Proper equipment grounding is essential to acquiring quality data. Electrical
engineers are taught at an early age to avoid ground loops and to ground equipment.
This can all sound easy, but in the real world it is not always possible to avoid
grounding problems.
Ideal Connection
Ground Referenced Sensor or Signal Source
The ideal situation is where the sensor is not referenced to ground. The
connections should be made as illustrated in Figure . In this
case the differential pair are connected to the respective sensor leads with
one side (typically the - lead) tied to the shield of the shielded twisted pair
cable.
It is critical that one lead be tied to the shield and that the shield be tied to analog ground at the instrumentation amplifier as illustrated in Figure . If the signal pair are left to float (i.e. not ground referenced), then the signal pair can float up near the supply voltage rails on the instrumentation amplifier (typically +/-15 Volts) where the instrumentation amplifier becomes non-linear and sensitive to common mode noise. Modern instrumentation amplifiers have extremely high input impedances which makes it easy for any small current to cause the signal pair to float up near the supply rails.
It is very important that a ground reference be provided for the signal lines to an instrumentation amplifier to prevent the signal lines from floating up near the power supply rails. An impedance of even several megohms to ground will suffice in most cases.
In real life it is not always possible to have an isolated sensor or signal
source. Engineers are taught to avoid ground loops so one is very tempted
to tie the shield of a shielded twisted pair to ground at only one end or
the other to avoid a ground loop. Unfortunately this frequently leads to
much higher noise levels from the instrumentation amplifier and corresponding
loss of accuracy in the measured quantity.
Tests by instrumentation manufacturers and others have shown that in many cases
significantly improved noise levels can be achieved by grounding the shield
at both ends as illustrated in Figure .
In some situations it can also help if a heavy ground strap is run with the signal cabling between the signal source and the instrumentation front-ends.