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I have made a reverse engineering about some medical devices nowadays that has pressure sensor. What is the purpose of extra excitation (Q2). When is Q2 on out rise about 150mV. Best Regards

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[Disclaimer: I haven't seen such arrangement before. Below is an educated guess.]

This looks like a self-test feature, rather than extra excitation. When Q2 is conducting, R11 appears in parallel with R3. That introduces a small imbalance in the bridge. If the measurement circuitry detects this imbalance, then self-test passed successfully.

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    \$\begingroup\$ I use a similar arrangement on bridge and strain gauge channels to create a temporary offset that can be used to prove the system works end-to-end. I can confirm your educated guess is probably correct. \$\endgroup\$ – Andy aka Apr 1 '15 at 7:26
  • \$\begingroup\$ That's flipping brilliant. Why the heck didn't they teach that in my sensors class? \$\endgroup\$ – Sean Boddy Apr 1 '15 at 8:12
  • \$\begingroup\$ It may also be for calibration : if the equipment has adjustments for zero and gain, you may adjust the zero trimmer until the meter reads zero, then press the button and adjust gain until the meter sits on its calibration mark (red line, for example). \$\endgroup\$ – Brian Drummond Apr 1 '15 at 10:32
  • \$\begingroup\$ Button is a symbolic. In fact, controlled by a microprocessor. 5-6 pulse (on-off) per second. \$\endgroup\$ – elektro Apr 1 '15 at 10:35

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