W. Stephen Woodward
University of North Carolina, Venable Hall, CB3300, Chapel Hill, NC 27599-3300; e-mail: woodward@unc.edu
ELECTRONIC DESIGN • December 18, 2000から引用
Regardless of its many
personality flaws, the
ordinary bi-metallic
thermocouple (TC) remains the dominant technology for industrial temperature sensing. TCs are
popular because of their
durability, simplicity, and
low cost, not to mention
their pedigree (they have
been around since John Seebeck discovered them in
1822).
These sensors continue
to thrive despite a multitude of irksome signal-conditioning challenges associated with their use. Some of
the worst TC quirks include
lowly millivolt-amplitude
signal levels, cold-junction
compensation (CJC) requirements, and nonlinearity that can become extreme at low temperature
(which is sometimes the
most problematic of all).
The signal-conditioning
circuit in Figure 1 addresses
TC foibles with a combination of three features, including chopper-stabilized “zero-drift”
amplification (which is derived from
A1, Linear Technology’s LTC1049). It
also relies on ∆VBE cold-junction compensation (using Q1, any generic
small-signal npn). An aggressive, digitally controlled
potentiometer (the Xicor X9C103 DCP) linearization loop
is employed as well. When joined with
a standard type-N TC, the circuit
becomes a “reasonably” accurate
Kelvin-scale analog thermometer. It
offers a 10-mV/K output scale factor
and untrimmed ±3% full-scale error
from 4K (the temperature of boiling
liquid helium) to 373K (boiling H2O).
Not all of the NIST standard TC
types (B, E, J, K, N, R, S, and T) will
work at cryogenic temperatures. One
that does, however, is the type-N TC
(Ni-Cr-Si alloy versus Ni-Si-Mg).
Type-N TCs are characterized
by temperatures ranging from
3K to 1570K (−270°C to
1370°C). They also have a
room-temperature Seebeck
coefficient of approximately
26 µV/K that’s “relatively” constant (±10%, which is good for
a TC) from 140K to 370K.
Over this 230° temperature
span, VOUT = [VTC + (26 µV ×
TQ1)]/0.0026 = 0.01 V/K. Here,
VTC is the TC Seebeck signal,
while TQ1 indicates the absolute temperature of CJC Q1.
Also, 26 µV × TQ1 represents
the temperature-compensating
∆VBE signal generated by Q1,
S2, S3, and the S2-modulated
Q1 collector-current ratio, (R1
+ R2)/R1.
Error-curve A in Figure 2
illustrates the performance of a
linear TC approximation.
Although acceptable for the upper portion of the temperature range, the simple first-order TC model goes to pot at
colder temperatures, ending up more
than 100° off at 4K! This is because the
type-N Seebeck coefficient starts to
nose-dive around 140K as it falls from
26 µV/K at 273K, to 20 µV/K at 140K,
and less than 2 µV/K near 0K. It’s at this
point that the DCP-based linearization
algorithm kicks in.
The linearization applied by the circuit in Figure 1 works by subtracting a
0- to 2.8-mV progressive correction
voltage (VDCP in Figure 2) from A1’s
(VTC + 26 µV/KTQ1) input signal when
VOUT is less than 1.4 V. According to
the rules for calculating VDCP correction, if VOUT is greater than 1.4 V (i.e.,
T > 140K), then comparator A2 holds
the DCP’s up/down count-direction
line low. Consequently, the DCP wiper
sticks at ground, forcing VDCP to zero.
But when VOUT is less than 1.4 V, A2
selectively enables DCP upcounting. By doing so, it closes a
feedback loop that maintains
VDCP = 0.002(1.4 − VOUT) and
thereby establishes VOUT = (VTC
+ (26 µV × TQ1) − 0.0028)/0.0006.
The net error of the composite
response is never more than
±10° away from the correct temperature over the entire 0K to
373K temperature span (Fig. 2,
curve C).
For both the DCP’s INC terminal and the S2/S3 CJC synchronous modulator/demodulator, the 208-Hz clock signal is
supplied by the multivibrator
implemented by A3/S1. The
208-Hz signal on S3 enhances
the circuit’s immunity to 60-Hz
pickup, which is further improved by the grounded-TC
input configuration used.
VDCP ripple, caused by dither
from the DCP wiper, is filtered by the
linearization-loop capacitors, C1 and
C2. Such filtering provides beneficial
interpolation of the 100 discrete DCP
steps. It also enables an effective continuum of VDCP correction levels.
Resistors used in the circuit should be
1% or better and have low temperature coefficients to preserve the thermometer’s no-trim error performance.
The total thermometer power consumption is typically 10 mW.
