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參數資料
| 型號： | AD7713AN |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | LC2MOS Loop-Powered Signal Conditioning ADC |
| 中文描述： | 3-CH 24-BIT DELTA-SIGMA ADC, SERIAL ACCESS, PDIP24 |
| 封裝： | 0.300 INCH, PLASTIC, DIP-24 |
| 文件頁數： | 25/28頁 |
| 文件大小： | 516K |
| 代理商： | AD7713AN |
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2
–25–
REV. C
AD7713
APPLICATIONS
Four-Wire RTD Configurations
Figure 20 shows a four-wire RTD application where the RTD
transducer is interfaced directly to the AD7713. In the four-wire
configuration, there are no errors associated with lead resis-
tances as no current flows in the measurement leads connected
to AIN1(+) and AIN1(–). One of the RTD current sources is
used to provide the excitation current for the RTD. A common
nominal resistance value for the RTD is 100
and, therefore,
the RTD will generate a 20 mV signal which can be handled di-
rectly by the analog input of the AD7713. In the circuit shown,
the second RTD excitation current is used to generate the refer-
ence voltage for the AD7713. This reference voltage is devel-
oped across R
REF
and applied to the differential reference
inputs. For the nominal reference voltage of +2.5 V, R
REF
is
12.5 k
. This scheme ensures that the analog input voltage span
remains ratiometric to the reference voltage. Any errors in the
analog input voltage due to the temperature drift of the RTD
current source is compensated for by the variation in the refer-
ence voltage. The typical matching between the two RTD cur-
rent sources is less than 3 ppm/
°
C.
AIN1(+)
AIN1(–)
AV
DD
AGND
DGND
A = 1 – 128
AD7713
RTD1
RTD
REF IN(+)
REF IN(–)
RTD2
200μA
+5V
R
REF
INTERNAL
CIRCUITRY
DV
DD
200μA
PGA
Figure 20. Four-Wire RTD Application with the AD7713
Three-Wire RTD Configurations
Figure 21 shows a three-wire RTD configuration using the
AD7713. In the three-wire configuration, the lead resistances
will result in errors if only one current source is used as the
200
μ
A will flow through R
L1
developing a voltage error between
AIN1(+) and AIN1(–). In the scheme outlined below, the sec-
ond RTD current source is used to compensate for the error in-
troduced by the 200
μ
A flowing through R
L1
. The second RTD
current flows through R
L2
. Assuming R
L1
and R
L2
are equal (the
leads would normally be of the same material and of equal
length) and RTD1 and RTD2 match, then the error voltage
across R
L2
equals the error voltage across R
L1
and no error volt-
age is developed between AIN1(+) and AIN1(–). Twice the
voltage is developed across R
L3
but since this is a common-mode
voltage it will not introduce any errors. The reference voltage is
derived from one of the current sources. This gives all the bene-
fits of eliminating RTD tempco errors as outlined in Figure 20.
The voltage on either RTD input can go to within 2 V of the
AV
DD
supply. The circuit is shown for a +2.5 V reference.
AIN(+)
AIN(–)
AV
DD
DV
DD
AGND
DGND
A = 1 – 128
AD7713
RTD1
RTD2
RTD
12.5k
INTERNAL
CIRCUITRY
REF IN(+)
REF IN(–)
200μA
PGA
R
L1
R
L2
R
L3
200μA
Figure 21. Three-Wire RTD Application with the AD7713
4–20 mA Loop
The AD7713’s high level input can be used to measure the cur-
rent in 4–20 mA loop applications as shown in Figure 22. In this
case, the system calibration capabilities of the AD7713 can be
used to remove the offset caused by the 4 mA flowing through
the 500
resistor. The AD7713 can handle an input span as
low as 3.2
×
V
REF
(= 8 V with a V
REF
of +2.5 V) even though the
nominal input voltage range for the input is 10 V. Therefore, the
full span of the A/D converter can be used for measuring the
current between 4 mA and 20 mA.
REF IN(+)
AIN1(+)
AIN1(–)
AIN3
AGND
DGND
A = 1 – 128
1μA
REF IN(–)
AD7713
4–20mA
LOOP
ANALOG +5V SUPPLY
500
VOLTAGE
ATTENUATION
AV
DD
AV
DD
DV
DD
INTERNAL
CIRCUITRY
M
U
X
PGA
Figure 22. 4–20 mA Measurement Using the AD7713
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