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AD7927 View Datasheet(PDF) - Unspecified

Part NameDescriptionManufacturer
AD7927 8-Channel, 200 kSPS, 12-Bit ADC with Sequencer in 20-Lead TSSOP ETC
Unspecified 
AD7927 Datasheet PDF : 20 Pages
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AD7927
ADC TRANSFER FUNCTION
The output coding of the AD7927 is either straight binary or
twos complement, depending on the status of the LSB in the
Control Register. The designed code transitions occur at suc-
cessive LSB values (i.e., 1 LSB, 2 LSBs, and so forth). The LSB
size is REFIN/4096 for the AD7927. The ideal transfer charac-
teristic for the AD7927 when straight binary coding is selected
is shown in Figure 8, and the ideal transfer characteristic for the
AD7927 when twos complement coding is selected is shown in
Figure 9.
111…111
111…110
111…000
011…111
000…010
000…001
000…000
0V 1 LSB
1LSB ؍VREF/4096
+VREF ؊ 1 LSB
ANALOG INPUT
NOTE: VREF IS EITHER REFIN OR 2 ؋ REFIN
Figure 8. Straight Binary Transfer Characteristic
011…111
011…110
000…001
000…000
111…111
100…010
100…001
100…000
1LSB ؍2 ؋ VREFր4096
–VREF ؉ 1LSB
+VREF ؊ 1LSB
VREF ؊ 1LSB
ANALOG INPUT
Figure 9. Twos Complement Transfer Characteristic with
REFIN ± REFIN Input Range
VREF
VDD
0.1F
REFINAVDD
VDRIVE
VDD
V
R4
AD7927
DSP/P
R3
TWOS
COMPLEMENT
R2
VIN0
DOUT
0V
V
VIN7
R1
+REFIN (= 2 ؋ REFIN)
011…111
R1 ؍R2 ؍R3 ؍R4
REFIN
000…000
(= 0V)
–REFIN
100…000
Figure 10. Handling Bipolar Signals
Handling Bipolar Input Signals
Figure 10 shows how useful the combination of the 2 ¥ REFIN
input range and the twos complement output coding scheme is
for handling bipolar input signals. If the bipolar input signal is
biased about REFIN and twos complement output coding is
selected, then REFIN becomes the zero code point, REFIN is
negative full scale and +REFIN becomes positive full scale, with
a dynamic range of 2 ¥ REFIN.
TYPICAL CONNECTION DIAGRAM
Figure 11 shows a typical connection diagram for the AD7927.
In this setup, the AGND pin is connected to the analog ground
plane of the system. In Figure 11, REFIN 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 AD7927 is con-
nected to a AVDD of 5 V, the serial interface is connected to a 3 V
microprocessor. The VDRIVE pin of the AD7927 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 one
leading zero, three address bits indicating which channel the
conversion result corresponds to, followed by the 12 bits of
conversion data. For applications where power consumption is
of concern, the power-down modes should be used between
conversions or bursts of several conversions to improve power
performance. (See the Modes of Operation section.)
0.1F 10F
5V
SUPPLY
SERIAL
INTERFACE
0V TO REFIN
VIN 0 AVDD
AD7927
VIN7
AGND
REFIN
SCLK
DOUT
CS
VDRIVE DIN
C/P
0.1F
2.5V
AD780
0.1F 10F
3V
SUPPLY
NOTE: ALL UNUSED INPUT CHANNELS SHOULD BE CONNECTED TO AGND
Figure 11. Typical Connection Diagram
REV. 0
–13–
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