AD7719BR Analog Devices Inc, AD7719BR Datasheet - Page 37

• Manufacturer Part Number: AD7719BR
• Description: IC ADC 16BIT 24BIT DUAL 28-SOIC
• Manufacturer: Analog Devices Inc

Specifications of AD7719BR

• Number Of Bits: 16/24
• Sampling Rate (per Second): 105
• Data Interface: DSP, MICROWIRE™, QSPI™, Serial, SPI™
• Number Of Converters: 2
• Power Dissipation (max): 4.5mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SOIC (0.300", 7.50mm Width)
• For Use With: EVAL-AD7719EB - BOARD EVAL FOR AD7719
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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.
REV. A
TRANSDUCER
RESISTIVE
Figure 21. 4-Wire RTD Temperature Measurement
Using the AD7719
Figure 22. Low Resistance Measurement AD7719
RL1
RL4
RL2
RL3
RTD
EMF2
EMF1
12.5k
R
R
CM
REF
A
R
REF
IOUT2
REFIN(+)
IOUT1
AIN1
DGND
REFIN(–)
AIN2
5V
AD7719
AGND
A
AV
REFIN(+)
REFIN(–)
IOUT2
DD
IOUT1
200 A
PWRGND
AIN2
AIN3
AIN1
AIN4
DV
XTAL1
XTAL2
DRDY
DOUT
SCLK
DIN
DD
CS
MUX1
AD7719
AV
CONTROLLER
DD
200 A
I1
BUF
AND
PGA
–37–
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 R
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.
Figure 23. 3-Wire RTD Configuration Using the AD7719
RL3
RL1
12.5k
RL2
RTD
R
CM
IOUT1
REFIN(+)
REFIN(–)
AIN1
AIN2
IOUT2
AGND
DGND
5V
200 A
200 A
AV
AD7719
DD
DV
XTAL1
XTAL2
DRDY
DOUT
SCLK
DD
DIN
CS
CONTROLLER
REF
AD7719
, which devel-