Download May 2000 Measure Resistances Easily, without Reference Resistor

DESIGN IDEAS
Measure Resistances Easily, without
Reference Resistor or Current Source
by Glen Brisebois
Measuring the resistance of a
device, for example a thermistor, usually requires biasing it with a precision
current source or combining it with
several other precision resistors in a
bridge. The circuit of Figure 1 shows
how to use the new LT1168 instrumentation amplifier to achieve a
precision resistance-to-voltage conversion as simply as possible.
Normally, the resistor across pins 1
and 8 is the gain-set resistor and the
voltage across pins 3 and 2 is the
variable to be measured. In this case,
however, the 1.25V reference establishes a fixed input voltage so that the
variable to be measured is now the
resistance. The equation for VOUT vs
RT is VOUT = 1.25V • 49.4kΩ/RT. Given
the limitation on output swing (with
the supply voltages shown), the smallest measurable resistance is about
4.5k. The highest resolvable resistance is limited to about 200M by the
300µV output offset voltage of the
LT1168. The 0.05% accuracy of the
LT1634 is not an issue here, because
it is subtracted at the Ref pin of the
LT1168 and only contributes to gain
error. Figure 2 shows output voltage
vs temperature for 10k and 100k (at
25°C) thermistors from two manufacturers. Thermistors are difficult to
linearize, so although the output is
still not linear with temperature, it
can, at least, be read directly by an
ADC and compared against a lookup
table. The circuit has good noise
immunity but does not tolerate
capacitance at pin 1 or 8 and so is not
ideal for resistive devices placed
remotely from the LT1168.
14
15V
12
+
8
1
RT
2
7
6
LT1168
–
4
5
REF
VOUT = 1.25 •
49.4kΩ
RT
–15V
LT16341.25
OUTPUT VOLTAGE (V)
3
THERMOMETRICS
DC95F103W
10
8
6
4
YSI #44006
2
22k
THERMOMETRICS
DC95G104Z
YSI #44011
0
–15V
–40 –20
Figure 1. Simple resistance-to-voltage converter
0
20 40 60 80
TEMPERATURE (°C)
100 120
Figure 2. Output voltage vs temperature for thermistors from
two manufacturers. Curves are approximations to aid design—
contact manufacturers for exact lookup tables:
YSI (800) 765-4974; Thermometrics (732) 287-2870.
http://www.linear-tech.com/ezone/zone.html
Articles, Design Ideas, Tips from the Lab…
36
Linear Technology Magazine • May 2000