AD7785 Analog Devices, AD7785 Datasheet - Page 29

• Manufacturer Part Number: AD7785
• Description: 3-Channel, Low Noise, Low Power, 20-Bit ?-? ADC with On-Chip In-Amp and Reference
• Manufacturer: Analog Devices

Specifications of AD7785

• Resolution (bits): 20bit
• # Chan: 3
• Sample Rate: n/a
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: ± (Vref/Gain)
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

APPLICATIONS INFORMATION
The AD7785 provides a low cost, high resolution analog-to-
digital function. Because the analog-to-digital function is
provided by a ∑-Δ architecture, the part is more immune to
noisy environments, making it ideal for use in sensor
measurement and industrial and process control applications.
TEMPERATURE MEASUREMENT USING A
THERMOCOUPLE
Figure 20 outlines a connection from a thermocouple to the
AD7785. In a thermocouple application, the voltage generated
by the thermocouple is measured with respect to an absolute
reference, so the internal reference is used for this conversion.
The cold junction measurement uses a ratiometric configuration,
so the reference is provided externally.
Because the signal from the thermocouple is small, the AD7785
is operated with the in-amp enabled to amplify the signal from
the thermocouple. As the input channel is buffered, large
THERMOCOUPLE
JUNCTION
R
R
C
AIN1(+)
AIN1(–)
R
REF
Figure 20. Thermocouple Measurement Using the AD7785
AIN2(+)
AIN2(–)
REFIN(+)
REFIN(–)
IOUT2
V
BIAS
GND
MUX
AV
AV
GND
Rev. 0 | Page 29 of 32
DD
DD
AV
DD
BUF
REFERENCE
decoupling capacitors can be placed on the front end to
eliminate any noise pickup that may be present in the
thermocouple leads. The AD7785 has a reduced common-
mode range with the in-amp enabled, so the bias voltage
generator provides a common-mode voltage so that the voltage
generated by the thermocouple is biased up to AV
The cold junction compensation is performed using a thermis-
tor. The on-chip excitation current supplies the thermistor. In
addition, the reference voltage for the cold junction
measurement is derived from a precision resistor in series with
the thermistor. This allows a ratiometric measure-ment 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).
BAND GAP
IN-AMP
INTERNAL
CLOCK
CLK
REFIN(+) REFIN(–)
ADC
Σ-Δ
GND
AD7785
INTERFACE
CONTROL
SERIAL
LOGIC
AND
DOUT/RDY
DIN
SCLK
CS
DV
DD
DD
AD7785
/2.