Hello @slava,
The HCNR200 contains a LED and two photodiodes. Linearity is increased by wrapping one of the diodes into a feedback loop. For your schematic:
- LED is the emitter for the optocoupler
- PD1 is the sensing (feedback) diode
- PD2 is the output photodiode
Note that this project is adjacent to a 2018 blog post.
Thoughts:
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The blog posts states " It only work with positive input voltages (VIN ≥ 0) regardless of single or dual supply of ap-amp." This may reflect the indented 4-20 mA industrial applications typically associated with the HCNR200.
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Eliminate the Q1 and D1. The op amp should be able to drive the LED directly as suggested in Application Note 5394 (Avago) as shown in the schematic.
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Note that the current through PD1 and PD2 should be identical. Note the values of R1 and R5 in the schematic below.
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When using BNC we are in a 50 Ω system. Be sure to provide a termination resistor directly to ground at the input BNC connector. Reflections in the BNC cable can cause problems at higher frequencies.
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Please provide the schematic for your circuit as the blog post diagram if very difficult to read as it does not follow a conventional left to right flow.
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With regards to frequency:
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Since PD1 and PD2 are identical, you could measure the frequency dependent voltage on PD1 and compare it to PD2. It should be the same.
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Assuming a non-linear frequency response (but compensated by the PD1 feedback loop) verify that the PD1 feedback signal isn’t “in the rails.”
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Please let us know if you were able to achieve your high frequency requirements.
Sincerely,
Aaron
