Datasheet

More detailed image.
Figure 1. Novato (MAXREFDES16#) block diagram.
The signal chain contains the MAX11213 (U1), 16-bit delta-sigma ADC with programmable gain and GPIO, the RL78/G13 Renesas'
microcontroller with 128kB flash memory (U6), and the 4–20mA current loop transmitter built on the MAX5216 and MAX9620 (U2 and U3).
The
DS8500 modem (U7) provides digital interface utilizing HART protocol over 4–20mA current loop. The MAX15006 (U5) and MAX6133
(U4) provide power and reference voltage to the entire circuitry.
For more detailed information about 4–20mA current loop transmitter refer to reference schematic 5610, "High-Performance, High-
Accuracy 4-20mA Current-Loop Transmitter Meets Toughest Industrial Requirements."
This design uses a ratiometric measurement of sensor resistance according to the ADC transfer function (Eq. 1 and Eq. 2):
ADCcode = ((V
AINP
- V
AINN
))/(V
REF
/gain) × (2
16
- 1)
(Eq. 1)
where
(V
AINP
- V
AINN
) = I
RTD
× R
RTD
is a voltage drop on RTD,
V
REF
= I
RTD
× R
REF
is a voltage drop on reference resistor R
REF
Equation 1 can be rewritten as:
ADCcode = R
RTD
/(R
REF
/gain) × (2
16
- 1)
(Eq. 2)
Then the ADC code is converted into temperature data according to the Callendar-Van Dusen equation (Eq. 3):
R(T) = R
0
(1 + A × T + B × T
2
– 100 × C × T
3
+ C × T
4
)
(Eq. 3)
where
R(T) is RTD resistance at temperature T (°C)
R
0
is RTD resistance at 0°C
A, B, and C constants are derived from experimentally determined parameters and regulated by the IEC751 standard; they also must be
provided by RTD manufacturers
For PT100 and temperature coefficient of resistance: α = 0.003850 [
α = (R
100
– R
0
)/(100 × R
0
)]
A = 3.90830 × 10
-3
B = -5.77500 × 10
-7
C = 4.18301 × 10
-12
for -200°C ² T ² 0°C
C = 0 for 0°C ² T ² 850°C.
Temperature T can be calculated by solving quadratic equation if we ignore coefficient C = 4.18301 × 10
-12
for negative temperature (Eq.
4):
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