EVAL-AD7709EB Analog Devices Inc, EVAL-AD7709EB Datasheet - Page 26

• Manufacturer Part Number: EVAL-AD7709EB
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD7709EB

• Lead Free Status / Rohs Status: Compliant

AD7709
A second key advantage to using the AD7709 in transducer-based
applications is that the on-chip low-side power switch can be fully
utilized in low power applications. The low-side power switch is
connected in series with the cold side of the bridge. In normal
operation, the switch is closed and measurements can be taken
from the bridge. In applications where power is a concern, the
AD7709 can be put into low power mode, substantially reducing
the power burned in the application. In addition to this, the power
switch can be opened while in low power mode, thus avoiding
the unnecessary burning of power in the front end transducer.
When taken back out of power-down, and the power switch is
closed, the user should ensure that the front end circuitry is fully
settled before attempting a read from the AD7709.
The circuit in Figure 19 shows a method that utilizes three
pseudo-differential input channels on the AD7709 to temperature-
compensate a pressure transducer.
In this application, pseudo-differential input channel AIN1/
AINCOM is used to measure the bridge output while pseuo-
differential channels AIN2/AINCOM and AIN3/AINCOM
measure the voltage across the bridge. The voltage measured
across the bridge will vary proportionally with temperature,
and the delta in this voltage can be used to temperature-
compensate the output of the pressure bridge.
PRESSURE
Figure 19. Temperature-Compensating a Pressure
Transducer
OUT(–)
BRIDGE
IN(+)
IN(–)
6.25k
250
OUT(+)
IOUT1
5V
AIN2
AIN1
AIN3
REFIN(+)
REFIN(–)
AINCOM
AD7709
V
DD
GND
I2
I1
XTAL1
XTAL2
–26–
Temperature Measurement
The AD7709 is also useful in temperature measurement appli-
cations. Figure 20 shows an RTD temperature measurement
application.
In this application, the transducer is an RTD (Resistive Tem-
perature 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 common-mode voltage.
There is no voltage drop across lead resistances RL2 and RL3
since the input current to the AD7709 is very low, looking into a
high input impedance buffer. R
input voltage to ensure that it lies within the common-mode
range (GND + 100 mV to V
application shown, the on-chip 200 mA current source provides
the excitation current for the PT100 and also generates the reference
voltage for the AD7709 via the 6.25 kW resistor. Variations in
the excitation current do not affect the circuit since both the
input voltage and the reference voltage vary ratiometrically with the
excitation current. However, the 6.25 kW resistor must have a low
temperature coefficient to avoid errors in the reference voltage
over temperature.
Figure 20. 4-Wire RTD Temperature Measurement
Using the AD7709
RL1
RL4
RL2
RL3
RTD
6.25k
R
R
CM
REF
REFIN(–)
REFIN(+)
IOUT1
IOUT2
AIN1
AIN2
AD7709
GND
DD
5V
CM
– 100 mV) of the ADC. In the
200 A
PWRGND
is included to shift the analog
XTAL1
XTAL2
DRDY
DOUT
SCLK
V
DIN
DD
CS
CONTROLLER
REV. A