AD7730LBR datasheet(40/52 Pages) AD

40 / 52 page

AD7730/AD7730L

–40–

REV. A

APPLICATIONS

The on-chip PGA allows the AD7730 to handle analog input

voltage ranges as low as 10 mV full scale. This allows the user to

connect a transducer directly to the input of the AD7730. The

AD7730 is primarily targeted for weigh-scale and load-cell

applications. The majority of the applications have a strain-

gage transducer whose resistance changes when subjected to

mechanical stress. Normally, the gages are configured in a

Wheatstone bridge arrangement. The strain gage is a passive

device and requires an excitation voltage (or in some cases a

current) to derive a voltage output. Two types of voltage excita-

tion can be provided for the bridge: dc excitation or ac excita-

tion. These are discussed in the following sections. While the

desire in most applications is to provide a single supply solution

(something that is aided by the AD7730’s single supply capabil-

ity), some applications provide a bipolar excitation voltage in

order to increase the output voltage from the bridge. In such

cases, the input voltage applied to the AD7730 can be slightly

negative with respect to ground. Figure 23 shows how to config-

ure the AD7730 to handle this type of input signal.

DC Excitation of Bridge

In dc-excitation applications, the excitation voltage provided for

the bridge is a fixed dc voltage. Connections between the AD7730

and the bridge are very straightforward in this type of applica-

tion as illustrated in Figure 23. The bridge configuration shown

is a six-lead configuration with separate return leads for the

reference lines. This allows a force/sense effect on the load cell

excitation voltage, eliminating voltage drops caused by the exci-

tation current flowing through the lead resistances. In applica-

tions where the lead lengths are short, a four-wire configuration

can be used with the excitation voltage and analog ground con-

nected local to the AD7730’s REF IN(+) and REF IN(–) termi-

nals. Illustrating a major advantage of the AD7730, the 5 V

excitation voltage for the bridge can be used directly as the refer-

ence voltage for the AD7730, eliminating the need for precision

matched resistors in generating a scaled-down reference.

The application is a ratiometric one with variations in the exci-

tation voltage being reflected in variations in the analog input

voltage and reference voltage of the AD7730. Because the

AD7730 is a truly ratiometric part, with the reference voltage

and excitation voltages equal, it is possible to evaluate its total

excitation voltage rejection. This is unlike other converters

which give a separate indication of the rejection of reference,

analog inputs and power supply. The combined (total) rejection

for the AD7730 when moving the excitation voltage (which was

also the power supply voltage) was better than 115 dB when

evaluated with a load cell simulator.

Drift considerations are a primary concern for load cell applica-

tions. It is recommended for these applications that the AD7730

is operated in CHOP mode to accrue the benefits of the excel-

lent drift performance of the part in CHOP mode. A common

source of unwanted drift effects are parasitic thermocouples.

Thermocouple effects are generated every time there is a junc-

tion of two dissimilar metals. All components in the signal path

should be chosen to minimize thermocouple effects. IC sockets

and link options should be avoided as much as possible. While

it is impossible to remove all thermocouple effects, attempts should

be made to equalize the thermocouples on each leg of the differen-

tial input to minimize the differential voltage generated.

Figure 23. Typical Connections for DC-Excited Bridge Application

SIGMA-

DELTA

MODULATOR

AVDD

DVDD

AD7730

6-BIT

DAC

SERIAL INTERFACE

AND CONTROL LOGIC

CLOCK

GENERATION

PROGRAMMABLE

DIGITAL

FILTER

SIGMA-DELTA A/D CONVERTER

BUFFER

PGA

AIN2(+)/D1

AIN2(–)/D0

ACX

STANDBY

SYNC

MCLK IN

MCLK OUT

SCLK

DIN

DOUT

RESET

RDY

POL

DGND

AGND

MUX

EXCITATION

CLOCK

CALIBRATION

MICROCONTROLLER

IN+

OUT–

IN–

OUT+

REF IN(+)

REF IN(–)

AIN1(+)

AIN1(–)

EXCITATION VOLTAGE = +5V

+/–

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