Datasheet

REV. A
AD7709
–26–
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.
5V
OUT(+)
OUT(–)
IN(–)
IN(+)
I1
I2
PRESSURE
BRIDGE
XTAL1
XTAL2
IOUT1
6.25k
V
DD
REFIN(+)
REFIN(–)
AIN2
AIN1
AIN3
AINCOM
GND
AD7709
250
Figure 19. Temperature-Compensating 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.
Temperature Measurement
The AD7709 is also useful in temperature measurement appli-
cations. Figure 20 shows an RTD temperature measurement
application.
REFIN(–)
IOUT1
5V
6.25k
AIN2
AIN1
AD7709
REFIN(+)
IOUT2
V
DD
GND PWRGND
DRDY
SCLK
DIN
DOUT
CS
XTAL1
XTAL2
R
REF
RL1
RL2
RL3
RL4
R
CM
RTD
200A
CONTROLLER
Figure 20. 4-Wire RTD Temperature Measurement
Using the AD7709
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
CM
is included to shift the analog
input voltage to ensure that it lies within the common-mode
range (GND + 100 mV to V
DD
– 100 mV) of the ADC. In the
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.