the amplifiers discussed above. The operation of this system can 

 be more readily understood from Figure 3-21, which shows only 

 one sensor channel. The inverting (-) input of the bridge -voltage 

 amplifier has an input resistance on the order of lOlO ohms so as 

 not to load the bridge. The effective input resistance, however, 

 is reduced by feedback to a value equal to R divided by the gain 

 of the bridge voltage amplifier, thus holding the voltage across 

 each sensor line virtually constant. Although this method causes 

 the voltage across the bridge to vary as sensor resistance varies, 

 recall that the conditions for null are independent of the supply 

 voltage across the bridge. In fact, it can readily be verified that 

 the conditions for d.c. balance of the bridge of Figure 3-21 are 

 identical to those for the bridge of Figure 3-19. 



The provisions of individual sensor amplifiers also yields other 

 advantages. Two advantages result from the fact that capacitor 

 C (Figure 3-21) allows us to set the high-frequency response of 

 each channel. We could thus increase the effective time-constant 

 of any sensor channel at will by suitable choice of C . Although 

 we do not, at present, plan to use this option, it could be advan- 

 tageous in avoiding aliasing if extremely long measurement inter- 

 vals were ever to be used. The main advantage of the high-fre- 

 quency roll-off is simply that, even when the shortest Measure- 

 ment Interval is used, the time constant provided by C can still 

 be set so that 60-Hz noise (and other electrical interference likely 

 to be floating around any substantial installation of electronic equip- 

 ment) will be considerably reduced. 



The inclusion of individual sensor amplifiers provides, for each 

 channel, a voltage signal representing instantaneous sensor resistance 

 (after low-pass filtering performed by the bridge voltage amplifier 

 with C ). This voltage is available at point P in Figure 3-21. It 

 is therefore possible to monitor any (or all) of the seven signals 

 from the Sensor Cable with an analog device, such as a stripchart 

 recorder, without disturbing normal operation of the Data Acquisi- 

 tion System. If, for example, significant mechanical oscillations 

 were to develop in the cable, it would be possible to accomplish 

 continuous, analog recording of pressure (i.e., depth) and tension 

 data without sacrificing normal digital measurement of all seven 

 inputs. A photograph of the digital measurement bridge is given 

 in Figure 3-22, The complete Data Acquisition System is shown 

 in Figure 3-23. 



217 



