INA321: ECG channel of instrumentation

Hi Vincent,

I am attaching the schematic from the datasheet for reference:

Using a shielded cable helps to significantly reduce the noise that will couple into your signal chain from the probes, however there is still parasitic capacitance between the sleeve of the cable and your inputs. This capacitance provides a path for leakage current between the shield and input terminals. This leakage current is proportional to the voltage across this capacitance, so you can effectively eliminate it by driving the sleeve to the same common-mode potential as your inputs. Notice in the schematic the INA inputs are lifted to 2.5V through the two 2MOhm resistors. This 2.5V reference is then buffered, and you will notice that the output of the OPA336 buffer is labeled as "Shield Drive." This is the potential you should connect your shield to. 

If you are using a single supply configuration, then yes, Vcc is V+ and V- would be ground. 

While we are not experts on the subject, we do have some material available on ECG applications that you may find useful. You can find it here: e2e.ti.com/.../2437.ecg-heart-signal-acquisition

Hi Zak,

Thank a lot for your response it allows me to clarify a lot of things. I have a last question related to the shield. For a PCB the mass plane is connected to V_R again or to GND?

Regards,
Vincent STRAGIER

P.S. : Sorry for my English I live in Belgium and my first language is French.

Hi Vincent,

No problem, that's what we're here for! I'm not quite sure what you mean by PCB mass plane, but I think you are referring to your power planes. If so, then yes the plane that connects to the negative supply of your amplifiers should be tied to ground.