Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
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?
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.
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.
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 :-)
Stage 2is an inverting band pass filter, but what doesStage 1do? And how does the diodes help to avoid clamping? \$\endgroup\$