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INADVERTANT GROUND:
-
_ _ _ A_S_A_F_E_T_Y_C_O_N_S_ID_E_R_A_TI_O_N---J[IJ
SENSOR
CURRENT LOOP TRANSMITTER
CONTROL
DEVICE
RECORDING
DEVICE
R
9000
SERIES 200
COMPUTER
SEE SPECIFICATIONS
Figure A-I. Be cautious using the ON/OFF switch.
This circuit is an example of some of the circuits you may be considering attaching your ADC to.
Please keep in mind any input's isolation from ground is only 1000 (one thousand) ohms when the
computer is turned off or IF POWER TO THE COMPUTER FAILS. If the resistor R is not sig-
nificantly smaller than 1000 ohms then the ADC will become a major secondary path to ground.
The resulting change in the voltage drop across R will affect other devices attached across R. In cer-
tain cases the effect could cause dangerous situations to develop because people or control devices
would react to faulty data.
There may be many other circuits in use which may cause a similar problem. Please consider the all
characteristics of the HP 9 8640A before adding it to a circuit.
A-I
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