Transmitter

The analog front-end combines the following:

RTD sensor PT1000

Ultra-precision, low-noise, zero-drift operational amplifier (MAX44248)

Ultra-low-power, high-accuracy delta-sigma ADC (MAX11200)

The low-power microcontroller (MAXQ615) is used to implement calibration and linearization.

The sensor used in the Monterey board is a platinum resistance thermometer (PT1000). The entire system offers excellent accuracy of

wide temperature range from -100°C to +100°C. The basic operation of this sensor block is to measure the temperature, which is

subsequently converted into current by the 4–20mA loop current source.

The variation in temperature creates a change in galvanic resistance of the wheatstone bridge, due to change in the resistivity of the RTD

element. As the temperature ranges from -100°C to +100°C, a 150mV differential voltage swing is observed across the wheatstone bridge

to INPUT A and INPUT B of the 50x amplifier.

The gain stage is used to amplify very minor differential voltage arising from the wheat-stone bridge. Jumpers J4 and J5 provide options to

use the amplifier stage or bypass it. When used, the differential voltage is gained 50 times by using the MAX44248 before digitized by the

MAX11200 ADC. A dual amplifier stage (MAX44248) is used as a 50x differential amplifier. The gain can be changed by changing the

resistors to match the desired signal range from the wheatstone bridge, thereby maintaining proper input signal range and output swing

from the amplifier driving the ADC.

Figure 2. Amplifier gain stage sensor block.

The MAXQ615 microcontroller then maps for the voltage represented to the calibrated temperature reading that the PT1000 is expected to

show. The MAXQ615 has a transfer function that compensates for the nonlinear function of the PT1000. The below equation represents

the compensation function:

where T is the temperature and dV is differential voltage

The above information only supports the PT1000 and for this specified wheatstone bridge configuration, sensor front-end component

selection. This mapped information is then sent to the transmitter through the SPI interface for loop-current generation.

The Transmitter Block

The transmitter block description and step-by-step design guide is discussed in the MAX5216LPT data sheet.

Evaluation of 4–20mA Loop Application

To evaluate the complete 4–20mA loop application, the following is required.

The Monterey Board (MAXREFDES15#), consisting of the smart sensor block and low-power transmitter

An analog front-end receiver (Santa Fe, Campbell, Fresno)

An interface board such as Munich USB2PMB1# can be used to facilitate the evaluation of this application.

The following step-by-step user guide explains testing the entire application using the

RTD Fluke 724 calibrator at the sensor end

Monterey Board MAXREFDES15#

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