Datasheet

AD7798/AD7799 Data Sheet
Rev. B | Page 26 of 28
APPLICATIONS INFORMATION
The AD7798/AD7799 provide a low cost, high resolution
analog-to-digital function. Because the analog-to-digital
function is provided by a ∑-∆ architecture, the parts are more
immune to noisy environments, making them ideal for use in
sensor measurement and industrial and process-control
applications.
WEIGH SCALES
Figure 19 shows the AD7798/AD7799 being used in a weigh
scale application. The load cell is arranged in a bridge network
and gives a differential output voltage between its OUT+ and
OUT– terminals. Assuming a 5 V excitation voltage, the full-
scale output range from the transducer is 10 mV when the
sensitivity is 2 mV/V. The excitation voltage for the bridge can
be used to directly provide the reference for the ADC because
the reference input range includes the supply voltage.
A second advantage of using the AD7798/AD7799 in transducer-
based applications is that the low-side power switch can be fully
utilized in low power applications. The low-side power switch is
connected in series with the cold side of the bridge. In normal
operation, the switch is closed and measurements can be taken.
In applications where power is of concern, the AD7798/AD7799
can be placed in standby mode, thus significantly reducing the
power consumed in the application. In addition, the low-side
power switch can be opened while in standby mode, thus
avoiding unnecessary power consumption by the front-end
transducer. When the part is taken out of standby mode and the
low-side power switch is closed, the user should ensure that the
front end circuitry is fully settled before attempting a read from
the AD7798/AD7799.
In Figure 19, temperature compensation is performed using a
thermistor. In addition, the reference voltage for the temperature
measurement is derived from a precision resistor in series with
the thermistor. This allows a ratiometric measurement—that is,
the ratio of the precision reference resistance to the thermistor
resistance is measured; therefore, variations of the reference
voltage do not affect the measurement.
DOUT/RDY
DIN
SCLK
CS
DV
DD
SERIAL
INTERFACE
AND
CONTROL
LOGIC
Σ-Δ
ADC
AD7798/AD7799
AIN2(+)
AIN2(–)
AIN1(+)
AIN1(–)
AV
DD
GND
MUX
INTERNAL
CLOCK
GND AV
DD
REFIN(–)
REFIN(+)
PSW
IN-AMP
REFERENCE
DETECT
IN+
IN–
OUT– OUT+
AV
DD
04856-011
Figure 19. Weigh Scales Using the AD7798/AD7799