Data acquisition and process control systems function more efficiently and are easier to install if their field inputs are supplied by sensor signal conditioning modules which have linear transfer functions. A linear transfer function means that outputs of a device are a "straight line" response to the inputs. This allows the receiving data acquisition and control systems to directly interpret field information by assigning the data a linear scale factor such as gallons-per-minute (gpm) per volt, degrees per milliamp, psi per volt, etc.

Many field sensors such as Thermocouples, Resistance Temperature Devices (RTDs), and Thermistors, have non-linear outputs and their unconditioned data must be linearized and scaled by the receiving devices. This linearizing of field data requires additional overhead in the receiver’s processor. Data acquisition and process control systems that utilize "pre-conditioned" field inputs, which have already been linearized, scaled, filtered, and isolated are free of this expensive computational overhead, are easier to upgrade, and more cost effective overall.

Linearity specifications define how close any set of data approximates a straight line. Conformity specifications, on the other hand, define how close a set of data approximates a specific functional curve (e.g., cubic, exponential, logarithmic, etc.).

Both linearity and conformity specifications have three category types: 1. Independent, 2. Terminal-based, and 3. Zero-based.

All specifications for linearity or conformity are established by averaging measurements taken by traversing the full range in each direction at least twice.

1. The independent specification is the maximum deviation of a set of data from a straight line or curve, which is positioned to minimize the maximum deviation.

2. The terminal specification is the maximum deviation of a set of data from a straight line or curve, which is positioned with the upper and lower values fixed to the upper and lower values of the data range.

3. The zero-based specification is the maximum deviation of a set of data from a straight line or curve, which is positioned with the lower value fixed to the lower value of the data range.

The defacto convention, unless otherwise indicated, provides that a "linearity" or "conformity" specification is understood to mean the independent type (Type 1 above).

Dataforth’s signal condition modules use a unique linearizing technique to provide you with "Instrument Class" sensor data. Essentially, we use circuitry to segment your sensor’s range into 10 areas in order to equalize any positive and negative conformity errors. Using this hardware technique Dataforth corrects to within ±0.015% of span (that’s correct, linear to an outstanding ±0.015%)! For a more complete description, please visit Dataforth’s website and examine our application note AN505 at the following link:

Dataforth’s Hardware Linearization of Non-Linear Signals (105Kb pdf file)

Remember, our Application Engineers can assist you with signal conditioner selection over the phone or via fax and email. Call us at our manufacturing facility in Tucson at 520-741-1404 (fax 520-741-0762) or Email us at techinfo@dataforth.com.