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Calibration

Thursday, September 01, 2005

To have the SDR make analog measurements, it must be calibrated.  If it is not, it will read 0.

Two values require calibration:

  1. Voltage Reference
  2. 4-20ma Resistor value

The values can be calibrated via menus in the front panel.  The pre-release versions (2.00 and 2.01) offer access to these via the Diagnostic menu.  Version 2.00 (analog development version) offers additional access via the Analog Settings submenu of the Analog Development menu.

First calibrate the reference by entering the “Analog VRef Cal” menu.

The menu will show the current calibration value.  If that value is 0, VRef has not been calibrated and analog measurements (except bridge) will read 0.

Hit set to calibrate.  The allowed values are 2.4 to 2.6 Volts.  0 can also be entered, effectively erasing the calibration.

When calibrating, VRef will default to zero, forcing the user to not select a default such as 2.5V.

Once set is hit, SDR turns on VRef.  Measure the voltage  between the Vref and Agnd pins on the terminal and enter that value.

“Analog 4-20mA Cal” menu should only be calibrated after VRef.  The current value is shown when the menu is entered.

To calibrate, follow these steps:

  1. Connect a highly accurate current meter to the VREF and the 4-20mA input pin.  This should create a 25 mA (approximate) current into the4-20 mA.
  2. Press set on the front panel to calibrate.  Pressing set will turn on the analog circuitry.
  3. Enter the current read by the meter into the SDR (expressed in mA).  The allowed range is from approx. 20 to 26mA.  SDR calculates the value of a resistor (ideally 100Ohms) by dividing the voltage it read on the 4-20mA pin by what the user entered.  That number becomes the value of the resistor.  The range of allowed resistance is 75 to 125Ohms.