AD7714YN datasheet(19/40 Pages) AD

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AD7714

REV. C

–19–

CIRCUIT DESCRIPTION

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.

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

±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

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 sinc

3 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 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.

SCLK

MCLK IN

DGND

DVDD

SYNC

RESET

DRDY

MCLK OUT

POL

DIN

DOUT

AIN1

AGND

AIN2

AIN6

AIN3

AIN5

AIN4

REF IN(+)

STANDBY

REF IN(–)

AVDD

BUFFER

AD7714

0.1 F

10 F

ANALOG

GROUND

DIFFERENTIAL

ANALOG INPUT 3

DIFFERENTIAL

ANALOG INPUT 2

DIFFERENTIAL

ANALOG INPUT 1

DIGITAL

GROUND

0.1 F

10 F

VOUT

VIN

GND

AD780

ANALOG

+5V SUPPLY

ANALOG

+5V SUPPLY

DATA

READY

RECEIVE

(READ)

SERIAL

DATA

SERIAL

CLOCK

CRYSTAL OR

CERAMIC

RESONATOR

+5V

Figure 2. Basic Connection Diagram

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