AD7714
CIRCUIT DESCRIPTION
modulator converts the sampled input signal into a digital pulse
train whose duty cycle contains the digital information. The
programmable gain function on the analog input is also
incorporated in this sigma-delta modulator with the input sam-
pling frequency of the modulator being modified to give the
higher gains. A sinc3 digital low-pass filter processes the output
of the sigma-delta modulator and updates the output register at
a rate determined by the first notch frequency of this filter. The
output data can be read from the serial port randomly or peri-
odically at any rate up to the output register update rate. The
first notch of this digital filter, its –3 dB frequency and its out-
put rate can be programmed via the filter high and filter low
registers. With a master clock frequency of 2.4576 MHz, the
programmable range for this first notch frequency and output
rate is from 4.8 Hz to 1.01 kHz giving a programmable range
for the –3 dB frequency of 1.26 Hz to 265 Hz.
The AD7714 is a sigma-delta A/D converter with on-chip digi-
tal filtering, intended for the measurement of wide dynamic
range, low frequency signals such as those in weigh-scale, pres-
sure transducer, industrial control or process control applica-
tions. It contains a sigma-delta (or charge-balancing) ADC, a
calibration microcontroller with on-chip static RAM, a clock
oscillator, a digital filter and a bidirectional serial communica-
tions port. The part consumes only 500 µA of power supply
current and features a standby mode which requires only 10 µA,
making it ideal for battery-powered or loop-powered instru-
ments. The part comes in two versions, the AD7714-5, which is
specified for operation from a nominal +5 V analog supply
(AVDD), and the AD7714-3, which is specified for operation
from a nominal +3.3 V analog supply. Both versions can be
operated with a digital supply (DVDD) voltage of either +3.3 V
or +5 V. AD7714Y grade parts operate with a nominal AVDD
of 3 V or 5 V and can be operated with a digital supply voltage
of either 3 V or 5 V.
2
The basic connection diagram for the part is shown in Figure 2.
This shows both the AVDD and DVDD pins of the AD7714 being
driven from the analog +3 V or +5 V supply. Some applications
will have AVDD and DVDD driven from separate supplies. In the
connection diagram shown, the AD7714’s analog inputs are
configured as three fully differential inputs. The part is set up
for unbuffered mode on the these analog inputs. An AD780,
precision +2.5 V reference, provides the reference source for the
part. On the digital side, the part is configured for three-wire
operation with CS tied to DGND. A quartz crystal or ceramic
resonator provides the master clock source for the part. It may
be 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 will vary depend-
ing on the manufacturer’s specifications.
The part contains three programmable-gain fully differential
analog input channels that can be reconfigured as five pseudo-
differential inputs. The gain range on all channels is from 1 to
128, allowing the part to accept unipolar signals of between
0 mV to +20 mV and 0 V to +2.5 V. In bipolar mode, the part
handles genuine bipolar signals of ±20 mV and quasi-bipolar
signals up to ±2.5 V when the reference input voltage equals
+2.5 V. With a reference voltage of +1.25 V, the input ranges
are from 0 mV to +10 mV to 0 V to +1.25 V in unipolar mode,
while in bipolar mode, the part handles genuine bipolar signals
of ±10 mV and quasi-bipolar signals up to ±1.25 V.
The part employs a sigma-delta conversion technique to realize
up to 24 bits of no missing codes performance. The sigma-delta
ANALOG
+5V SUPPLY
10F
0.1F
0.1F
AV
DV
DD
DD
DATA
READY
AIN1
AIN2
DIFFERENTIAL
ANALOG INPUT 1
DRDY
CS
AIN3
AIN4
DIFFERENTIAL
ANALOG INPUT 2
RECEIVE
(READ)
DOUT
DIN
SERIAL
DATA
SERIAL
CLOCK
AIN5
AIN6
DIFFERENTIAL
ANALOG INPUT 3
SCLK
AD7714
ANALOG
GROUND
AGND
+5V
RESET
ANALOG
+5V SUPPLY
SYNC
DIGITAL
GROUND
DGND
STANDBY
V
IN
POL
V
REF IN(+)
OUT
10F
0.1F
AD780
GND
MCLK IN
REF IN(–)
MCLK OUT
BUFFER
CRYSTAL OR
CERAMIC
RESONATOR
Figure 2. Basic Connection Diagram
REV. C
–19–
ADI [ ADI ]
