Could an analogue electronics expert please explain to me how the circuit below works (circled in red)?
The circuit was displayed on instructables.com as a bioimpedance measurement pcb.
I know that a transcounductance amplifier acts as a constant current - voltage driven - source, but since there are 2 electrodes that lead to the human body (source electrodes), and the INA128 amplifies the difference between the 2 electrodes that are glued to another part of the body (destination electrodes), how exactly does the voltage difference happen (the excitation signal is an AC one)? How does the Rprotect between the constant-current lead and constant-voltage work?
It's a tricky one and I'm not really good in the analogue world!
The TL072 is an opamp. Transconductance just
means "variable gain." The "subject's" body
resistance is used to control the opamp's
circuit gain. It looks like one strange idea to
me.
Herb
This is a four terminal AC resistance measurement. Current is passed through the body and the voltage drop is measured. Rprotect protects the circuit if the electrodes are not attached.
"Bioimpedance" is a made up term, which suggests that some amount of hokum is involved, as we expect from Instructables.
I would have to speculate that electrode #1
is the source of the stimulation current.
Without shielded and probably guarded sense
leads to the response electrodes, you are
not going to have any success with this
thing!
