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參數資料
| 型號： | AD7719 |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | ADC |
| 英文描述： | Low Voltage, Low Power, Factory-Calibrated 16-/24-Bit Dual ADC |
| 中文描述： | 低電壓，低功耗，工廠校準，16-/24-Bit雙ADC |
| 文件頁數： | 37/40頁 |
| 文件大?。?/td> | 367K |
| 代理商： | AD7719 |
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REV. 0
AD7719
–37–
Temperature Measurement
The AD7719 is also useful in temperature measurement appli-
cations, Figure 21 shows an RTD temperature measurement
application. In this application, the transducer is an RTD (Re-
sistive Temperature Device), a PT100. The arrangement is a
4-lead RTD configuration. There are voltage drops across the
lead resistances RL1 and RL4, but these simply shift the com-
mon-mode voltage. There is no voltage drop across lead resistances
RL2 and RL3 as the input current to the AD7719 is very low,
looking into a high input impedance buffer. R
CM
is included to
shift the analog input voltage to ensure that it lies within the
common- mode range (AGND + 100 mV to AV
DD
–
100 mV) of
the ADC. In the application shown, the on-chip 200
μ
A cur-
rent source provides the excitation current for the PT100 and
also generates the reference voltage for the AD7719 via the
12.5 k
resistor. Variations in the excitation current do not
affect the circuit as both the input voltage and the reference
voltage vary ratiometrically with the excitation current. However,
the 12.5 k
resistor must have a low temperature coefficient to
avoid errors in the reference voltage over temperature.
REFIN(
–
)
IOUT1
5V
12.5k
AV
DD
AIN2
AIN1
AD7719
REFIN(+)
CONTROLLER
IOUT2
DV
DD
DGND
AGND
PWRGND
DRDY
SCLK
DIN
DOUT
CS
XTAL1
XTAL2
R
REF
RL1
RL2
RL3
RL4
R
CM
RTD
200 A
Figure 21. 4-Wire RTD Temperature Masurement Using
the AD7719
Figure 22 shows a further enhancement to the circuit shown in
Figure 21. Generally dc excitation has been accepted as the
normal method of exciting resistive-based sensors like RTD
’
s
(Resistance Temperature Detectors) in temperature measure-
ment applications.
With dc excitation, the excitation current through the sensor
must be large enough so that the smallest temperature/resistance
change to be measured results in a voltage change that is larger
than the system noise, offset, and drift of the system. The purpose
of switching the excitation source is to eliminate dc-induced
REFIN(
–
)
IOUT1
IOUT2
AV
DD
AIN2
AIN1
AIN3
AIN4
AD7719
REFIN(+)
MUX1
R
REF
A
A
BUF
&
PGA
200 A
I1
EMF1
RESISTIVE
TRANSDUCER
EMF2
Figure 22. Low Resistance Measurement AD7719
errors. DC errors (EMF1 and EMF2) due to parasitic thermo-
couples produced by differential metal connections (solder and
copper track) within the circuit are also eliminated when using
this switching arrangement. This excitation is a form of synchro-
nous detection where the sensor is excited with an alternating
excitation source and the ADC only measures information in
the same phase as the excitation source.
The switched polarity current source is developed using the on-
chip current sources and external phase control switches (A and
A
) driven from the controller. During the conversion process the
AD7719 takes two conversion results, one on each phase. During
Phase 1 the on-chip current source is directed to IOUT1 and
flows top to bottom through the sensor and switch controlled by
A
. In Phase 2, the current source is directed to IOUT2 and flows
in the opposite direction through the sensor and through switch
controlled by A. In all cases the current flows in the same direc-
tion through the reference resistor to develop the reference voltage
for the ADC. All measurements are ratiometrically derived. The
results of both conversions are combined within the microcon-
troller to produce one output measurement representing the
resistance or temperature of the transducer. For example, if the
RTD output during Phase 1 is 10 mV, a 1 mV circuit-induced
dc error exists due to parasitic thermocouples, the ADC mea-
sures 11 mV. During the second phase, the excitation current is
reversed and the ADC measures
–
10 mV from the RTD and
again sees 1 mV dc error, giving an ADC output of
–
9 mV
during this phase. These measurements are processed in the
controller (11 mV
–
(
–
9 mV)/2 = 10 mV) thus removing the
dc-induced errors within the system.
In the circuit shown in Figure 22, the resistance measurement is
made using ratiometric techniques. Resistor R
REF
, which devel-
ops the ADC reference, must be stable over temperature to
prevent reference-induced errors in the measurement output.
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