MLX90323 Datasheet, PDF (8/25 Pages) Melexis Microelectronic Systems – 4 – 20 mA Loop Sensor Interface with Signal Conditioning and EEPROM

MLX90323
4 – 20 mA Loop Sensor Interface
with Signal Conditioning and EEPROM
7 Detailed General Description
7.1 Understanding 4-20mA current loop interface
MLX90323 IC is optimized for 4 - 20 mA industry standard current loop interface. The 4 - 20mA current loop
shown in Figure 1 is a common method of transmitting sensor information in many industrial applications.
Transmitting sensor information via a current loop is particularly useful when the information has to be sent to
a remote location over long distances. The loop operation is straightforward: a sensor’s output voltage is first
converted to a proportional current, with 4mA normally representing the sensor’s zero-level output, and 20mA
representing the sensor’s full-scale output. Then, a receiver at the remote end converts the 4-20mA current
back into a voltage which in turn can be further processed by a controller module.
Sensor
MLX90323
Signal Line
VB
SLP Positive Transmission Line
INP
INM
GND
4 – 20 mA
SLN Signal Line
Negative
Signal processor / Controller
Current loop
E ^{power source}
R
Output voltage for
further processing
Figure 1. Current loop interface diagram
7.2 Analog features
Supply Regulator
A bandgap-stabilized supply-regulator is on-chip while the pass-transistor is external. The bridge-type sensor
is typically powered by the regulated supply (typically 4.75V).
Oscillator
The MLX90323 contains a programmable on-chip RC oscillator. No external components are needed to set
the frequency (87.8 kHz +/-1%). The MCU-clock is generated by a PLL (phase locked loop tuned for 614 kHz
or 2.46 MHz) which locks on the basic oscillator.
The frequency of the internal clock is stabilized over the full temperature range, which is divided into three
regions, each region having a separate digital clock setting. All of the clock frequency programming is done
by Melexis during final test of the component. The device uses the internal temperature sensor to determine
which temperature range setting to use.
Power-On Reset
The Power-On Reset (POR) initializes the state of the digital part after power up. The reset circuitry is
completely internal. The chip is completely reset and fully operational 3.5 ms from the time the supply
crosses 3.5 volts. The POR circuitry will issue another POR if the supply voltage goes below this threshold for
1.0 us.
Temperature Sensor
The temperature measurement, TPO, is generated from the external or internal temperature sensor. This is
converted to a 10-bit number for use in calculating the signal compensation factors. A 2-bit coarse adjustment
GNTP[1:0] is used for the temperature signal gain & offset adjustment.
3901090323
Rev 003
Page 8 of 25
Data Sheet
Feb/12