Rev. B | Page 43 of 52
The AD7707 provides a low cost, high resolution analog-to-
digital function with two low level input channels and one high
level input channel. 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 AD7707 in a system allows the system designer to achieve
a much higher level of resolution because noise performance of
the AD7707 is better than that of the integrating ADCs.
The on-chip PGA allows the AD7707 to handle an analog input
voltage ranges as low as 10 mV full scale with VREF = 1.25 V.
The pseudo differential input capability of the low level channel
allows this analog input range to have an absolute value anywhere
between AGND − 100 mV and AVDD + 30 mV when the part is
operated in unbuffered mode. It allows the user to connect the
transducer directly to the input of the AD7707.
In addition, the 3-wire digital interface on the AD7707 allows
data acquisition front ends to be isolated with just three wires.
The AD7707 can be operated from a single 3 V or 5 V, and its
low power operation ensures that very little power needs to be
brought across the isolation barrier in an isolated application.
Figure 25 shows a data acquisition system in which the low level
input channel is used to digitize signals from a thermocouple
and the high level input channel converts process control signals
up to ±10 V in amplitude. This application shows the high level
input channel being used to convert a number of input signals
provided through an external mux controlled by the system
microcontroller. Switching channels on the external multiplexer
is equivalent to providing a step change on the AIN3 input. It takes
three or four updates before the correct output code correspond-
ing to the new analog input appears at the output. Therefore,
when switching between channels on the external mux, the first
three outputs should be ignored following the channel change,
or the FSYNC bit in the setup register should be used to reset
the digital filter, and ensure that the DRDY is set high until a
valid result is available in the output register.
SMART VALVE/ACTUATOR CONTROL
Another area where the low power, single supply and high
voltage input capability is of benefit is in smart valve and
actuator control circuits. The AD7707 monitors the signal from
the control valve. The controller and the AD7707 form a closed-
loop control circuit. Figure 26 shows a block diagram of a smart
actuator control circuit, which includes the AD7707. The AD7707
monitors the valve position via a high quality servo potentiometer
whose output is ±10 V.
Similar applications for the AD7707 include smart transmitters
(see Figure 27). Here, the entire smart transmitter must operate
from the 4 mA to 20 mA loop.
Tolerances in the loop mean that the amount of current availa-
ble to power the transmitter is as low as 3.5 mA. The AD7707
consumes only 280 μA, leaving at least 3 mA available for the
rest of the transmitter. Figure 27 shows a block diagram of a
smart transmitter, which includes the AD7707.
0V TO 5V INPUT
4mA TO 20mA
0mA TO 20mA
P1 P2 P3
0V TO 10V INPUT
Figure 25. Data Acquisition System Using the AD7707