AN3649| Application Note
SENSOR SIGNAL CONDITIONERS
TEMPERATURE SENSORS and THERMAL MANAGEMENT Sep 23, 2005
Keywords: Compensation algorithm, Multi-channel signal conditioner, digital signal conditioner, High performance signal conditioner, Low cost signal conditioner, Precision Sensor Signal Conditioner, Pressure sensor compensation, PRT sensor compensation, Thick
APPLICATION NOTE 3649
MAX1464 Signal-Conditioner, Sensor Compensation Algorithm
The MAX1464 is a fully digital, high-performance signal conditioner with multichannel inputs, analog and digital outputs, and support for 4-20mA output applications. The MAX1464 can be programmed to correct the nonlinearities and temperature dependent characteristics of sensors. This document describes the procedures to compensate and calibrate a sensor signal that is applied to one of the MAX1464's ADC channels. The algorithm provides offset and span correction over the specified temperature range. It is assumed that the MAX1464 VDD supply voltage does not change during the compensation process and operation of the device. The data presented in this document is real data acquired from a piezoresistive pressure sensor used to demonstrate the compensation algorithm.
1. Input Parameters
The user must define some input parameters for the application. They are: Tmin Tintr Tmax Pmin Pmax Vmin Vmax = minimum temperature, in degrees Celsius = intermediate temperature, in degrees Celsius = maximum temperature, in degrees Celsius = minimum pressure = maximum pressure = desired MAX1464 output, at the minimum pressure, in volts = desired MAX1464 output, at the maximum pressure, in volts
The next limits are application dependent, and can differ for other applications:
Tmin Tintr Tmax 40 25 125 Pmin Pmax 0 15 Vmin Vmax 0.5 4.5
For pressure nonlinearity correction, we define Pmed as the sensor excitation midpoint:
Pmed Pmax 2 Pmed = 7.5 Pmin Pmin
For better performance and to maximize the range of the MAX1464's ADC converter, you must adjust the appropriate coarse offset and PGA settings. Select the temperature for which the sensor sensitivity is the highest, and apply minimum and maximum sensor excitation. The user should then select the PGA gain and coarse offset settings that maximize the ADC output at these conditions. The ADC acquired data must then be entered in the matrix below. Each row of data has the normalized ADC reading for the minimum, medium, and maximum sensor excitation at the indicated temperature. Each row also has the ADC reading for the MAX1464's internal temperature sensor, and the DAC output voltage (through either the small or large op amp) for a fixed, normalized digital input of -0.5 (DACinM) and +0.5 (DACinP).
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