AD5300
–8–
REV.
GENERAL DESCRIPTION
D/A Section
The AD5300 DAC is fabricated on a CMOS process. The archi-
tecture consists of a string DAC followed by an output buffer
amplifier. Since there is no reference input pin, the power
supply (V
DD
) acts as the reference. Figure 20 shows a block
diagram of the DAC architecture.
V
DD
V
OUT
GND
RESISTOR
STRING
REF (+)
REF (–)
OUTPUT
AMPLIFIER
DAC REGISTER
Figure 20. DAC Architecture
Since the input coding to the DAC is straight binary, the ideal
output voltage is given by
V
OUT
=V
DD
×
D
256
where D = decimal equivalent of the binary code that is loaded
to the DAC register; D can range from 0 to 255.
Resistor String
The resistor string section is shown in Figure 21. It is simply a
string of resistors, each of value R. The code loaded to the
DAC register determines at which node on the string the voltage
is tapped off to be fed into the output amplifier. The voltage is
tapped off by closing one of the switches connecting the string
to the amplifier. Because it is a string of resistors, it is guaran-
teed monotonic.
R
R
TO OUTPUT
AMPLIFIER
R
R
R
Figure 21. Resistor String
Output Amplifier
The output buffer amplifier is capable of generating rail-to-rail
voltages on its output, which gives an output range of 0 V to
V
DD
. It is capable of driving a load of 2 kΩ in parallel with
1000 pF to GND. The source and sink capabilities of the output
amplifier can be seen in Figures 8 and 9. The slew rate is 1 V/µs
with a half-scale settling time of 4 µs with the output loaded.
SERIAL INTERFACE
The AD5300 has a 3-wire serial interface (SYNC, SCLK, and
DIN), which is compatible with SPI, QSPI, and MICROWIRE
interface standards as well as most DSPs. See Figure 1 for a
timing diagram of a typical write sequence.
The write sequence begins by bringing the SYNC line low. Data
from the DIN line is clocked into the 16-bit shift register on the
falling edge of SCLK. The serial clock frequency can be as high
as 30 MHz, making the AD5300 compatible with high speed
DSPs. On the 16th falling clock edge, the last data bit is clocked
in and the programmed function is executed (i.e., a change in
DAC register contents and/or a change in the mode of operation).
At this stage, the SYNC line may be kept low or be brought
high. In either case, it must be brought high for a minimum of
33 ns (V
DD
= 3.6 V to 5.5 V) or 50 ns (V
DD
= 2.7 V to 3.6 V)
before the next write sequence so that a falling edge of SYNC
can initiate the next write sequence. Since the SYNC buffer
draws more current when V
IN
= 2.4 V than it does when V
IN
=
0.8 V, SYNC should be idled low between write sequences for
even lower power operation of the part. As previously men-
tioned, however, it must be brought high again just before the
next write sequence.
Input Shift Register
The input shift register is 16 bits wide (see Figure 22). The first
two bits are Don’t Cares. The next two are control bits that
control which mode of operation the part is in (normal mode or
any one of three power-down modes). There is a more complete
description of the various modes in the Power-Down Modes
section. The next eight bits are the data bits. These are transferred
to the DAC register on the 16th falling edge of SCLK. Finally, the
last four bits are Don’t Cares.
DB0 (LSB)DB15 (MSB)
00NORMAL OPERATION
011k⍀ TO GND
10100k⍀ TO GND
11THREE-STATE
POWER-DOWN MODES
DATA BITS
XXPD1 PD0 D7 D6 D5D4D3D2D1D0 X X X X
Figure 22. Input Register Contents
