AD7705/AD7706
APPLICATIONS
The AD7705 provides a dual-channel, 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, thus making it ideal for use in industrial and
process-control applications. It also provides a programmable
gain amplifier, digital filter, and calibration options. Therefore,
it provides far more system level functionality than off-the-shelf
integrating ADCs, but without the disadvantage of needing to
supply a high quality integrating capacitor. In addition, using
the AD7705 in a system allows the designer to achieve a much
higher level of resolution, because noise performance of the
AD7705 is better than that of the integrating ADCs.
PRESSURE MEASUREMENT
One typical application of the AD7705 is pressure measure-
ment. Figure 25 shows the AD7705 used with a pressure
transducer, the BP01 from SenSym. The pressure transducer is
arranged in a bridge network and provides a differential output
voltage between it’s 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 it’s IN(+) and IN(−)
terminals). Assuming a 5 V excitation voltage, the full-scale
output from the transducer is 15 mV. The excitation voltage for
the bridge is also used to generate the reference voltage for the
AD7705. Therefore, variations in the excitation voltage do not
introduce errors in the system. Choosing resistor values of 24 kΩ
and 15 kΩ, as per Figure 25, results in a 1.92 V reference voltage
for the AD7705 when the excitation voltage is 5 V.
The on-chip PGA allows the AD7705 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 the absolute value of this
analog input range to be between GND and VDD when the
part is operated in unbuffered mode. It allows the user to
connect the transducer directly to the input of the AD7705.
The programmable-gain front end on the AD7705 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.
Using the part with a programmed gain of 128 results in the
full-scale input span of the AD7705 being 15 mV, which
corresponds with the output span from the transducer. The
second channel on the AD7705 can be used as an auxiliary
channel to measure a secondary variable, such as temperature,
as shown in Figure 25. This secondary channel can be used as a
means of adjusting the output of the primary channel, thus
removing temperature effects in the system.
EXCITATION VOLTAGE = 5V
IN+
5V
V
DD
OUT(+)
AIN1(+)
OUT(–)
AIN1(–)
IN–
MCLK IN
AD7705
24kΩ
AIN2(+)
AIN2(–)
THERMOCOUPLE
JUNCTION
MCLK OUT
REF IN(+)
REF IN(–)
RESET
DRDY
15kΩ
GND
DOUT DIN CS SCLK
Figure 25. Pressure Measurement Using the AD7705
Rev. C | Page 38 of 44
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