Rev. E | Page 36 of 40
The AD7715 provides a low cost, high resolution analog-to-
digital function. Because the analog-to-digital function is
provided by a Σ-Δ architecture, it makes the part more immune
to noisy environments thus making the part ideal for use in
industrial and process control applications. It also provides a
programmable gain amplifier, a digital filter and calibration
options. Thus, it provides far more system level functionality
than off-the-shelf integrating ADCs without the disadvantage
of having to supply a high quality integrating capacitor. In
addition, using the AD7715 in a system allows the system
designer to achieve a much higher level of resolution because
noise performance of the AD7715 is significantly better than
that of the integrating ADCs.
The on-chip PGA allows the AD7715 to handle an analog input
voltage range as low as 10 mV full-scale with VREF = 1.25 V. The
differential inputs of the part allow this analog input range to
have an absolute value anywhere between AGND and AVDD
when the part is operated in unbuffered mode. It allows the user
to connect the transducer directly to the input of the AD7715.
The programmable gain front end on the AD7715 allows the
part to handle unipolar analog input ranges from 0 mV to
20 mV to 0 V to 2.5 V and bipolar inputs of ±20 mV to
±2.5 V. Because the part operates from a single supply, these
bipolar ranges are with respect to a biased-up differential input.
One typical application of the AD7715 is pressure measurement.
Figure 14 shows the AD7715 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 (that is, 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 is also used to generate the reference voltage for the
AD7715. Therefore, variations in the excitation voltage do not
introduce errors in the system. Choosing resistor values of 24 kΩ
and 15 kΩ as per the diagram give a 1.92 V reference voltage for
the AD7715 when the excitation voltage is 5 V.
Using the part with a programmed gain of 128 results in the
full-scale input span of the AD7715 being 15 mV which
corresponds with the output span from the transducer.
CHARGE BALANCING ADC
EXCITATION VOLTAGE = +5V
A = 1 TO 128
Figure 14. Pressure Measurement Using the AD7715