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Another Weekly Status Update

A project log for Photo-acoustic Spectroscopy

Creating an at home photo-acoustic spectroscopy system

JasMoHJasMoH 12/04/2017 at 20:300 Comments

Progress from last week:

LNA and transimpedance amplifier schematic complete

Power supply complete

Initial high gain amplifier design complete. Gain selection scheme complete.

Selected mechanical switches for user input.

ADA2200 added, input protection and indication created.

Connector scheme complete

RF can for LNA selected

Design for manufacture/test begun. (test point addition, 0 ohm resistors to isolate or modify circuit, resistor / capacitor component minimization).

Found some real cute 0603 smt test points that’ll let me sprinkle tones of TPs without taking up too much board space for down the line debugging.

Outstanding todo:

PLL - find/ implement/ build. It’s looking like I’ll need to do at least some signal conditioning on the front to accommodate flexible input signals, and may need to build large portions of it due to low frequency range

Line filter - choose a 60Hz line filter implementation. It’s for sure going to be put at the input to the high gain amplifier chain, after the LNA. Debating stand alone active filter, integrating into the high gain amp, others.

Op amp selection review - I have some “lazy” op amp choices. I’m currently using the OP1177 as a “performance” amplifier and the lm358 as the “cheapo” amplifier. The OP1177 is expensive (~$2.50), and the lm358 is kinda crap (1.1MHz GBWP). I’m adding a lot more op amp applications than expected. These include: reference voltage buffer, guard trace driver, and amp overload indicator. I expect that I may need a few more to complete the design, such as an active line filter.  I would like to get a cheaper “performance” amp and a better “cheapo” amp than my current selection.. I’m considering the lm833, which is similar in price to the lm358 and has similar performance numbers (GBWP, input noise) to the OP1177.

Select power connector: just something cheap, dual supply (+-12V), and polarized so it can’t be inserted backwards...

Design for test / manufacture:

Sprinkle more test points

Add 0 ohm resistors for isolating subcircuits, adding flexibility, etc.

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