AC VOLTAGE OR CURRENT INPUT

By far the greatest majority of AC measurements encountered relate to measurements concerning the primary AC power mains.

The factors that influence the cost of signal conditioning can be defined without knowing the exact source of the signal.

RMS VERSUS AVERAGE MEASUREMENT

If the waveshape of the input signal is consistent, the least expensive method of converting the input into a proportional DC signal is through a simple rectifier circuit and filtering out the AC component to create a DC signal proportional to the average value of the input. This DC signal can then be scaled to represent any parameter desired such as RMS or peak value of the input. However, if the wave shape changes, the accuracy also changes.

If the waveshape of the input changes and an accurate measurement is required, an RMS converter is required to get a DC signal proportional to the RMS value of the input.

The conversion requires a costly integrated circuit that performs the equation to square the input, integrate the results, and then take the square root of the DC from the filter.

If an RMS measurement is required, a parameter called crest factor becomes important. Crest factor is defined as the ratio of the peak to RMS value of the input. The conditioner must be designed to process the highest peak required to get an accurate measurement. Normal crest factors range from about 5 to 10. If an input had a normal full scale of 1V RMS and a crest factor of 8, the conditioner would have to process the 8V peak without clipping to get an accurate measurement. Crest factor dictates the supply voltages used on the amplifiers in the conditioner.

INPUT IMPEDANCE FOR VOLTAGE INPUT

The load presented by the conditioner to the signal source must be high enough to not load the source and create an excessive drop in the signal level.

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.

INPUT IMPEDANCE FOR CURRENT INPUT

Signal conditioners that process AC current inputs are almost always a voltage input conditioner that has a resistor connected across its input to convert the current input signal into a proportional AC voltage.

Two limitations dictate the resistor value. The resistor must be large enough to give a reasonable signal level. It must be small enough so its power dissipation is limited to reasonable levels.

Current shunts can be purchased that produce 50 mV and 100 mV signals at their rated current.

Self-contained shunts can be mounted inside the conditioner to a point.

One of the problems with internal shunts, that can be unplugged from the current source, is the tendency of the current source to keep raising its voltage to maintain current. It is possible to draw arcs that can melt screws and terminals. It is also possible to get voltage levels that are dangerous to personnel.

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 usually 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 cost 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 available components.

An AC input conditioner can use AC coupled circuits to the point where the AC signal is converted to a proportional DC signal. Once the DC signal is being processed, all of the stability factors enter into the design requirement.

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 AC 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.
5. Average or RMS measurement required.