Specifications

Sensors
Temperature sensing
72 Maxim Industrial Solutions
MAX1402, MAX4236/
MAX4237
Any appreciable resistance in the RTD’s
leads will cause errors in temperature
measurement. Therefore, for long wire
lengths use a 3- or 4-wire connection
to eliminate lead-resistance errors. The
circuit in Figure A is a 4-wire RTD
interface using the MAX1402 over-
sampling ADC. The MAX1402 has two
matched current sources, which signifi-
cantly reduce the IC count in an RTD
converter. One of the current sources
provides excitation current for the RTD,
in this case, a Pt100. Because the
excitation current does not flow
through the sense leads, lead resistance
will not affect the temperature-
measurement accuracy. The second
current source drives a precision resistor
to generate the reference voltage for
the ADC, thereby eliminating the need
for an external voltage reference.
For best accuracy when using an
RTD, apply linearity correction to the
acquired data to compensate for the
Pt100’s nonlinearity. Also use gain
correction to compensate for both the
tolerance of the reference resistor and
mismatch between the current sources.
The digital linearity correction can be
avoided if a small amount of positive
feedback is applied to an amplifier
circuit, as shown in Figure B. The
resulting linearity error from -100°C to
+200°C is less than ±0.05°C. This circuit
does not compensate for long leads,
so it should be used when the RTD is
located near the measurement circuitry.
For more details, refer to Maxim’s
application note 3450, “Positive
Analog Feedback Compensates
Pt100 Transducer.”
Simple, integrated RTD-to-digital conversion
Figure A. Circuit diagram shows the MAX1402 ADC in a 4-wire
interface for a Pt100 RTD.
Figure B. A Pt100 linearizer circuit. Pt100 is compensated by one additional resistor. R2 provides
a small positive feedback.
V+
200µA
5V
V
DD
OUT2
REFIN+
AIN1
AIN2
AGND
DGND
PGA
MODULATOR
A = 1 TO 128
200µA
RTD
R
REF
REFIN-
OUT1
MAX1402
R2
11.8kΩ
R1
11kΩ
R3
105kΩ
V
OUT
Pt100
100Ω
R5
3.01kΩ
5V
V1
R4
12.4kΩ
+
−
MAX4236
MAX4237A