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| 型号： | EVAL-AD7719EBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 31/40页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION FOR AD7719 |
| 标准包装： | 1 |
| ADC 的数量： | 2 |
| 位数： | 16，24 |
| 采样率（每秒）： | 105 |
| 数据接口： | 串行 |
| 输入范围： | 2 Vpp |
| 在以下条件下的电源（标准）： | 4.85mW @ 105SPS |
| 工作温度： | -40°C ~ 85°C |
| 已用 IC / 零件： | AD7719 |
| 已供物品： | 板 |
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REV. A
AD7719
–37–
REFIN(–)
IOUT1
5V
12.5k
AVDD
AIN2
AIN1
AD7719
REFIN(+)
CONTROLLER
IOUT2
DVDD
DGND
AGND
PWRGND
DRDY
SCLK
DIN
DOUT
CS
XTAL1
XTAL2
RREF
RL1
RL2
RL3
RL4
RCM
RTD
200 A
Figure 21. 4-Wire RTD Temperature Measurement
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 RTDs
(resistance temperature detectors) in temperature measurement
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
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.
REFIN(–)
IOUT1
IOUT2
AVDD
AIN2
AIN1
AIN3
AIN4
AD7719
REFIN(+)
MUX1
RREF
A
BUF
AND
PGA
200 A
I1
EMF1
RESISTIVE
TRANSDUCER
EMF2
Figure 22. Low Resistance Measurement AD7719
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 direction 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 microcontroller 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, and the ADC measures 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 mea-
surements 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 RREF, which devel-
ops the ADC reference, must be stable over temperature to
prevent reference-induced errors in the measurement output.
3-Wire RTD Configurations
To fully optimize a 3-wire RTD configuration, two identically
matched current sources are required. The AD7719, which
contains two well-matched current sources, is ideally suited to
these applications. One possible 3-wire configuration using the
AD7719 is outlined in Figure 23.
REFIN(–)
IOUT1
DGND
AGND
5V
12.5k
AVDD
AIN2
AIN1
AD7719
RL3
RCM
REFIN(+)
CONTROLLER
IOUT2
DVDD
DRDY
SCLK
DIN
DOUT
CS
XTAL1
XTAL2
RL2
RTD
200 A
RL1
Figure 23. 3-Wire RTD Configuration Using the AD7719
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