You need two voltage readings and two corresponding data points in order to calibrate an input to a sensor. You can usually get sufficient data from the specifications published by the manufacturer of the sensor. You can also derive the data by taking calibration readings. Most of the time, zero volts represents zero data units. If this is the case, you can enter 0 and 0 for reading #1. Next, look for the full scale reading of the sensor. It may be specified as 100psi @ 10VDC for example. If this is the case, your voltage reading #2 would be 10 and data reading #2 would be 100.

Signal type should be selected as volts or milliamps. If milliamps is selected, you must also enter the value of the external resistor you will use to convert milliamps to volts. When calibrating milliamps, enter 4mA as 4, enter 20mA as 20, etc. When calibrating volts, enter 10V as 10, etc.

External resistor values recommended are 523 ohms for a 10V signal, or 255 ohms for a 5V signal. A 10V signal will take advantage of the full resolution of the input; however, many devices that generate a 20mA signal are not capable of driving more than a 250 ohm load. If this is the case, use the recommended resistor, and apply the slope and intercept calculated above to compensate accordingly.

The external resistor has effectively a 16.8K ohm resistor in parallel with it inside the AddMe III hardware. The requirement for an external resistor was chosen to minimize repairs needed. If the 24V power is accidentally connected to a current input, the dropping resistor will burn out if it is less than a 5 watt resistor. It was not considered desirable to penalize all inputs with the added cost and bulk of a 5 watt resistor.

The relatively low input impedance (16.8K ohms) was chosen to maximize noise immunity. The current required by the input at 10VDC is less than 1mA, well within the drive capability of most active sensors. The thermistor input is sourced by internal excitation (5VDC).

Once you have calculated a slope and intercept, you should enter those values in the approprate boxes back in the Analog Inputs setup page. The input should now produce scaled data units in the input's floating point registers. Note that the integer registers will always reflect raw A/D counts (except for temperature which is a special case).

During the calculation process, you can test hypothetical input values by entering numbers for input signal and/or A/D counts at the bottom of the above table. Click calculate again to see the results using your calculated slope and intercept.

Check "Track input signal" to automatically used the calculated A/D counts to derive display data when testing. If you wish to test raw counts directly, uncheck this box.