Model AA72-2 Two Channel DC Strain Gage Conditioner
is a general-purpose two channel conditioner for use with DC-excited load cells, pressure sensors, and any other conventional strain gage transducer employing a 4-arm bridge of nominal 350 Ω or higher, with a full-scale range of 0.75, 1.50, or 3.00 mV/V. The AA72’s advanced design techniques overcome errors that traditionally plague the strain-gage conditioning process. Separate excitation for each channel uses remote sensing of bridge voltage and is slaved to a common System Reference Voltage. The result is consistently stable ratio metric measurement, unaffected by possible power supply drift. Input impedances in excess of 10,000 MΩ are presented to signal leads to eliminate cable resistance as a source of error. Allowable cable length has virtually no practical limits.
The AA72 features selectable per-channel excitation (1, 5, or 10 V-DC). Using low excitation helps reduce gage heating effects in stress analysis of materials with low thermal conductivity. Table 1 gives the full-scale mV/V ranges that correspond to each excitation level. Like most Daytronic “Advanced Analog” (“AA”) cards, the AA72 features optional PROGRAMMABLE LOW-PASS ACTIVE FILTERING for the removal of unwanted high-frequency measurement-signal components. Selectable analog filtering is offered for the AA72 either from 0.2 through 200 Hz in 16 steps (“F1” filtering) or from 2 through 2000 Hz in 16 steps (“F2” filtering). Or, if desired, a fixed filter of either 10 or 50 Hz (F010, F050) for all channels may be specified at the time of order. When the AA72 is used in System 10, its filter cutoff values are set by means of an on-board 16-position switch for each channel.
A nominal ±5-V ANALOG OUTPUT is produced by each active AA72 input channel, for purposes of real-time signal monitoring. Each of these “Auxiliary Outputs” is available on a corresponding mainframe wire-wrap pin. Each output may be individually set, if desired, to represent the prefiltered value of the corresponding input. When such is the case, the output bandwidth is limited only by that of the AA72 card. A convenient shunt calibration technique is provided. Each channel’s shunt resistor may be switched in and out by software command or by means of logic-level inputs through the rear I/O CONNECTOR.