MAX11410 Datasheet, PDF (93/95 Pages) Maxim Integrated Products – 24-Bit Multi-Channel Low-Power

MAX11410
24-Bit Multi-Channel Low-Power
1.9ksps Delta-Sigma ADC with PGA
Two-RTD Temperature Measurement Circuit
(2- 3-, and 4-Wire)
In Typical Application Circuit A, AIN1 and AIN2 serve as
the analog inputs for measuring the voltage across the
first RTD, with AIN0 and AIN3 providing the RTD excitation
currents. AIN5 and AIN6 are the analog inputs for the second
RTD, with AIN4 and AIN7 providing the RTD excitation
current. Up to 1000Ω (at 0°C) RTDs, such as Pt1000s can
be measured over a full 850°C operating range.
The 1kΩ resistors provide over-voltage protection for
the inputs. Although not shown, a 100nF filter capacitor
will normally be connected across REF1P and REF1N.
A 100nF filter capacitor will be connected across AIN1/
AIN2, with a 10nF from AIN1 to GND and from AIN2 to
GND. Capacitors will be similarly connected to AIN5 and
AIN6.
If the first RTD is a 4-wire or 2-wire unit, set IDAC0 to
source 200μA from AIN0. This current will flow through
the RTD and through RREF, creating a voltage drop of
800mV across RREF. The voltage across RREF serves
as the reference voltage for measurement of the RTD
resistance. Because the same current flows through the
RTD and RREF, the conversion data will be the ratio of
the RTD resistance to RREF. Note that any error in the
value of RREF will directly affect the accuracy of the
RTD measurement, so use a low-drift, accurate resistor
for RREF. If a singletemperature system calibration is
performed, RREF may have a relaxed initial tolerance.
Note that, while the 4-wire connection can eliminate errors
due to cable resistance, any lead resistance will add to
the apparent RTD resistance measurement when using
a 2-wire connection. Therefore, the 2-wire connection is
normally used only when the RTD is close to the circuit.
If a 3-wire RTD is used, IDAC0 will again source current
from AIN0, and IDAC1 will source current from AIN3. If the
lead resistances are equal, the voltage drops across the
two upper leads will be equal, and therefore the voltage
measured between AIN1 and AIN2 will be equal to the
RTD voltage. Because both excitation currents will flow
through RREF, the current values should be reduced to
150μA to maintain voltage headroom. The output code
will be half the ratio of the RTD resistance to RREF
because 300μA flows through RREF, but only 150μA
flows through the RTD.
Thermocouple Measurement Circuit
Measuring temperature with a thermocouple requires two
measurements. The thermocouple voltage is measured
using a precision voltage reference. In addition, a separate
sensor must measure the temperature at the “cold junction”
– the point at which the thermocouple wires make contact
with copper at the input connector. Cold-junction temperature
may be measured in a number of different ways—with a
stand-alone temperature sensor, a thermistor, an RTD, or
a diode-connected transistor. Typical Application Circuit B
uses an RTD to measure cold-junction temperature.
1kΩ protection resistors connect the thermocouple output
to AIN4 and AIN5. Set the PGA gain to an appropriate
value for the thermocouple being used. For example, a
K-type thermocouple produces a maximum output voltage
of about 54mV. Setting the PGA gain to 32 results in a
maximum PGA output voltage of about 1.7V, which is
appropriate for use with the 2.5V reference shown. Bias
the thermocouple to V_DD/2 using the internal bias voltage
generator. Select AIN5 as the pin to which the internal
bias generator is connected. To detect an unconnected
thermocouple or a broken thermocouple wire, enable the
burnout current generator. An open circuit will result in an
overrange input.
To measure the cold-junction temperature using an RTD,
set IDAC0 to source 200μA from AIN8. This current will flow
through the RTD and through RREF, creating a voltage
drop of 800mV across RREF. The voltage across RREF
serves as the reference voltage for measurement of the
RTD resistance. Because the same current flows through
the RTD and RREF, the conversion data will be the ratio
of the RTD resistance to RREF. Note that any error in
the value of RREF will directly affect the accuracy of the
RTD measurement, so use a low-drift, accurate resistor
for RREF. If a single-temperature system calibration is
performed, RREF may have a relaxed initial tolerance.
Because the RTD is close to the ADC, a 2-wire RTD may
be used.
Although not shown, a 100nF filter capacitor will normally
be connected across REF1P and REF1N. A 100nF filter
capacitor will be connected across AIN4/AIN5, with a 10nF
from AIN4 to GND and from AIN5 to GND. Capacitors
will be similarly connected to AIN8 and AIN9. Additional
thermocouples may be connected to the unused inputs.
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