RESISTANCE INPUT

Resistance input signal conditioners create an output proportional to the resistance of a 2 terminal resistor.

The source is often the slide and one end of a potentiometer, but can include many resistive devices such as CdS photo cells, single leg strain gauge, simple DC conductivity measurement, thermistors, and others.

The resistor is usually excited by a constant current source and the DC voltage developed across the resistor is conditioned to create the desired output.

INPUT IMPEDANCE

The load presented by the conditioner to the signal source must be high enough to not load the source and create a nonlinear response of output versus resistance.

Wiring between the source and the load has a finite resistance that also contributes to the drop in signal at the signal conditioner. In addition, the lead resistance will change with temperature and this change in resistance must be small in relation to the conditioner's input impedance so the input level will not change excessively with ambient temperature changes.

COMMON MODE REJECTION

If the same signal is put on both input terminals of a signal conditioner, there would be no output if the unit had perfect common mode rejection.

Common mode rejection is usually best at DC and deteriorates as the frequency of the input common mode signal increases.

Common mode rejection is valuable for voltage input conditioners when the two input leads pass sources of electrical noise that capacitively couple the noise to both leads as a common mode signal. The conditioner's rejection capability can be very effective in reducing the influence of this noise to acceptable levels.

A conditioner designed for high common mode rejection costs more than one with less rejection. The application and installation reasonably dictates the need or not.

Wiring practices between the signal source and conditioner input can have a great influence on the amount of noise at the input. Good wiring practices can eliminate the need for a high common mode rejection in the conditioner by reducing the common mode signal to an acceptable level.

STABILITY VERSUS AMBIENT TEMPERATURE

Every component used in the design and manufacture of a signal conditioner has a temperature coefficient that influences the stability of the output versus ambient temperature.

For best cost/performance ratio, the design should be done with the most accessible components. The factor that controls this parameter the most is input signal level.

As an example, assume a conditioner is designed to accept a 1 volt input and the drift on the output, for a 30 Deg C ambient temperature change, is 1% of the output. If the input could be changed to 2 volts, the output drift could be cut almost in half.

Depending on the actual input level, the first amplifier can vary very widely in cost (10 to 20 times).

SPEED OF RESPONSE OR BANDWIDTH

The speed at which a signal conditioner responds to input signal level changes is determined by its bandwidth.

The common method of specifying bandwidth is to state the frequency at which the output level has dropped 3db(to 70.7%) of its DC value.

The response to a step change is also a common method of specifying the speed of response. If the specification is stated as "time constant of x seconds", it is assumed the response moves about 63% toward its final value. For a change to move 99% toward its final value requires about 5 time constants; 99.9% requires about 7 time constants.

Wider bandwidth requires better wiring practices to keep noise pickup to a minimum.

Lower bandwidths are effective in reducing noise by not allowing it to pass through the conditioner.

COST FACTORS OF RESISTANCE INPUT SIGNAL CONDITIONER

1. Absolute accuracy of measurement.
2. Stability versus ambient temperature.
3. Environment (hazardous, humid, corrosive, etc.)
4. Span of measurement. (Span is the difference between the zero scale signal and full scale signal.) The narrower the span the more costly it is to meet stability requirements.