AD7457 Datasheet, PDF (11/20 Pages) Analog Devices – Low Power, Pseudo Differential, 100 kSPS 12-Bit ADC in an 8-Lead SOT-23

THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7457 is a 12-bit, low power, single supply, successive
approximation analog-to-digital converter (ADC) with a
pseudo differential analog input. It operates with a single 2.7 V
to 5.25 V power supply and is capable of throughput rates up to
100 kSPS. It requires an external reference to be applied to the
VREF pin.
The AD7457 has an on-chip differential track-and-hold
amplifier, a successive approximation (SAR) ADC, and a serial
interface, housed in an 8-lead SOT-23 package. The serial clock
input accesses data from the part and provides the clock source
for the successive approximation ADC. The AD7457 automati-
cally powers down after conversion, resulting in low power
consumption.
CONVERTER OPERATION
The AD7457 is a successive approximation ADC based around
two capacitive DACs. Figure 14 and Figure 15 show simplified
schematics of the ADC in the acquisition phase and the conver-
sion phase, respectively. The ADC is comprised of control logic,
a SAR, and two capacitive DACs. In Figure 14 (acquisition
phase), SW3 is closed, SW1 and SW2 are in Position A, the
comparator is held in a balanced condition, and the sampling
capacitor arrays acquire the differential signal on the input.
V_IN+
V_IN–
B
C_S
A SW1
A SW2
B
C_S
V_REF
CAPACITIVE
DAC
SW3
COMPARATOR
CONTROL
LOGIC
CAPACITIVE
DAC
Figure 14. ADC Acquisition Phase
When the ADC starts a conversion (Figure 15), SW3 opens, and
SW1 and SW2 move to Position B, causing the comparator to
become unbalanced. Both inputs are disconnected once the
conversion begins. The control logic and the charge redistribu-
tion DACs are used to add and subtract fixed amounts of charge
from the sampling capacitor arrays to bring the comparator
back into a balanced condition. When the comparator is
rebalanced, the conversion is complete. The control logic
generates the ADC’s output code. The output impedances of the
sources driving the VIN+ and the VIN– pins must be matched;
otherwise the two inputs have different settling times, resulting
in errors.
AD7457
V_IN+
V_IN–
B
C_S
A SW1
A SW2
B
V_REF
C_S
CAPACITIVE
DAC
SW3
COMPARATOR
CONTROL
LOGIC
CAPACITIVE
DAC
Figure 15. ADC Conversion Phase
ADC TRANSFER FUNCTION
The output coding for the AD7457 is straight (natural) binary.
The designed code transitions occur at successive LSB values
(1 LSB, 2 LSB, and so on). The LSB size is VREF/4096. The ideal
transfer characteristics of the AD7457 are shown in Figure 16.
111...11
111...10
1LSB = V_REF/4096
111...00
011...11
000...10
000...01
000...00
0V ^1LSB
V_REF –1LSB
ANALOG INPUT
Figure 16. Ideal Transfer Characteristics
TYPICAL CONNECTION DIAGRAM
Figure 17 shows a typical connection diagram for the AD7457.
In this setup, the GND pin is connected to the analog ground
plane of the system. The VREF pin is connected to the AD780, a
2.5 V decoupled reference source. The signal source is con-
nected to the VIN+ analog input via a unity gain buffer. A dc
voltage is connected to the VIN– pin to provide a pseudo ground
for the VIN+ input. The VDD pin should be decoupled to AGND
with a 10 µF tantalum capacitor in parallel with a 0.1 µF
ceramic capacitor. The reference pin should be decoupled to
AGND with a capacitor of at least 0.33 µF. The conversion result
is output in a 16-bit word with four leading zeros followed by
the MSB of the 12-bit result.
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