AD7476/AD7477/AD7478
Rev. F | Page 13 of 24
THEORY OF OPERATION
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.
Each AD7476/AD7477/AD7478 provides an on-chip, track-
and-hold ADC and a serial interface housed in a tiny 6-lead
SOT-23 package, which offers considerable space-saving
advantages. The serial clock input accesses data from the part
and 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 provides 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
CONVERTER OPERATION
The AD7476/AD7477/AD7478 are successive-approximation
analog-to-digital converters based around a charge redistribu-
of the ADC.
Figure 10 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.
0102
4-
0
10
COMPARATOR
SAMPLING
CAPACITOR
ACQUISITION
PHASE
A
B
AGND
SW1
SW2
VIN
VDD/2
CHARGE
REDISTRIBUTION
DAC
CONTROL
LOGIC
Figure 10. ADC Acquisition Phase
When the ADC starts a conversion (see
Figure 11), SW2 opens
and SW1 moves 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 rebal-
anced, the conversion is complete. The control logic generates
transfer function.
01
02
4-
01
1
COMPARATOR
SAMPLING
CAPACITOR
CONVERSION
PHASE
A
B
AGND
SW1
SW2
VIN
VDD/2
CHARGE
REDISTRIBUTION
DAC
CONTROL
LOGIC
Figure 11. ADC Conversion Phase
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, such as
LSB, 1 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
For the AD7478, designed code transitions occur midway
between successive integer LSB values, such as 1 LSB, 2 LSB,
and so on. The LSB size for the AD7478 is V/256. The ideal
transfer characteristic for the AD7478 is shown in
Figure 13.
0
102
4-
01
2
ANALOG INPUT
111 ... 111
0V
0.5LSB
+VDD – 1.5LSB
ADC
CO
DE
111 ... 110
111 ... 000
011 ... 111
000 ... 010
000 ... 001
000 ... 000
1LSB = VDD/4096 (AD7476)
1LSB = VDD/1024 (AD7477)
Figure 12. Transfer Characteristic for the AD7476/AD7477
010
24-
013
ANALOG INPUT
111 ... 111
0V
1LSB
+VDD – 1LSB
ADC
CO
DE
111 ... 110
111 ... 000
011 ... 111
000 ... 010
000 ... 001
000 ... 000
1LSB = VDD/256 (AD7478)
Figure 13. Transfer Characteristic for AD7478