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| 型號(hào)： | AD7457BRT-REEL7 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | Low Power, Pseudo Differential, 100 kSPS 12-Bit ADC in an 8-Lead SOT-23 |
| 中文描述： | 1-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, SERIAL ACCESS, PDSO8 |
| 封裝： | MO-178BA, SOT-23, 8 PIN |
| 文件頁數(shù)： | 3/20頁 |
| 文件大小： | 532K |
| 代理商： | AD7457BRT-REEL7 |
AD7457
Rev. A | Page 11 of 20
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
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.
VIN+
VIN–
A
B
SW1
SW3
COMPARATOR
CONTROL
LOGIC
CAPACITIVE
DAC
CAPACITIVE
DAC
CS
VREF
SW2
B
A
03157-0-003
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 rebal-
anced, 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.
VIN+
VIN–
A
B
SW1
SW3
COMPARATOR
CONTROL
LOGIC
CAPACITIVE
DAC
CAPACITIVE
DAC
CS
VREF
SW2
B
A
03157-0-004
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.
000...00
0V
ADC
CODE
ANALOG INPUT
111...11
000...01
111...00
011...11
1LSB
VREF –1LSB
1LSB = VREF/4096
111...10
000...10
03157-0-005
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
connected 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 F tantalum capacitor in parallel with a 0.1 F
ceramic capacitor. The reference pin should be decoupled to
AGND with a capacitor of at least 0.33 F. 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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