
REV. 0
AD7719
–36–
Pressure Measurement
One typical application of the AD7719 is pressure measure-
ment. Figure 19 shows the AD7719 used with a pressure
transducer, the BP01 from Sensym.
The pressure transducer is arranged in a bridge network and
gives a differential output voltage between its OUT(+) and
OUT(
–
) terminals. With rated full-scale pressure (in this case
300 mmHg) on the transducer, the differential output voltage is
3 mV/V of the input voltage (i.e., the voltage between its IN(+)
and IN(
–
) terminals).
Assuming a 5 V excitation voltage, the full-scale output range
from the transducer is 15 mV. The excitation voltage for the
bridge can be used to directly provide the reference for the ADC
as the reference input range includes the supplies. Alternatively,
a suitable resistor divider can be implemented that allows the
full dynamic range of the input to be utilized in these applica-
tion. This implementation is fully ratiometric, so variations in
the excitation voltage do not introduce errors in the system.
Choosing resistor values of 20 k
and 12 k
as per Figure 19
give a 1.875 V reference voltage for the AD7719 when the exci-
tation voltage is 5 V.
AD7719
IN+
OUT+
OUT
–
IN
–
20k
12k
EXCITATION VOLTAGE = 5V
AV
DD
DV
DD
AIN1
AIN2
REFIN1(+)
P1
PWRGND
DGND
AGND
REFIN2(
–
)
Figure 19. Pressure Measurement Using AD7719
Using the part with a programmed gain of 128 results in the
full-scale input span of the AD7719 being 15 mV, which corre-
sponds with the output span from the transducer.
A second key advantage to using the AD7719 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 of con-
cern, the AD7719 can be put in 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 cir-
cuitry is fully settled before attempting a read from the AD7719.
The circuit in Figure 20 shows a method that utilizes all three
pseudo-differential input channels on the AD7719 main channel
to temperature-compensate a pressure transducer.
5V
OUT(+)
OUT(
–
)
IN(
–
)
IN(+)
I1
I2
PRESSURE
BRIDGE
XTAL1
XTAL2
IOUT1
6.25k
AV
DD
REFIN(+)
REFIN(
–
)
AIN2
AIN1
AIN3
AIN4
AGND
AD7719
250
Figure 20. Temperature-Compensating a Pressure
Transducer
In this application, pseudo-differential input channel AIN1/AIN4
is used to measure the bridge output while pseudo-differential
channels AIN2/AIN4 and AIN3/AIN4 measure the voltage
across the bridge. The voltage measured across the bridge will
vary proportionally with temperature, and the delta in this volt-
age can be used to temperature-compensate the output of the
pressure bridge.