REV. A
AD8300
–7–
Table I. Control Logic Truth Table
CS CLK CLR LD Serial Shift Register Function DAC Register Function
H X H H No Effect Latched
L L H H No Effect Latched
L H H H No Effect Latched
L ↑ H H Shift-Register-Data Advanced One Bit Latched
↑ L H H No Effect Latched
HX H ↓ No Effect Updated with Current Shift Register Contents
H X H L No Effect Transparent
H X L X No Effect Loaded with All Zeros
HX ↑ H No Effect Latched All Zeros
NOTES
1. ↑ = Positive Logic Transition; ↓ = Negative Logic Transition; X = Don’t Care.
2. Do not clock in serial data while LD is LOW.
3. Data loads MSB first.
OPERATION
The AD8300 is a complete ready to use 12-bit digital-to-analog
converter. Only one +3 V power supply is necessary for opera-
tion. It contains a 12-bit laser-trimmed digital-to-analog
converter, a curvature-corrected bandgap reference, rail-to-rail
output op amp, serial-input register, and DAC register. The
serial data interface consists of a serial-data-input (SDI) clock
(CLK), and load strobe pins (LD) with an active low CS strobe.
In addition an asynchronous CLR pin will set all DAC register
bits to zero causing the V
OUT
to become zero volts. This func-
tion is useful for power on reset or system failure recovery to a
known state.
D/A CONVERTER SECTION
The internal DAC is a 12-bit device with an output that swings
from GND potential to 0.4 volt generated from the internal band-
gap voltage, see Figure 20. It uses a laser-trimmed segmented
R-2R ladder which is switched by N-channel MOSFETs. The
output voltage of the DAC has a constant resistance indepen-
dent of digital input code. The DAC output is internally con-
nected to the rail-to-rail output op amp.
AMPLIFIER SECTION
The internal DAC’s output is buffered by a low power con-
sumption precision amplifier. This low power amplifier contains
a differential PNP pair input stage that provides low offset volt-
age and low noise, as well as the ability to amplify the zero-scale
DAC output voltages. The rail-to-rail amplifier is configured
with a gain of approximately five in order to set the 2.0475 volt
full-scale output (0.5 mV/LSB). See Figure 20 for an equivalent
circuit schematic of the analog section.
12-BIT DAC
R1
R2
V
OUT
2.047V
FS
1.2V
0.4V
0.4V
FS
BANDGAP
REF
Figure 20. Equivalent AD8300 Schematic of Analog Portion
The op amp has a 2 µs typical settling time to 0.4% of full scale.
There are slight differences in settling time for negative slewing
signals versus positive. Also negative transition settling time to
within the last 6 LSB of zero volts has an extended settling time.
See the oscilloscope photos in the typical performances section
of this data sheet.
OUTPUT SECTION
The rail-to-rail output stage of this amplifier has been designed
to provide precision performance while operating near either
power supply. Figure 21 shows an equivalent output schematic
of the rail-to-rail amplifier with its N-channel pull-down FETs
that will pull an output load directly to GND. The output
sourcing current is provided by a P-channel pull-up device that
can source current to GND terminated loads.
P-CH
N-CH
V
DD
V
OUT
AGND
Figure 21. Equivalent Analog Output Circuit
The rail-to-rail output stage achieves the minimum operating
supply voltage capability shown in Figure 2. The N-channel
output pull-down MOSFET shown in Figure 21 has a 35 Ω on
resistance which sets the sink current capability near ground. In
addition to resistive load driving capability, the amplifier has
also been carefully designed and characterized for up to 500 pF
capacitive load driving capability.
REFERENCE SECTION
The internal curvature-corrected bandgap voltage reference is
laser trimmed for both initial accuracy and low temperature
coefficient. Figure 18 provides a histogram of total output per-
formance of full-scale vs. temperature which is dominated by
the reference performance.
POWER SUPPLY
The very low power consumption of the AD8300 is a direct
result of a circuit design optimizing use of a CBCMOS process.
By using the low power characteristics of the CMOS for the
logic, and the low noise, tight matching of the complementary
bipolar transistors, good analog accuracy is achieved.
For power-consumption sensitive applications it is important to
note that the internal power consumption of the AD8300 is
strongly dependent on the actual logic input voltage levels
present on the SDI, CLK, CS, LD, and CLR pins. Since these
inputs are standard CMOS logic structures, they contribute
static power dissipation dependent on the actual driving logic
