AN135 データシートの表示（PDF） - Xicor -> Intersil
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AN135 Datasheet PDF : 5 Pages
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Xicor Application Note
AN135
pressure sensor that is compatible (thanks to DCP1 and
2) with full automation of the calibration process, is very
low in total power draw (< 1 milliampere, most of which
goes to transducer excitation), and (equally important) is
low in cost.
PRTD SIGNAL CONDITIONING CIRCUIT
Among temperature transducers, the platinum
resistance temperature detector (PRTD) is generally
accepted as the ‘gold standard’. PRTDs are ubiquitous
and find wide application in the aviation, environmental,
industrial, and scientific instrumentation areas. The
circuit in Figure 5 uses the PRTD in a bridge circuit
whose output is amplified by a high performance
instrumentation amplifier (IA). Amongst the problems
associated with this traditional approach is the lack of
variablility to account for sensor variations, lack of a
linearization scheme, and the high cost of the
instrumentation amplifier.
The PRTD temperature response consists of resistance
variations of the order of only tenths of ohms/°C. Hence
strict attention must be paid to the effects of transducer
lead wire resistance. The magnitude of the excitation
current must also be severely limited, otherwise
excessive I2R PRTD power dissipation will cause
unacceptably large self-heating measurement errors.
Low excitation currents and small resistance changes
combine to mean that the signal developed by the PRTD
will typically be of the order of tens of µV/°C generating
a requirement for stable high gain DC amplification in
the signal chain. In addition, the PRTD temperature
coefficient is only ‘reasonably’ invariant with
temperature and, as a result, the PRTD’s response is
significantly nonlinear. The accurate measurement of
temperature over a wide range depends on the
provision for linearization of the PRTD signal. These
design considerations are incorporated in the circuit of
Figure 6 and result in a precision thermometer with
output span of –1V to +3.5V corresponding to a
temperature range of –100 to +350°C. The maximum
error over this span can be adjusted to ±0.02°C at 0°C
and ±0.05°C elsewhere.
Current excitation (approximately 250µA) for the PRTD
is sourced by the 2.5V voltage reference VR1 via R1.
The 256 tap digitally controlled potentiometer DCP1
provides for automated adjustment of the thermometer
scale factor and span. A1 is a noninverting amplifier with
a gain of 100 which scales up the raw 100µV/°C PRTD
temperature signal to 0.01V/°C. The DCP2 network
implements a high resolution zero adjustment. Each
increment in DCP2’s setting will result in a 200µV shift in
Al’s output which is equivalent to a 0.02°C zero
adjustment. The symmetry of the R6-R9 network
surrounding DCP2 causes zero adjustment to have no
effect on A1’s gain and therefore no effect on the
thermometer’s span/scale factor. Likewise, span
adjustments via changes in the VR1 reference allow no
interaction between DCP1 and the zero calibration
established by DCP2.
Positive feedback provided by R2 linearizes the
thermometer’s response curve by providing a Thevenin
equivalent of a negative amplifier input resistance of –
2064 ohms in parallel with R1. This introduces a positive
gain slope (roughly +0.016%/°C) which effectively
cancels the tendency of the PRTD temperature
coefficient to decline with increasing temperature. The
result is better than a factor of 100 improvement in
linearity over the raw PRTD response.
The net result of the combination of A1 and the
associated circuit is a signal conditioning, precision
temperature sensor that is compatible (thanks to DCP1
and 2) with full automation of the calibration process,
low in total power draw, and low in cost.
VBIAS
PRTD
∝ VOUT °CV
IA
Figure 5. PRTD Sensor Circuit – Basic
AN135-4
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