AD7799 Analog Devices, AD7799 Datasheet - Page 26

• Manufacturer Part Number: AD7799
• Description: 3-Channel, Low Noise, Low Power, 24-Bit, Sigma Delta ADC with On-Chip In-Amp
• Manufacturer: Analog Devices

Specifications of AD7799

• Resolution (bits): 24bit
• # Chan: 3
• Sample Rate: n/a
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref/PGA Gain) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

AD7798/AD7799
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 ∑-Δ 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
OUT–
IN+
IN–
AV
DD
OUT+
REFIN(+)
AIN1(+)
AIN1(–)
AIN2(+)
AIN2(–)
REFIN(–)
PSW
Figure 19. Weigh Scales Using the AD7798/AD7799
GND
MUX
AV
DD
AV
GND
Rev. A | Page 26 of 28
AD7798/AD7799
DD
IN-AMP
INTERNAL
CLOCK
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.
ADC
Σ-Δ
REFERENCE
INTERFACE
DETECT
CONTROL
SERIAL
LOGIC
AND
DOUT/RDY
DIN
SCLK
CS
DV
DD