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CN-0271
Circuit Note
4.9 V
? ? 6.6 × (32 nV/ Hz ) × 122.4 × 1.57 × 800 Hz ? ?
CIRCUIT DESCRIPTION
The thermocouple is a simple, widely used component for
measuring temperature. It consists of a junction of two dissimilar
metals. These metals are connected at one end to form the
=
? ?
log ? ?
log(2)
= 12.4 bits
measurement junction, also called the hot junction. The other
end of the thermocouple is connected to the metal lines that
lead to the measurement electronics. This connection forms a
second junction: the reference junction, also called the cold
junction. To derive the temperature at the measurement junction
(T MJ ), the user must know the differential voltage created by
the thermocouple. The user must also know the error voltage
generated by the temperature at the reference junction (T RJ ).
Compensating for the reference junction error voltage is called
cold junction compensation. The electronics must compensate
for any changes in temperature at the reference (cold) junction
so that the output voltage is an accurate representation of the
hot junction measurement.
The circuit uses the AD8495 thermocouple amplifier on a single
5 V supply. The output voltage of the AD8495 is calibrated for
5 mV/°C. On a single 5 V supply, the output is linear between
approximately 75 mV and 4.75 V, corresponding to a temperature
range of 15°C to 950°C. The output of the AD8495 drives the
noninverting input of the AD8476 unity-gain differential amplifier,
which converts the single-ended input to differential outputs for
driving the AD7790 16-bit, Σ-Δ ADC.
A low-pass differential and common-mode filter before the
input of the AD8495 prevents RF signals, which, if allowed to
reach the AD8495 , can be rectified and appear as temperature
fluctuations. The two 100 ? resistors and the 1 μF capacitor
form a differential filter with a cutoff frequency of 800 Hz. The
two 0.01 μF capacitors form common-mode filters with a cutoff
frequency of 160 kHz. A similar filter is used at the output of
the AD8476 differential amplifier before the signal is applied
to the AD7790 ADC.
The AD8495 inputs are protected from input voltage excursions
up to 25 V from the opposite supply rail. For example, in this
The AD8476 is a very low power, fully differential precision
amplifier with integrated thin film, laser trimmed 10 k? gain
resistors for unity gain. It is an ideal choice for this application
because it presents a relatively high impedance load to the
The AD7790 is a low power, complete analog front end for low
frequency measurement applications. It contains a low noise,
16-bit, Σ-Δ ADC with one differential input that can be buffered or
unbuffered.
Test Results
An important measure of the performance of the circuit is the
amount of linearity error. The AD8495 output is accurate to
within 2°C from ?25°C to +400°C. To achieve even greater
accuracy when operating at or outside of this range, a linearity
correction algorithm must be implemented in software. The
CN-0271 evaluation software uses NIST thermoelectric voltage
lookup tables to achieve an output error within 1°C from 15°C
to 950°C.
CN-0271 system, and the result of applying the linearization
correction to the ADC output. For details on how the algorithm
was implemented in the software, see the AN-1087 Application
AD8495/AD8496/AD8497 .
2.0
1.5
1.0
0.5
0
circuit, with a 5 V positive supply rail and the negative supply
rail tied to GND, the part can safely withstand voltages at the
inputs from ?20 V to +25 V. Voltages at the reference and sense
–0.5
–1.0
AD8495
CN-0271
CN-0271 WITH
NONLINEARITY CORRECTION
pins should not go beyond 0.3 V of the supply rails. This feature
is of particular importance in applications with power supply
sequencing issues that can cause the signal source to be active
before the supplies to the amplifier are applied.
The theoretical resolution of the system can be calculated from
–1.5
–2.0
JUNCTION TEMPERATURE (°C)
Figure 2. Output Error of AD8495 , Total CN-0271 Circuit Error, and
Total CN-0271 Circuit Error with Thermocouple Nonlinearity Correction
V OUT MAX
The peak-to-peak (noise free code) resolution in bits is
Noise Free Bits =
? ?
log ? ?
? ? 6.6 × Voltage Noise Density × Gain × 1 . 57 × Bandwidth ? ?
log( 2 )
Rev. B | Page 2 of 5
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