AD8401 View Datasheet(PDF) - Analog Devices
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AD8401 Datasheet PDF : 12 Pages
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AD8401
OPERATION
The AD8401 is a complete data acquisition and control system.
It contains the DAC, a four channel input multiplexer, a track/
hold, an ADC, as well as an internal bandgap reference. It inter-
faces to the microcontroller via an 8-bit digital I/O port.
D/A CONVERTER SECTION
The DAC is an 8-bit voltage mode DAC with an output that
swings from AGDAC to the 1.25 volt bandgap voltage. It uses an
R-2R ladder fed by PNP current sources which allow the output
to swing to ground so that the DAC operates in a unipolar mode.
AMPLIFIER SECTION
The DAC’s output is buffered by an internal high speed op
amp. The op amps output range is set at 0 V to 2.5 V. The op
amp has a 500 ns typical settling time to 0.2% for positive
slewing signals. There are differences in settling time for nega-
tive slewing signals. Signals going to zero volts will settle slightly
slower to ground than is seen in the positive direction.
VDD
20Ω
20Ω
n-CH
VOUT
AG DAC
Figure 3. Equivalent Amplifier Output Stage
Current sinking capability is also limited near zero volts in single
supply operation. Figure 3 provides an equivalent amplifier out-
put stage schematic.
INTERNAL REFERENCE
An on-chip bandgap is provided as a voltage reference to both
the DAC and the ADC. This reference is internal to the
AD8401 and is not accessible to the user. It is laser trimmed for
both absolute accuracy and temperature coefficients. The refer-
ence is internally buffered by a separate control amplifier for both
the DAC and ADC to improve isolation between the converters.
DIGITAL I/O
The 8-bit parallel data I/O port on the AD8401 provides access
to both the DAC and the ADC. This port is TTL/CMOS com-
patible with three-state outputs that are ESD protected.
The data format is binary. This data coding applies to both the
DAC and the ADC. See the applications information section.
ADC SECTION
A fast successive approximation ADC is used to attain a conver-
sion time of 2 microseconds. Start of conversion is initiated by
CS and RD. Following a Start command the BUSY signal will
become active and another Start command should not be given
until the conversion is complete.
The RESET (RS) input does not affect A/D conversion, but the
INT (Interrupt or conversion complete) which normally goes
active low at the end of a conversion will be forced high by
RESET asynchronously.
Figure 4 shows the wave forms for a conversion cycle. The track
and hold begins holding the input voltage VIN approximately
50 ns after the falling edge of the Start command. The MSB de-
cision is made approximately 50 ns after the second falling edge
of the CLK. If tX is greater than 50 ns, then the falling edge of
the CLK will be seen as the first falling clock edge. If tX is less
than 50 ns, the first MSB conversion will not occur until one
clock cycle later. The following bits will each be converted in a
similar manner 50 ns after each CLK edge until all eight bits
have been converted. After the end of conversion the contents of
the ADC SAR register are transferred to the output data latch,
the track and hold is returned to the track mode, INT goes low
and the SAR is reset.
CS , RD
OR ST
BUSY
VIN
50ns TYP
100ns
TYP
CLK
tx
50ns TYP
MSB DECISION
DB7
LSB DECISION
DB0
Figure 4. Operating Waveforms Using the External Clock
ANALOG INPUT
The analog inputs of the AD8401 are fed into resistor voltage
divider networks with a typical value of 8.5 kΩ. The amplifiers
driving these inputs must have an output resistance low enough
to drive these nodes without losing accuracy. Taps from the
voltage dividers are connected to the track and hold amplifier by
the multiplexer switches.
VINA
5kΩ
3.57kΩ
MUX
T/H
VIND
5kΩ
3.57kΩ
AGADC
Figure 5. Equivalent Analog Input Circuit
TRACK-AND-HOLD AMPLIFIER
Following the resistive divider at the input of the AD8401 is a
track-and-hold amplifier that captures input signals accurately
up to the 200 kHz Nyquist frequency of the ADC. To attain this
performance the T/H amplifier must have a much greater band-
width than the signal of interest. Because of this the user must
be careful to band limit the input signal to avoid aliasing high
frequency components and noise into the passband.
The track-and-hold amplifier is internally controlled by the Start
command and is not directly available to the user. After the
Start command signal the track-and-hold is placed into the hold
mode; it returns to the track mode after the conversion is
complete.
–6–
REV. 0
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