參數(shù)資料
型號: AD7707EB
廠商: Analog Devices, Inc.
英文描述: 3 V/5 V 610 V Input Range 1 mW 3-Channel 16-Bit Sigma-Delta ADC(316.51 k)
中文描述: 3伏/ 5伏610 V輸入范圍1毫瓦3通道16位Σ-Δ模數(shù)轉(zhuǎn)換器(316.51十一)
文件頁數(shù): 36/40頁
文件大小: 316K
代理商: AD7707EB
REV. A
AD7707
–36–
RTD Measurement
Figure 29 shows another temperature measurement application
for the AD7707. In this case, the transducer is an RTD (Resis-
tive Temperature Device), a PT100. The arrangement is a 4-
lead RTD configuration. There are voltage drops across the lead
resistances R
L1
and R
L4
but these simply shift the common-
mode voltage. There is no voltage drop across lead resistances
R
L2
and R
L3
as the input current to the AD7707 is very low
.
The
lead resistances present a small source impedance so it would
not generally be necessary to turn on the buffer on the AD7707.
If the buffer is required, the common-mode voltage should be
set accordingly by inserting a small resistance between the bot-
tom end of the RTD and GND of the AD7707. In the applica-
tion shown, an external 400
μ
A current source provides the
excitation current for the PT100 and also generates the refer-
ence voltage for the AD7707 via the 6.25 k
resistor. Variations
in the excitation current do not affect the circuit as both the
input voltage and the reference voltage vary radiometrically with
the excitation current. However, the 6.25 k
resistor must have
a low temperature coefficient to avoid errors in the reference
voltage over temperature.
AD7707
DV
DD
+5V
REF IN(–)
AGND
MCLK IN
MCLK OUT
DRDY
RTD
AV
DD
AIN1
LOCOM
DGND
SCLK
DIN
DOUT
CS
CONTROLLER
R
L3
6.25k
V
R
L2
R
L4
R
L1
REF IN(+)
400
m
A
REF IN(+)
REF IN(–)
Figure 29. RTD Measurement Using the AD7707
Chart Recorders
Another area where both high and low level input channels are
usually found is in chart recorder applications. Circular chart
recorders generally have two requirements. The first which
would utilize the low level input channels of the AD7707 to
measure inputs from thermocouples, RTDs and pressure sen-
sors. The second requirement is to be able to measure dc input
voltage ranges up to
±
10 V. The high level input channel is
ideally suited to this measurement as there is no external signal
conditioning required to accommodate these high level input
signals.
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