AD7476 データシートの表示（PDF） - Analog Devices
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AD7476 Datasheet PDF : 20 Pages
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AD7476/AD7477/AD7478
CIRCUIT INFORMATION
The AD7476/AD7477/AD7478 are, respectively, 12-bit, 10-bit,
and 8-bit, fast, micropower, single-supply ADCs. The parts can be
operated from a 2.35 V to 5.25 V supply. When operated from
either a 5 V supply or a 3 V supply, the AD7476/AD7477/AD7478
are capable of throughput rates of 1 MSPS when provided with
a 20 MHz clock.
The AD7476/AD7477/AD7478 provide the user with an on-chip,
track-and-hold ADC, and a serial interface housed in a tiny
6-lead SOT-23 package, which offers the user considerable
space saving advantages over alternative solutions. The serial
clock input accesses data from the part and also provides the
clock source for the successive-approximation ADC. The analog
input range is 0 V to VDD. An external reference is not required
for the ADC, nor is there a reference on-chip. The reference for
the AD7476/AD7477/AD7478 is derived from the power supply
and thus gives the widest dynamic input range.
The AD7476/AD7477/AD7478 also feature a power-down option
to save power between conversions. The power-down feature is
implemented across the standard serial interface as described in
the Modes of Operation section.
CONVERTER OPERATION
The AD7476/AD7477/AD7478 are successive-approximation
analog-to-digital converters based around a charge redistribution
DAC. Figures 2 and 3 show simplified schematics of the ADC.
Figure 2 shows the ADC during its acquisition phase. SW2 is
closed and SW1 is in position A, the comparator is held in a
balanced condition, and the sampling capacitor acquires the
signal on VIN.
ADC TRANSFER FUNCTION
The output coding of the AD7476/AD7477/AD7478 is straight
binary. For the AD7476/AD7477, designed code transitions
occur midway between successive integer LSB values (i.e.,
1/2 LSB, 3/2 LSB, and so on). The LSB size for the AD7476
is VDD/4096 and the LSB size for the AD7477 is VDD/1024. The
ideal transfer characteristic for the AD7476/AD7477 is shown
in Figure 4.
For the AD7478, designed code transitions occur midway between
successive integer LSB values (i.e., 1 LSB, 2 LSB, and so on).
The LSB size for the AD7478 is VDD/256. The ideal transfer
characteristic for the AD7478 is shown in Figure 5.
111 ... 111
111 ... 110
111 ... 000
011 ... 111
1LSB = VDD/4096 (AD7476)
1LSB = VDD/1024 (AD7477)
000 ... 010
000 ... 001
000 ... 000
0V
0.5LSB
؉VDD–1.5LSB
ANALOG INPUT
Figure 4. Transfer Characteristic for the AD7476/AD7477
CHARGE
REDISTRIBUTION
VIN
SAMPLING
A CAPACITOR
COMPARATOR
DAC
SW1
B
AGND
ACQUISITION
PHASE
SW2
CONTROL
LOGIC
VDD/2
Figure 2. ADC Acquisition Phase
When the ADC starts a conversion (see Figure 3), SW2 will open
and SW1 will move to Position B, causing the comparator to
become unbalanced. The Control Logic and the Charge Redistri-
bution DAC are used to add and subtract fixed amounts of charge
from the sampling capacitor to bring the comparator back into a
balanced condition. When the comparator is rebalanced, the
conversion is complete. The Control Logic generates the ADC
output code. Figures 4 and 5 show the ADC transfer function.
SAMPLING
VIN
A CAPACITOR
SW1
B
CONVERSION
PHASE
AGND
CHARGE
REDISTRIBUTION
DAC
COMPARATOR
SW2
CONTROL
LOGIC
VDD/2
Figure 3. ADC Conversion Phase
111 ... 111
111 ... 110
111 ... 000
011 ... 111
1LSB = VDD/256 (AD7478)
000 ... 010
000 ... 001
000 ... 000
0V
1LSB
؉VDD–1LSB
ANALOG INPUT
Figure 5. Transfer Characteristic for AD7478
TYPICAL CONNECTION DIAGRAM
Figure 6 shows a typical connection diagram for the AD7476/
AD7477/AD7478. VREF is taken internally from VDD and as
such, VDD should be well decoupled. This provides an analog
input range of 0 V to VDD. The conversion result is output in a
16-bit word with four leading zeros followed by the MSB of the
12-bit, 10-bit, or 8-bit result. The 10-bit result from the AD7477
will be followed by two trailing zeros. The 8-bit result from
the AD7478 will be followed by four trailing zeros.
–10–
REV. D
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