參數(shù)資料
型號: AD7707BRU
廠商: ANALOG DEVICES INC
元件分類: ADC
英文描述: 3 V/5 V, +-10 V Input Range, 1 mW 3-Channel 16-Bit, Sigma-Delta ADC
中文描述: 3-CH 16-BIT DELTA-SIGMA ADC, SERIAL ACCESS, PDSO20
封裝: LOW PROFILE, TSSOP-20
文件頁數(shù): 33/40頁
文件大?。?/td> 316K
代理商: AD7707BRU
REV. A
AD7707
–33–
APPLICATIONS
The AD7707 provides a low cost, high resolution analog-to-
digital function with two low level input channels and one high
level input channel. Because the analog-to-digital function is
provided by a sigma-delta architecture, it makes the part more
immune to noisy environments thus making the part ideal for
use in industrial and process control applications. It also provides
a programmable gain amplifier, a digital filter and calibration
options. Thus, it provides far more system level functionality
than off-the-shelf integrating ADCs without the disadvantage of
having to supply a high quality integrating capacitor. In addi-
tion, using the AD7707 in a system allows the system designer
to achieve a much higher level of resolution because noise per-
formance of the AD7707 is better than that of the integrating
ADCs.
The on-chip PGA allows the AD7707 to handle an analog input
voltage ranges as low as 10 mV full-scale with V
REF
= +1.25 V.
The pseudo-differential input capability of the low level channel
allows this analog input range to have an absolute value any-
where between AGND – 100 mV and AV
DD
+ 30 mV when the
part is operated in unbuffered mode. It allows the user to con-
nect the transducer directly to the input of the AD7707.
In addition, the three-wire digital interface on the AD7707
allows data acquisition front ends to be isolated with just three
wires. The AD7707 can be operated from a single 3 V or 5 V
and its low power operation ensures that very little power needs
to be brought across the isolation barrier in an isolated application.
Data Acquisition
Figure 24 shows a data acquisition system showing in which the
low level input channel is used to digitize signals from a thermo-
couple and the high level input channel converts process control
signals up to
±
10 V in amplitude. This application shows the
high level input channel being used to convert a number of
input signals provided through an external mux controlled by
the system microcontroller. Switching channels on the external
multiplexer is equivalent to providing a step change on the
AIN3 input. It takes 3 or 4 updates before the correct output
code corresponding to the new analog input appears at the
output. Therefore, when switching between channels on the
external mux the first three outputs should be ignored following
the channel change or the FSYNC bit in the setup register
should be used to reset the digital filter and ensure that the
DRDY
is set high until a valid result is available in the output
register.
Smart Valve/Actuator Control
Another area where the low power, single supply and high volt-
age input capability is of benefit is in smart valve and and actua-
tor control circuits. The AD7707 monitors the signal from the
control valve. The controller and the AD7707 form a closed-
loop control circuit. Figure 25 shows a block diagram of a smart
actual or control circuit which includes the AD7707. The AD7707
monitors the valve position via a high quality servo pot whose
output is
±
10 V.
Similar applications for the AD7707 would be in the area of
smart transmitters. Here, the entire smart transmitter must
operate from the 4 mA to 20 mA loop.
Tolerances in the loop mean that the amount of current avail-
able to power the transmitter is as low as 3.5 mA. The AD7707
consumes only 280
μ
A, leaving at least 3 mA available for the
rest of the transmitter. Figure 26 shows a block diagram of a
smart transmitter which includes the AD7707.
AD7707
DV
DD
+5V
AIN2
REF IN(–)
AGND
MCLK IN
MCLK OUT
CS
REF IN(+)
VBIAS
8.2k
V
CJC
AV
DD
AD590
+5V
AIN1
LOCOM
2.5V
AIN3
HICOM
DGND
IN1
IN2
IN3
IN4
IN5
250
V
OUT
250
V
IN6
VSS
A0
A1
A2
SCLK
DIN
DOUT
+15V
–15V
6
10V INPUT
0V TO 10V INPUT
6
5V INPUT
0V TO 5V INPUT
4-20mA
0-20mA
THERMOCOUPLE
JUNCTION
MICRO-
CONTROLLER
SCLK
PO
DOUT
DIN
P1
P2
P3
ANALOG
MULTIPLEXER
VDD
Figure 24. Data Acquisition System Using the AD7707
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