Being a phase-sensitive carrier-demodulator instrument (rather than fully DC), the single-channel Model 4078 is intended for applications involving transformer coupling to the transducer bridge (as with rotary-transformer torque sensors) and for applications requiring high sensitivity with optimum signal-to-noise characteristics—as, for example, where the electrical environment is especially noisy and there is a need for high amplification of low signal levels.
Responding only to the modulated carrier frequency, the 4078 rejects extraneous voltages that can cause errors in DC systems. Excitation is fixed at 3 VAC (rms) at 3280 Hz.
Other important 4078 features include
- remote sensing and regulation of bridge excitation—eliminates errors from temperature effects on cable resistance and yields consistently stable ratiometric measurement, unaffected by possible power-supply drift
- high input impedance preserves the validity of factory calibration, prevents conversion of common-mode to normal-mode signals, and eliminates remaining errors attributable to cable resistance. Allowable cable length has virtually no practical limits.
- user-adjustable phase and symmetry controls for accurate calibration
As with most standard 4000 Series models, both active low-pass filtering and user-selectable digital smoothing eliminate dynamic components arising from vibration, power impulses, etc., that might prevent stable digital conversion or control action.
Simple two-point "zero and span" calibration is provided for the 4078's input channel. In addition, a 59-kΩ, 0.1% shunt resistor is supplied. You can use this resistor—or one or your own—to apply an "equivalent input" for calibration purposes, when the transducer's full-scale mV/V sensitivity is accurately known. The calibration shunt may be switched in and out for either a positive or negative up-scale reading by simple mnemonic commands issued via the Interface Port or optional Model 10P80D Extended Keyboard. (Note that the special "MVV" command used with the Model 4077 DC Strain Gage Instrument does not apply to the Model 4078.) For a 4000 Series instrument based on both the Model 4078 and the Model 4040 Frequency Instrument, and dedicated to the measurement of torque, rpm, and horsepower, see the Model 4K/HP Horsepower Instrument.