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參數資料
| 型號： | AD7920BRM-REEL7 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | 250 kSPS, 10-/12-Bit ADCs in 6-Lead SC70 |
| 中文描述： | 1-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, SERIAL ACCESS, PDSO8 |
| 封裝： | MO-187AA, MSOP-8 |
| 文件頁數： | 11/20頁 |
| 文件大小： | 1091K |
| 代理商： | AD7920BRM-REEL7 |
REV. B
AD7910/AD7920
–11–
When the ADC starts a conversion (see Figure 5), SW2 opens
and SW1 moves to position B, causing the comparator to become
unbalanced. The control logic and charge redistribution 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.
Figure 6 shows the ADC transfer function.
CHARGE
REDISTRIBUTION
DAC
CONTROL
LOGIC
COMPARATOR
SW2
SAMPLING
CAPACITOR
CONVERSION
PHASE
SW1
A
B
AGND
V
DD
/2
V
IN
Figure 5. ADC Conversion Phase
ADC Transfer Function
The output coding of the AD7910/AD7920 is straight binary.
The designed code transitions occur at the successive integer
LSB values, i.e., 1 LSB, 2 LSBs, and so on. The LSB size is
V
DD
/4096 for the AD7920 and V
DD
/1024 for the AD7910. The
ideal transfer characteristic for the AD7910/AD7920 is shown
in Figure 6.
000...000
0V
A
ANALOG INPUT
111...111
111...110
000...001
000...010
111...000
011...111
1LSB
+V
DD
–1LSB
1LSB = V
DD
/1024 (AD7910)
1LSB = V
DD
/4096 (AD7920)
Figure 6. Transfer Characteristic
Typical Connection Diagram
Figure 7 shows a typical connection diagram for the AD7910/
AD7920. V
REF
is taken internally from V
DD
and, as such, V
DD
should be well decoupled. This provides an analog input range of
0 V to V
DD
. The conversion result is output in a 16-bit word with
four leading zeros followed by the MSB of the 12-bit or 10-bit
result. The 10-bit result from the AD7910 will be followed by two
trailing zeros.
Alternatively, because the supply current required by the AD7910/
AD7920 is so low, a precision reference can be used as the
supply source to the AD7910/AD7920. An REF19x voltage
reference (REF195 for 5 V or REF193 for 3 V) can be used to
supply the required voltage to the ADC (see Figure 7). This con-
figuration is especially useful if the power supply is quite noisy
or if the system supply voltages are at a value other than 5 V or
3 V (e.g., 15 V). The REF19x will output a steady voltage to the
AD7910/AD7920. If the low dropout REF193 is used, the current
it needs to supply to the AD7910/AD7920 is typically 1.2 mA.
When the ADC is converting at a rate of 250 kSPS the REF193
needs to supply a maximum of 1.4 mA to the AD7910/AD7920.
The load regulation of the REF193 is typically 10 ppm/mA
(REF193, V
S
= 5 V), which results in an error of 14 ppm (42
m
V)
for the 1.4 mA
drawn from it. This corresponds to a 0.057 LSB
error for the AD7920 with V
DD
= 3 V from the REF193 and a
0.014 LSB error for the AD7910. For applications where power
consumption is of concern, the power-down mode of the ADC
and the sleep mode of the REF19x reference should be used to
improve power performance. See the Modes of Operation section.
AD7910/
AD7920/
SCLK
SDATA
CS
V
IN
GND
0V TO V
INPUT
V
DD
C/ P
SERIAL
INTERFACE
0.1 F
1 F
TANT
REF193
1.2mA
680nF
10 F
0.1 F
3V
5V
SUPPLY
Figure 7. REF193 as Power Supply
Table I provides typical performance data with various references
used as a V
DD
source for a 100 kHz input tone at room temper-
ature under the same setup conditions.
Table I. AD7920 Typical Performance
for Various Voltage References IC
Reference
Tied to V
DD
AD780 @ 3 V
REF193
AD780 @ 2.5 V
REF192
REF43
AD7920 SNR
Performance (dB)
72.65
72.35
72.5
72.2
72.6
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