12/5/17 by Dave

Sensor module with RI issue

A client reached out for help on a radiated immunity issue. We resolved it without ever seeing or testing the part.

Analog sensors can be particularly susceptible to RF interference. This high volume automotive sensor used an op-amp to amplify the signal of a photodiode. The output signal shifted when exposed to certain radio frequencies, causing a radiated susceptibility failure at the certified lab. This is the baseline output of the sensor when subjected to RF interference:

Baseline

Since the photodiode can potentially rectify RF and produce a DC offset, that was immediately a suspect. Additionally, op-amps require good power supply decoupling for reliable operation.

photosensor

After reviewing the design, we recommended two changes: Adding a capacitor across the op-amp supply pins and a via to tie the photodiode anode to the op-amp VSS pin. The capacitor improves high frequency power supply decoupling to the op-amp and tying the photodiode anode to the op-amp Vss prevents rectification to the inverting input of the op-amp. Adding the cap might be obvious, but why does the via fix the rectification? The best way to understand it is to see the op-amp VSS as the 0V reference and replace the photodiode "ground" with an RF noise source. The noise source will produce a DC voltage across the diode resulting in a dip in the signal output. Tying the two points together pegs the photodiode anode to the 0V reference so there is no RF noise source to rectify. The client was able to mockup the proposed changes to the design and retest in their in-house lab.

This is the new test result after adding just the decoupling cap:

Plus Cap

This is the new test result with both the decoupling cap and the via:

cap and via

We were able to import the design files, make the modifications and send back fab files to make the production design. The revised design passed immunity testing in the certified outside lab and was approved for mass production.

By: Dave


Dave is an expert in circuit design, PCB layout, and EMC - skills that have served him well consulting for companies big and small for the last decade. In addition to 10+ years as an electronics consultant, Dave spent his career in the automotive industry from engineering to upper management. Dave is a registered Professional Engineer (Mechanical). He also holds a MS in Computer Science (Laurence Tech 06') and a BS in Mechanical Engineering (GMI 86').

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