Being a phase-sensitive carrier-demodulator instrument (rather than fully DC), the Model 3578 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 3578 rejects extraneous voltages that can cause errors in DC systems. Excitation is fixed at 3 VAC (rms) at 3280 Hz.
Other important 3578 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-settable phase and symmetry controls for accurate calibration
- selectable active low-pass filtering smoothes unwanted dynamic signal 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 3578's input channel. In addition, a 100-kΩ, 0.1% shunt resistor is supplied. You can use this resistor—or one of 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 via simple commands issued to the RS232/RS485 port.
[NOTE: Unlike the Model 3570 DC Strain Gage Conditioner, the 3578 employs a special Conditioner Connector, which requires direct solder-terminal attachment of cable leads, and does not permit the input of logic-level signals for control of shunt calibration. Also the Model 3578 cannot be calibrated by means of the software MVV command.]
The 3578 offers all standard 3500 Series features, including
- selectable digital smoothing
- custom 15-segment linearization
- HI/LO limit monitoring with logic I/O
- scalable analog output
- analog +peak capture
- automatic tare offset
- digital track and hold
- either single-node (RS232) or multinode network (RS485) communications
The instrument can be completely set up and operated either through the front-panel keypad or via simple mnemonic commands received from an external computer or terminal through the RS232/RS485 Interface Port.