Electronic – Connecting the Shield of a strain gage cable connector to PCB with ADC

adcshieldingstrain-gagewheatstone-bridge

I have a half-bridge strain gage sensor (pre-assembled product) with a 30cm cable, which I am trying to interface with my PCB containing a 24-bit ADC.

The connector at the cable end toward the ADC (i.e., the end away from the strain gage) has the following four pins, based on my reading of the resistances:

  • Pin 1: V_Excitation
  • Pin 2: Doesn't seem to be connected to anything
  • Pin 3: V_signal
  • Pin 4: Ground

All good so far.

However, in addition, there is the metallic shield constituting the body of the connector.

Being new to strain gages and ADC measurements, I am unsure of the shield's significance — I presume it has something to do with minimizing interference-based offsets. (Also, I don't want to break open the cable to see what's inside but I'm guessing it is a standard twisted-pair cable.)

Question A: What role exactly does the shield play in a strain gage / ADC application?

Question B: And thus which of the following is good practice to follow with the shield? :

  1. Just isolate shield from GND (i.e., leave shield alone) ?
  2. Or connect shield directly to the Ground plane on my ADC pcb ?
  3. Or place 4.7 nF Capacitor and 1M resistor between shield and Ground, as suggested for USB connector shield in this question ?
  4. Or connect shield to metal chassis of my enclosure only, as suggested in this question ?

Best Answer

As a general guideline, it is preferable to connect any cable shields to the metal chassis (not PCB ground), at just one end. To keep things simple when multiple shielded cables connect out from some central device, the shield connection should be done only at the "hub" device, and left open at the "spoke" devices.

Similarly, for a chain of devices connected by shielded cable, each shield ought to be connected to the chassis of the upstream device, and left open at the downstream end. Yes, this does mean that the shields of different links of such a chain might be at different potentials, depending on how well earthed the individual devices are, but this is generally not a problem.

A basic, inexpensive addition incorporated in many consumer device cables today, is a clamp-on ferrite bead or "split bead" at each end of the cable, close to the connector. This reduces high frequency EMI off the shield with minimal complexity. A good document about such RFI beads is here.

Note that the chassis is usually connected to "earth" of the location, not to ground of your circuit board.

Of course, this is a simplification of a fairly complex subject, but it serves the purpose for designs at frequencies where surface effects do not predominate. As frequency of signals involved rises into GHz, other factors need addressing.

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