AD7705/AD7706
CIRCUIT DESCRIPTION
cycle contains the digital information. The programmable gain
function on the analog input is also incorporated in this Σ-Δ
modulator, with the input sampling frequency being modified
to provide higher gains. A sinc3, digital, low-pass filter processes
the output of the Σ-Δ modulator and updates the output register
at a rate determined by the first notch frequency of this filter.
The AD7705/AD7706 are Σ-Δ analog-to-digital converters (ADC)
with on-chip digital filtering, intended for the measurement of
wide, dynamic range, low frequency signals, such as those in
industrial-control or process-control applications. Each contains
a Σ-Δ (or charge-balancing) ADC, a calibration microcontroller
with on-chip static RAM, a clock oscillator, a digital filter, and a
bidirectional serial communication port. The parts consume only
320 μA of power supply current, making them ideal for battery-
powered or loop-powered instruments. These parts operate with
a supply voltage of 2.7 V to 3.3 V or 4.75 V to 5.25 V.
The output data can be read from the serial port randomly or
periodically at any rate up to the output register update rate.
The frequency of the first notch of the digital filter ranges from
50 Hz to 500 Hz; therefore, the programmable range for the
−3 dB frequency is 13.1 Hz to 131 Hz. With a master clock
frequency of 1 MHz, the programmable range for this first
notch frequency is 20 Hz to 200 Hz, giving a programmable
range for the −3 dB frequency of 5.24 Hz to 52.4 Hz.
The AD7705 contains two programmable-gain, fully differential
analog input channels, and the AD7706 contains three pseudo
differential analog input channels. The selectable gains on these
inputs are 1, 2, 4, 8, 16, 32, 64, and 128, allowing the parts to accept
unipolar signals of 0 mV to 20 mV and 0 V to 2.5 V, or bipolar
signals in the range of 20 mV to 2.5 V when the reference input
voltage equals 2.5 V. With a reference voltage of 1.225 V, the input
ranges are from 0 mV to 10 mV and 0 V to 1.225 V in unipolar
mode, and from 10 mV to 1.225 V in bipolar mode. Note that
the bipolar ranges are with respect to AIN(−) on the AD7705,
and with respect to COMMON on the AD7706, but not with
respect to GND.
The AD7705 basic connection diagram is shown in Figure 13.
It shows the AD7705 driven from an analog 5 V supply. An
AD780 or REF192 precision 2.5 V reference provides the reference
source for the part. On the digital side, the part is configured for
CS
3-wire operation with
tied to GND. A quartz crystal or ceramic
resonator provides the master clock source for the part. In most
cases, it is necessary to connect capacitors on the crystal or
resonator to ensure that it does not oscillate at overtones of its
fundamental operating frequency. The values of capacitors vary,
depending on the manufacturer’s specifications. The same setup
applies to the AD7706.
The input signal to the analog input is continuously sampled at a
rate determined by the frequency of the master clock, MCLK IN,
and the selected gain. A charge-balancing ADC (∑-Δ modulator)
converts the sampled signal into a digital pulse train whose duty
ANALOG
5V SUPPLY
10μF
0.1μF
V
AD7705
DD
DIFFERENTIAL
DATA READY
DRDY
AIN1(+)
AIN1(–)
ANALOG
INPUT
RECEIVE (READ)
SERIAL DATA
SERIAL CLOCK
DOUT
DIN
DIFFERENTIAL
ANALOG
AIN2(+)
AIN2(–)
INPUT
ANALOG 5V
SUPPLY
SCLK
GND
5V
RESET
CS
V
IN
V
REF IN(+)
OUT
10μF
0.1μF
AD780/
REF192
REF IN(–)
MCLK IN
CRYSTAL OR
CERAMIC
RESONATOR
GND
MCLK OUT
Figure 13. AD7705 Basic Connection Diagram
Rev. C | Page 21 of 44
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