AD7398/AD7399
Rev. C | Page 15 of 24
OPERATION WITH V
REF
EQUAL TO THE SUPPLY
The AD7398/AD7399 are designed to approach the full output
voltage swing from ground to V
DD
or V
SS
. The maximum output
swing is achieved when the corresponding V
REF
input pin is tied
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 or REF02. The output swing is limited
by the internal buffer offset voltage and the output drive current
capability of the output stage. Users should at least budget the V
ZSE
offset voltage as the closest the output voltage can get to either
supply voltage under a no load condition. Under a loaded output,
degrade the headroom by a factor of 2 mV per 1 mA of load
current. Also note that the internal op amp has an offset voltage
so that the first eight codes of AD7398 may not respond at the
supply voltage or at ground until the internal DAC voltage
exceeds the offset voltage of the output buffers. Similarly, the first
two codes of AD7399 should not be used.
POWER SUPPLY SEQUENCING
V
DD
/V
SS
of AD7398/AD7399 should be powered from the system
analog supplies. The external reference input can be supplied from
the same supply to avoid a possible latch-up when the reference is
powered on prior to V
DD
/V
SS
, or powered off subsequent to
V
DD
/V
SS
. If V
DD
/V
SS
and V
REF
have separate power sources, ensure
the power-up sequence is GND, V
DD
, V
SS
, V
REF
/digital input/digital
output. The reverse sequence applies to the power-down sequence.
The order of V
REF
and digital input/digital output is not important.
In addition, V
REF
pins of the unused DACs should be connected to
GND or some other power sources to ensure a similar power-
up/power-down sequence.
PROGRAMMABLE POWER SHUTDOWN
The two MSBs of the serial input register, SA and SD, are used
to program various shutdown modes. If SA is set to Logic 1, all
DACs are placed in shutdown mode. If SA = 0 and SD = 1, a
corresponding DAC is shutdown addressed by Bit A0 and
Bit A1 (see the Input Registers section).
WORST CASE ACCURACY
Assuming a perfect reference, the worst-case output voltage can
be calculated from the following equation:
INLVVV
D
V
ZSEFSEREF
N
OUT
2
(3)
where:
D = decimal code loaded to DAC ranges 0 ≤ D ≤ 2
N
–1.
N = number of bits.
V
REF
= applied reference voltage.
V
FSE
= full-scale error in volts.
V
ZSE
= zero-scale error in volts.
INL = integral nonlinearity in volts. INL is 0 at full scale or zero
scale.
SERIAL DATA INTERFACE
The AD7398/AD7399 uses a 3-wire (
CS
, SDI, CLK) SPI-
compatible serial data interface. Serial data of the AD7398 and
AD7399 is clocked into the serial input register in a 16-bit and 14-
bit data-word format, respectively. MSBs are loaded first. The Input
Registers section defines the 16 data-word bits for AD7398 and the
14 data-word bits for the AD7399. Data is placed on the SDI pin,
and clocked into the register on the positive clock edge of CLK,
subject to the data setup and data hold time requirements specified
in the Specifications section. Data can only be clocked in while the
CS
chip select pin is active low. For the AD7398, only the last 16
bits clocked into the serial register are interrogated when the
CS
pin
returns to the logic high state, and extra data bits are ignored. For
the AD7399, only the last 14 bits clocked into the serial register are
interrogated when the
CS
pin returns to the logic high state.
Because most microcontrollers output serial data is in eight-bit
bytes, two right-justified data bytes can be written to the AD7398
and AD7399. Keeping the
CS
line low between the first and second
byte transfers results in a successful serial register update.
Once the data is properly aligned in the shift register, the positive
edge of the
CS
initiates the transfer of new data to the target DAC
register, determined by the decoding of Address Bit A1 and
Address Bit A0. For the AD7398, Table 5, Table 6, the Input
Registers section, Figure 3, and Figure 4 define the characteristics
of the serial interface. For the AD7399, Table 5, Table 6, the Input
Registers section, and Figure 4 (with a 14-bit exception) define the
characteristics of the serial interface. Figure 27 and Figure 28 show
the equivalent logic interface for the key digital control pins for
AD7398 and AD7399.
An asynchronous
RS
provides hardware control reset to zero-
code state over the preset function and DAC register loading. If
this function is not needed, the
RS
pin can be tied to logic high.
02179-027
EN
CLK
SDI
SHIFT
REGISTER
ADDRESS
DECODER
A
B
C
D
CS
TO INPUT REGISTER
Figure 27. Equivalent Logic Interface
