAC

VSS

Figure 25. Simplified Block Diagram

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

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

DAC has its own independent multiplying reference input. Both

the AD7398 and AD7399 use a 3-wire, SPI-compatible serial

data interface, with an asynchronous RS pin for zero-scale reset.

In addition, an LDAC strobe enables four-channel simultaneous

updates for hardware-synchronized output voltage changes.

VDD

AD7398/AD7399

VREF

VOUTA

R

R

GND

VSS

Figure 26. Simplified DAC Channel

DAC OPERATION

The internal R-2R ladder of the AD7398/AD7399 operates in

the voltage switching mode, maintaining an output voltage that

is the same polarity as the input reference voltage. A proprietary

scaling technique is used to attenuate the input reference voltage in

the DAC. The output buffer amplifies the internal DAC output to

achieve a VREF to VOUT gain of unity.

The nominal DAC output voltage is determined by the

externally applied VREF and the digital data (D) as

VOUT = VREF × D/4096 (For AD7398)

(1)

VOUT = VREF × D/1024 (For AD7399)

(2)

where:

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

VREF is the externally applied reference voltage.

In order to maintain good analog performance, the user should

bypass power supplies with 0.01 μF ceramic capacitors (mount

them close to the supply pins) and 1 μF to 10 μF tantalum

capacitors in parallel. In addition, clean power supplies with low

ripple voltage capability should be used. Switching power supplies

can be used for this application, but beware of its higher ripple

voltage and PSS frequency-dependent characteristics. It is also

best to supply power to the AD7398/AD7399 from the system’s

analog supply voltages. Do not use the digital 5 V supply.

The reference input resistance is code dependent, exhibiting

worst case 35 kΩ for AD7398 when the DAC is loaded with

alternating codes 010101010101. Similarly, the reference input

resistance is 40 kΩ for AD7399 when the DAC is loaded with

0101010101.

Rev. C | Page 14 of 24

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