Datasheet
AD7398/AD7399
Rev. C | Page 14 of 24
THEORY OF OPERATION
0
2179-025
V
REF
A
V
DD
V
REF
B
V
REF
C
V
REF
D
DAC A
DAC
REGISTER
INPUT
REGISTER
V
OUT
A
DAC B
DAC
REGISTER
INPUT
REGISTER
V
OUT
B
DAC C
DAC
REGISTER
INPUT
REGISTER
V
OUT
C
DAC D
DAC
REGISTER
INPUT
REGISTER
V
OUT
D
SERIAL
REGISTER
CLK
SDI
CS
AD7398/AD7399
POWER
ON RESET
V
SS
GNDRS LDAC
ADDRESS
DECODE
4
12/10
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.
02179-026
AD7398/AD7399
V
OUT
A
V
REF
GND V
SS
V
DD
R
R
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 V
REF
to V
OUT
gain of unity.
The nominal DAC output voltage is determined by the
externally applied V
REF
and the digital data (D) as
V
OUT
= V
REF
× D/4096 (For AD7398) (1)
V
OUT
= V
REF
× D/1024 (For AD7399) (2)
where:
D is the 12-bit or 10-bit decimal equivalent of the data word.
V
REF
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.
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