Applications Information; Flowmeter - Analog Devices AD7194 Manual

APPLICATIONS INFORMATION

The AD7194 provides a low cost, high resolution analog-to-
digital function. Because the analog-to-digital function is
provided by a Σ-Δ architecture, the part is more immune to noisy
environments, making it ideal for use in sensor measurement and
industrial and process control applications.

FLOWMETER

Figure 67 shows the AD7194 being used in a flowmeter application
that consists of two pressure transducers with the rate of flow
being equal to the pressure difference. The pressure transducers
are arranged in a bridge network and give 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
(that is, the voltage between the 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 supply voltage.
IN+
OUT–
IN–
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5V
REFIN1(+)
IN+
AIN1
OUT+
OUT–
AIN2
IN–
OUT+
AIN5
AIN6
AIN7
AIN8
REFIN2(+)
R
REF
REFIN2(–)
REFIN1(–)
Figure 67. Typical Application (Flowmeter)
In Figure 67, temperature compensation is performed using
a thermistor. In addition, the reference voltage for the temperature
measurement is derived from a precision resistor in series with
the thermistor. This allows a ratiometric measurement so that
variation of the excitation voltage has no affect on the measurement
(it is the ratio of the precision reference resistance to the thermistor
resistance that is measured).
For simplicity, external filters are not shown in Figure 67;
however, an R-C antialias filter must be included on each analog
input. This is required because the on-chip digital filter does
not provide any rejection around the modulator sampling
frequency or multiples of this frequency. Suitable values are a
100 Ω resistor in series with each analog input, a 0.1 μF capacitor
between the analog input pins, and a 0.01 μF capacitor from
each analog input pin to AGND.
AGND AV
DD
AV
DD
MUX
PGA
AGND
AD7194
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DV DGND
DD
REFERENCE
DETECT
DOUT/RDY
SERIAL
INTERFACE
DIN
Σ-Δ ADC
AND
CONTROL SCLK
LOGIC
CS
SYNC
CLOCK
CIRCUITRY
MCLK1 MCLK2
AD7194