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I read this application note from Texas Instruments to learn something about the design of an analog frontend for PIR sensors, but I don´t know the function of the first OpAmp circuit.

What is the purpose of the two diodes in the feedback loop of the OPAmp?

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    \$\begingroup\$ From the document. Feedback diodes, D1 and D2 provide clamping so that the op amps in both filter stages stay out of saturation for motion events which are close to the sensor. \$\endgroup\$ – Peter Smith Feb 23 at 11:19
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    \$\begingroup\$ Okay and that is the function of the OpAmp in this case? The OpAmp from Stage 2 is an inverting band pass filter, but what does Stage 1 do? And how does the diodes help to avoid clamping? \$\endgroup\$ – Kampi Feb 23 at 11:25
  • \$\begingroup\$ First of all clamping, also it's a buffer and a preamplifier for the crazy output impedance of the sensor. I guess that's not a run-of-the-mill opamp but some fet input precision thing \$\endgroup\$ – Lorenzo Marcantonio Feb 23 at 13:07
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The op-amp, with the aid of the external parts, acts as a bandpass filter and high-gain AC amplifier, with clamping.

The diodes lower the gain for large (over 500mV-ish) output to about 1. AC gain in mid-frequency band is approximately 1+R13/R11 = 221.

C34 and R13 set the high frequency cutoff at 1/2\$\pi\$fC = 10.6Hz.

R11 and C33 set the lower frequency cutoff similarly at 0.7Hz

This frequency band (0.7 to 10.6Hz) is related to how fast we are expecting a warm object to be moving across the facets of Fresnel lens of the detector.

The detector itself is an AC device, incidentally, and will have its own upper and lower cutoff frequencies and an output bias voltage that may change with ambient lighting and supply voltage. The output is typically the source of a JFET follower. We also want to reject 100/120Hz noise from mains-powered ambient lighting.

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To me, it looks like a non inverting amplifier with a gain of 220. C34 is there because of R13 being a large value resistor. Diodes for clamping and C33 i guess because the input is AC, so no DC get amplified.

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Agree - the diodes are clamping. May I add to what @Hedgehog writes that it is AC gain only. The principle of a PIR sensor is to have a sensor which is sensitive to infrared light but its sensitivity is not uniform in space. Which means that the signal received varies depending on where the object is.

Look at it this way: Imagine the PIR sensor chip is replaced by a light source. The pattern illuminated will have several bright and dark regions - bright means areas where there is good sight for the PIR sensor. Dark means poor sight.

When an object traverses these zones, the detector will see a varying signal. A varying signal means AC signal.

So now you know how to cheat a PIR sensor: Move so slowly that the signal variation stays below the lower cut-off frequency of the bandpass filter. Or so fast that frequency is above the higher band frequency :-)

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