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參數資料
| 型號： | AD7918 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 8-Channel, 1 MSPS, 8-/10-/12-Bit ADCs with Sequencer in 20-Lead TSSOP |
| 中文描述： | 8通道，1 MSPS的，8-/10-/12-Bit與序列ADC的20引腳TSSOP封裝 |
| 文件頁數： | 17/24頁 |
| 文件大小： | 607K |
| 代理商： | AD7918 |
AD7908/AD7918/AD7928
REV. A
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum source
impedance will depend on the amount of total harmonic distortion
(THD) that can be tolerated. The THD will increase as the source
impedance increases, and performance will degrade. (See TPC 5.)
V
IN
C1
4pF
C2
30pF
R1
D1
D2
AV
DD
CONVERSION PHASE: SWITCH OPEN
TRACK PHASE: SWITCH CLOSED
Figure 7. Equivalent Analog Input Circuit
ADC TRANSFER FUNCTION
The output coding of the AD7908/AD7918/AD7928 is either
straight binary or twos complement, depending on the status of
the LSB in the Control Register. The designed code transitions
occur at successive LSB values (i.e., 1 LSB, 2 LSBs, and so on).
The LSB size is REF
IN
/256 for the AD7908, REF
IN
/1024 for the
AD7918, and REF
IN
/4096 for the AD7928. The ideal transfer
characteristic for the AD7908/AD7918/AD7928 when straight
binary coding is selected is shown in Figure 8, and the ideal
transfer characteristic for the AD7908/AD7918/AD7928 when
twos complement coding is selected is shown in Figure 9.
000…000
0V
ANALOG INPUT
111…111
111…110
111…000
011…111
000…001
000…010
1 LSB
+V
REF
1 LSB
1LSB V
REF
/256 AD7908
1LSB V
REF
/1024 AD7918
1LSB V
REF
/4096 AD7928
NOTE: V
REF
IS EITHER REF
IN
OR 2 REF
IN
Figure 8. Straight Binary Transfer Characteristic
–V
REF
1 LSB
A
ANALOG INPUT
+V
REF
1 LSB
1LSB 2 V
REF
256 AD7908
1LSB 2 V
REF
1024 AD7918
1LSB 2 V
REF
4096 AD7928
V
REF
1LSB
100…000
011…111
011…110
000…001
000…000
100…001
100…010
111…111
Figure 9. Twos Complement Transfer Characteristic with
REF
IN
± REF
IN
Input Range
Handling Bipolar Input Signals
Figure 10 shows how useful the combination of the 2 REF
IN
input range and the twos complement output coding scheme is
for handling bipolar input signals. If the bipolar input signal is
biased about REF
IN
and twos complement output coding is
selected, then REF
IN
becomes the zero code point,
–
REF
IN
is
negative full scale and +REF
IN
becomes positive full scale, with
a dynamic range of 2 REF
IN
.
TYPICAL CONNECTION DIAGRAM
Figure 11
shows a typical connection diagram for the AD7908/
AD7918/AD7928. In this setup, the AGND pin is connected to
the analog ground plane of the system. In Figure 11, REF
IN
is
connected to a decoupled 2.5 V supply from a reference source,
the AD780, to provide an analog input range of 0 V to 2.5 V
(if RANGE bit is 1) or 0 V to 5 V (if RANGE bit is 0). Although
the AD7908/AD7918/AD7928 is connected to a V
DD
of 5 V, the
serial interface is connected to a 3 V microprocessor. The V
DRIVE
pin of the AD7908/AD7918/AD7928 is connected to the same 3 V
supply of the microprocessor to allow a 3 V logic interface (see
the Digital Inputs section). The conversion result is output in a
16-bit word. This 16-bit data stream consists of a leading zero,
three address bits indicating which channel the conversion result
corresponds to, followed by the 12 bits of conversion data for
the AD7928 (10 bits of data for the AD7918 and 8 bits of data for
the AD7908, each followed by two and four trailing zeros, respec-
tively). For applications where power consumption is of
R3
R2
R4
REF
IN
V
IN
7
AD7908/
AD7918/
AD7928
DSP/ P
V
DD
0.1 F
V
AV
DD
V
DRIVE
DOUT
TWOS
COMPLEMENT
+REF
IN
REF
IN
–REF
IN
011…111
000…000
100…000
(= 0V)
(= 2 REF
IN
)
0V
V
R1
R1 R2 R3 R4
V
DD
V
REF
V
IN
0
Figure 10. Handling Bipolar Signals
–17–
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