AD7719BRUZ Analog Devices Inc, AD7719BRUZ Datasheet - Page 36

• Manufacturer Part Number: AD7719BRUZ
• Description: Dual 16-Bit & 24-Bit SD ADC
• Manufacturer: Analog Devices Inc

Specifications of AD7719BRUZ

• 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-TSSOP (0.173", 4.40mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD7719EB - BOARD EVAL FOR AD7719
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

AD7719
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 kΩ and 12 kΩ as per Figure 19
give a 1.875 V reference voltage for the AD7719 when the exci-
tation voltage is 5 V.
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 the AD7719 is 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 AD7719.
Figure 19. Pressure Measurement Using AD7719
OUT–
EXCITATION VOLTAGE = 5V
IN–
IN+
12k
OUT+
20k
AIN1
AIN2
REFIN1(+)
REFIN2(–)
DGND
P1
AGND
AV
DD
AD7719
DV
PWRGND
DD
–36–
The circuit in Figure 20 shows a method that utilizes all three
pseudodifferential input channels on the AD7719 main channel
to temperature-compensate a pressure transducer.
In this application, pseudodifferential input channel AIN1/AIN4
is used to measure the bridge output while pseudodifferential
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.
Temperature Measurement
The AD7719 is also useful in temperature measurement applica-
tions; Figure 21 shows an RTD temperature measurement
application. In this application, the transducer is an RTD (resistive
temperature 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 as
the input current to the AD7719 is very low, looking into a high
input impedance buffer. R
voltage to ensure that it lies within the common-mode range
(AGND + 100 mV to AV
application shown, the on-chip 200 µA current source provides the
excitation current for the PT100 and also generates the reference
voltage for the AD7719 via the 12.5 kΩ resistor. Variations in
the excitation current do not affect the circuit as both the input
voltage and the reference voltage vary ratiometrically with the
excitation current. However, the 12.5 kΩ resistor must have a low
temperature coefficient to avoid errors in the reference voltage
over temperature.
Figure 20. Temperature-Compensating a Pressure
Transducer
PRESSURE
OUT(–)
BRIDGE
IN(+)
IN(–)
6.25k
250
OUT(+)
DD
CM
IOUT1
– 100 mV) of the ADC. In the
is included to shift the analog input
5V
AIN2
AIN1
AIN4
AIN3
REFIN(+)
REFIN(–)
AD7719
AV
DD
AGND
I2
I1
XTAL1
XTAL2
REV. A