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FIGURE 1. Thermocouple Application Using a MUX to Scale the Input Range of an Analog-to-Digital Converter in Order
to Increase Accuracy as well as Resolution.
3
4
1
1997 Burr-Brown Corporation
AB-110
Printed in U.S.A. February, 1997
APPLICATION BULLE TIN
Mailing Address: PO Box 11400, Tucson, AZ 85734 Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 Tel: (520) 746-1111
Telex: 066-6491 FAX (520) 889-1510 Product Info: (800) 548-6132 Internet: www.burr-brown.com/ FAXLine: (800) 548-6133
Temperature sensing can be one of the more challenging
physical entities to measure if a variety of ranges and levels
of accuracy are required by one electrical system. The circuit
in Figure 1 uses a low power, single-supply analog-to-digital
converter, ADS7816, configured with a thermocouple and
CJC sub-circuit to achieve the desired range of responsive-
ness as well as high accuracy. The thermocouple is inter-
faced with a 12-bit A/D converter that allows for an adjust-
able input range. This range is adjusted with the voltage
reference input. The range of the voltage reference input of
the ADS7816 A/D converter is from the supply voltage
(+5V typically) all the way down to 0.1V. The voltage
reference range mimics a gain stage for the 12-bit converter.
With this converter a suto-gain stage provides 17.6-bit
virtual resolution against a 5V scale. When this circuit is
properly calibrated for the desired temperature range, the
effective resolution can be increased by 50x over a system
with a standard 5V full-scale range.
Other converters in the same family include the ADS1286
and ADS7817 (see Table I). All three of these products have
the unique characteristics of having a variable input range,
while also being low power, low cost and available in 8-pin
packages (such as PDIP, SOIC and MSOP). This product
line, also known as the fly-spec converters, is only the
VOLTAGE REFERENCE SCALING TECHNIQUES
Increase the Accuracy of the Converter as well as Resolution
By Bonnie Baker
beginning of a new family of converters that take advantage
of the versatile reference function and compact architecture.
This application bulletin addresses the uniqueness of the
voltage reference function to the A/D conversion process.
ADS1286
ADS7816
ADS7817
Input
Configuration
Single-Ended
w/Sense Pin
Single-Ended
w/Sense Pin
Full
Differential
Maximum
Sampling Rate
20kHz
200kHz
200kHz
Voltage
Reference
Range
100mV to
Supply
100mV to
Supply
50mV to
Supply
Analog Input
Voltage Range
V
REF
V
REF
2V
REF
Supply Voltage
Range
4.5V - 5.25V
4.5V - 5.25V
4.75V - 5.25V
Minimum LSB
Size to 12 Bits
24.4
μ
V
24.4
μ
V
24.4
μ
V
Minimum
Achievable LSB
51.79
μ
V
51.79
μ
V
51.79
μ
V
TABLE I. Flyspec 12-Bit, Single Supply Converter Product
Line Capable of Achieving Very Small Actual
LSB Values.
ADS7816
μP
DCLOCK
D
OUT
CS/SHDN
A
0
A
1
U
3
U
4
U
1
U
2
E-Type Thermocouple
MUX
OPA237
0.3V
0.4V
0.2V
0.1V
+5V
R
2
59k
R
4
1k
R
3
1M
R
5
1k
100
C
5
0.1μF
C
4
0.1μF
C
3
0.1μF
R
6
1M
R
1
150k
D
1
TC
2
TC
1
TC
3
+5V
C
2
0.1μF
C
1
10μF
+5V
R
8
460k
R
9
1k
R
10
1k
R
11
1k
R
12
1k
V
REF
3-Wire
Interface
Constantan
Chromel
ISO
Thermal
Block
Copper