AD7795 Analog Devices, AD7795 Datasheet - Page 35

• Manufacturer Part Number: AD7795
• Description: 6-Channel, Low Noise, Low Power, 16-Bit Sigma Delta ADC with On-Chip In-Amp and Reference
• Manufacturer: Analog Devices

Specifications of AD7795

• Resolution (bits): 16bit
• # Chan: 6
• 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

APPLICATIONS INFORMATION
The AD7794/AD7795 offer low cost, high resolution analog-to-
digital functions. Because the analog-to-digital function is
provided by a ∑-Δ architecture, it makes the parts more immune
to noisy environments, making them ideal for use in sensor
measurement, and industrial and process control applications.
FLOWMETER
Figure 24 shows the AD7794/AD7795 being used in a
flowmeter application that consists of two pressure transducers,
with the rate of flow being equal to the pressure difference. The
pressure transducers shown are the BP01 from Sensym. The
pressure transducers are arranged in a bridge network and give
a differential output voltage between its OUT+ and OUT–
terminals. With rated full-scale pressure (in this case
300 mmHg) on the transducer, the differential output voltage is
3 mV/V of the input voltage (that is, the voltage between the
IN(+) and IN(–) terminals).
Assuming a 5 V excitation voltage, the full-scale output range
from the transducer is 15 mV. The excitation voltage for the
bridge can be used to directly provide the reference for the
ADC, as the reference input range includes the supply voltage.
OUT–
IN+
IN–
OUT+
OUT–
IN+
IN–
V
DD
R
CM
OUT+
REFIN1(+)
AIN1(+)
AIN1(–)
AIN2(+)
AIN2(–)
AIN3(+)
AIN3(–)
REFIN2(+)
REFIN2(–)
IOUT1
REFIN1(–)
PSW
Figure 24. Typical Application (Flowmeter)
GND
Rev. D | Page 35 of 36
MUX
GND
AV
GND
V
DD
DD
BUF
V
A second advantage of using the AD7794/AD7795 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 bridges. In normal
operation, the switch is closed and measurements can be taken.
In applications where power is of concern, the AD7794/AD7795
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
transducers. When the parts are 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 AD7794/AD7795.
In the diagram, temperature compensation is performed using a
thermistor. The on-chip excitation current supplies the 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 so that variation of the
excitation current has no effect on the measurement (it is the
ratio of the precision reference resistance to the thermistor
resistance that is measured).
DD
IN-AMP
CLK
AD7794/AD7795
ADC
INTERNAL
Σ-Δ
CLOCK
INTERFACE
CONTROL
SERIAL
LOGIC
AND
AD7794/AD7795
DOUT/RDY
DIN
SCLK
CS
DV
DD