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ADR3440 Datasheet(PDF) 3 Page - Analog Devices

No. de pieza ADR3440
Descripción Electrónicos  Devices Connected
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ADR3440 Datasheet(HTML) 3 Page - Analog Devices

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Circuit Note
CN-0287
Rev. C | Page 3 of 9
reference resistor that is driven with the same current (i.e. a
ratiometric measurement). Minimizing the leakage current
through the current path is important for achieving high accuracy
because the excitation current is typically only a few hundred
microamps to prevent self heating.
For the industrial field applications both high performance as
well as protection against both high-voltage transient events and
dc over-voltage conditions are important design considerations.
How this Circuit Works
The circuit shown in Figure 1 is designed for precision
temperature measurement applications in the industrial field
environment and is optimized for flexibility, performance,
robustness, and cost. This circuit uses the AD7193, low noise,
24-bit sigma-delta ADC to ensure high resolution and linearity
for the entire circuit.
The AD5201, 33-position digital potentiometer, AD8603 op
amp, and ADG702 single channel switch constitute a simple
programmable current source and bias voltage buffer for the RTD
and thermocouple measurements. The ADG738 routes the
current source to the active RTD channel and allows wire
resistance compensation for the 3-W RTD configuration.
The ADT7310 digital SPI temperature sensor has ±0.8°C
maximum accuracy (+5 V supply) from −40°C to +105°C and is
used for cold-junction compensation for the thermocouple
measurement. The ADR3440 is a low noise and high accuracy
4.096 V reference connected to REFIN1(+)/REFIN1(−) of the
AD7193 for the thermocouple measurements.
Analog-to-Digital Converter
The AD7193 is a low noise, complete analog front end for high
precision measurement applications. It contains a low noise, 24-
bit sigma-delta (Σ-Δ) analog-to-digital converter (ADC). This
ADC achieves high resolution, low non-linearity, and low noise
performance as well as very high 50 Hz/60 Hz rejection. The
data output rate can be varied from 4.7 Hz (24 bits effective
resolution, Gain = 1), to 4.8 kHz (18.6 bits effective resolution,
Gain = 1). The on-chip low noise PGA amplifies the small
differential signal from the thermocouple or RTD with a gain
programmable from 1 up to 128, thereby allowing a direct
interface. The gain stage buffer has high input impedance and
limits the input leakage current to ± 3 nA maximum. The gain
of theAD7193 must be configured properly depending on the
temperature range and type of sensors. The on-chip multiplexer
allows four differential input channels to be shared with the
same ADC core, saving both space and cost.
Programmable Current Source for RTDs and Bias
Voltage Generator Circuit for Thermocouples
RTD measurements require a low noise current source that
drives the RTD and a reference resistor. Thermocouple
measurements, on the other hand, need a common-mode bias
voltage that shifts the small thermocouple voltage into the input
range of the AD7193. The circuit shown in Figure 2 meets both
requirements and utilizes the AD8603 a low noise CMOS rail-
to-rail input/output op amp with only 1 pA maximum input
bias current and 50 μV maximum offset voltage, combined with
the ADG702 single channel, CMOS low voltage 2 Ω SPST
switch, and the ADG738 eight-channel matrix switch.
Figure 2. External Programmable Current Source and Bias Voltage Generator
With the ADG738 opened and the ADG702 closed, the
AD8603 acts as a low noise, low output impedance unity-gain
buffer for the thermocouple application. The voltage from the
AD5201 digital potentiometer is buffered and is used for the
thermocouple common-mode voltage, usually 2.5 V, which is
one-half the supply voltage. The 33-position AD5201 digital
potentiometer is driven with the ADR3440 low drift (5 ppm/°C)
4.096 V reference for accuracy.
With the ADG738 closed and the ADG702 opened, the
AD8603 generates the RTD excitation current, IEXC = VW/RREF.
AD7193
REFIN2(–)
REFIN2(+)
AIN2
AIN1
D
S
ADG702
1kΩ
R3
C2
D
S1
ADG738
A
W
B
AD5201
+4.096V
AD8603
+5V
RTD
TC
VW
IEXC
IEXC =
VW
RREF
RREF


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