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? V IN
log ?
?
? Noise Vp ? p
?
? =
log ? ?
? ?
? 0.449 × 10 Vp ? p ? = 13.4 bits
V OUT = ( T MJ × 5 mV/°C) + V REF
Circuit Note
The noise performance of the system is also important to the
accuracy of the circuit. Figure 3 shows a histogram of 1,000
measurement samples. This data was taken with the CN-0271
evaluation board connected to the EVAL-SDP-CB1Z System
Demonstration Platform (SDP-B) evaluation board. Details
section.
The measured peak-to-peak noise is approximately 6 LSBs
(1 LSB = 4.9 V ÷ 65536 = 74.8 μV), corresponding to
0.449 mV p-p and 13.4 bits of noise free resolution.
?
MAX
?
Noise Free Bits =
log(2)
? 4.9 V ?
? 3
log(2)
This shows that the converter does not decrease the noise
free resolution because the measured resolution of a fixed
thermocouple input voltage results in approximately the same
number of noise free bits as predicted by the theoretical output
500
450
400
350
300
250
200
150
100
CN-0271
COMMON VARIATIONS
To measure negative temperatures, apply a voltage at the
reference pin to offset the output voltage at 0°C. The output
voltage of the AD8495 is
The complete K type thermocouple range of ?200°C to +1250°C
can be measured by modifying the circuit to run on dual supplies.
When operating the AD8495 on a single supply, measurement
of temperatures less than ambient become nonlinear because
the output starts to saturate close to the supply rail. To maintain
accuracy at lower temperatures, use dual supplies or level-shift
the output by applying the appropriate offset voltage to the
reference pin.
The AD8494 is calibrated for J type thermocouples. Both the
AD8494 and AD8495 are optimized for reference junctions
between 0°C and 50°C.
The AD8496 (J type) and AD8497 (K type) are optimized for
reference junctions between 25°C and 100°C.
The circuit is proven to work with good stability and accuracy.
CIRCUIT EVALUATION AND TEST
This circuit uses the EVAL-CN0271-SDPZ circuit board and
the System Demonstration Platform (SDP-B) controller board
connectors, allowing for the quick setup and evaluation of the
performance of the circuit. The EVAL-CN0271-SDPZ contains
the circuit to be evaluated, as described in this note, and the
SDP-B controller board is used with the CN-0271 evaluation
software to capture the data from the EVAL-CN0271-SDPZ
circuit board.
Equipment Needed
The following equipment is needed:
50
?
A PC with a USB port and Windows? XP or Windows
0
4CC4
4CC5
4CC6 4CC7
ADC CODE IN HEX
4CC8
4CC9
?
Vista? (32-bit), or Windows? 7 (32-bit)
The EVAL-CN0271-SDPZ circuit evaluation board
Figure 3. Histogram of Codes for 1,000 Samples at 120 Hz
A complete design support package for this circuit note can be
found at www.analog.com/CN0271-DesignSupport.
?
?
?
The SDP-B controller board ( EVAL-SDP-CB1Z ) or the
The CN-0271 SDP evaluation software
The 6 V power supply (EVAL-CFTL-6V-PWRZ) or
equivalent dc power supply
Getting Started
Load the evaluation software by placing the CN-0271 evaluation
software CD in the CD drive of the PC. Using My Computer ,
locate the drive that contains the evaluation software.
Functional Block Diagram
See Figure 1 of this circuit note for the circuit block diagram and
the EVAL-CN0271-SDPZ-SCH-RevA.pdf file for the circuit
Rev. B | Page 3 of 5
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