EE 311: Electrical Engineering Junior Lab Grounding and EMI ...

The circuit shown below illustrates a common situation: a sensor (thermocouple, strain gauge, etc.) is located some
distance from it's amplifier. There is some ground potential difference due to stray current flow. Connecting the
common point of Vi to ground grid 1 and the polarity side of Vi to a simple amplifier would cause amplification of
both the sensor voltage and the ground potential difference. Reconnection of the Vi common to ground grid 2 could
remedy this problem- but now the sensor is floating with some potential difference to the local ground voltage, which
can be a safety problem. The third approach we will see is the use of a differential amplifier, which allows Vi to be
tied to local ground but amplifies only Vi and not the ground potential difference.
Figure 1. A sensor at one location to be connected to an amplifier at another location.
Figure 2. Proto-board 1 schematic
On your laboratory bench, you are provided with a variety of equipment. Each piece of equipment is tied to the
bench ground wire (the green wire) through the third prong of the plug. In addition, the electronics common point of
some of this equipment is tied to the case, and hence to the green wire. In particular, you can count on any standard
oscilloscope to have the common lead tied to case. Also on this bench, the counter and the signal generator are
reference to case. The measurement terminals on digital voltmeters are typically NOT tied to case, and this is true
for both DVM’s you will be using in this experiment. You will also be using a signal generator from the instrument