ADC formula

All the analogue to digital MPs have some specific common parameters shown below.

Common ADC MP parameters

Parameter	Description
Input Range	Selects the input range.
Measuring Range	Selects the measuring (output) range.
Advanced button	Presents the formula used for measurement calculation where x is the input reading.

Input ranges

The input ranges are predefined and depend on the type of the ADC MP hardware specification.

Typical ranges are 0-20mA. 4-20mA or 0-10V.

Measuring ranges

The measuring ranges can be defined as described in Measuring Ranges chapter.

ADC conversion formula

To define the ADC conversion formula, select the Input Range which defines the range of the raw values read from the sensor and then the Measuring Range which defines the output range of the measurements after conversion.

The system allows to use only a linear equation formula in the form of:

y = a*x + b

Please note that the system creates the formula automatically taking into account the units used in the measurement raw value which may differ depending on the ADC module used e.g. raw current readings may be reported in mA or uA. This is shown in detail in example 1 and 2 below.

Example 1

A GMP251 Vaisala CO2 sensor (FC014 ) is added to the system. The sensor has 0-20mA output where increase of 1mA corresponds to increase of CO2 concentration of 1% so the scale is 1mA/1%.

The sensor is connected to one of the channels of NWIN004 WIN 4xADC 4-20mA module. This module measures the current from 0 to 20mA despite its B3 name and returns the readings in mA.

The input range is set to 0-20mA and the measuring range to 0-20.

The formula in B3 is:

y = (x*(20-0)/20)+0

After shortening, the mathematical formula is:

y = x

Example 2

A GMP251 Vaisala CO2 sensor (FC014 ) is added to the system. The sensor has 0-20mA output where 1mA increase corresponds to CO2 concentration increase of 1% so the scale is 1mA/1%.

The sensor is connected to one of S2 3xAnalogue input module channels. The module measures the current from 0 to 20mA and returns the readings in uA.

The input range is set to 0-20mA and the measuring range to 0-20.

The formula in B3 is:

y = (x*20/20000)+0

After shortening, the mathematical formula looks as below:

y = x/1000

The raw value must be divided by 1000 to convert raw value in uA to mA.

Example 3

An H2 sensor which characteristics is shown below, is added to the system.

The sensor has 4-20mA output where increase of 2mA corresponds to H2 concentration increase of 1% so the scale is 2mA/1%.

The input range is set to 4-20mA which gives the current span of 20-4 = 16mA and the measuring range is set to 0-8.

The formula in B3 is:

y = ((x-4)*(8-0)/16)+0

After shortening, the formula is:

y = (x-4)/2

We may check the result for e.g. x = 8mA for which the H2 concentration shall equal 2%.

y = (8-4)/2 = 4/2 = 2

Example 4

A sensor with mA output is added to the system. The sensor has 4-20mA output which shall be mapped to the MP readings from -50 to +50.

The input span of 20 - 4 = 16mA shall be mapped to output span of 50 - (-50) = 100 so 1mA increase shall equal to output increase of 100/16 = 6.25. The output for x = 4 shall be -50 and for x = 20 shall be 50.

The sensor is connected to one of the channels of NWIN004 WIN 4xADC 4-20mA module.

The input range is set to 4-20mA and the measuring range to -50 to +50.

The formula in B3 is:

y = ((x-4)*(50+50)/16)-50

After shortening, the mathematical formula looks as below:

y = 6.25*(x-4)-50

We may check the result for x = 4mA for which the measurement shall be equal -50.

y = 6.25*(4-4)-50 = 6.25*0-50 = -50

The result for x = 20mA for which the measurement shall be equal 50.

y = 6.25*(20-4)-50 = 6.25*16-50 = 100-50 = 50