3 ms·

ah hadn't noticed those - the big problem with this circuit is that if need to need to be able manufacture something it needs to be reliably repeatable - I've f

ah hadn't noticed those - the big problem with this circuit is that if need to need to be able manufacture something it needs to be reliably repeatable - I've found I need to buy a reel made by a particular manufacturer and tweek the circuit just so.

magicalhippo 3y ago
Ah I guess the manufacturing-dependent parasitics plays a role, given the floating base and such. Fortunately they're quite cheap tho :)
cushychicken 3y ago
A roll of 5k MMBTs from a sole source would probably cost you less than $500 if you looked around a bit. Another option: characterize a bunch of different vendors, then selectively populate resistors for proper biasing based on test results from each vendor. What are you tuning? The bias resistor? Edit: wow you can do even better than I thought. You could get 30k MMBTs for $500!! https://www.digikey.com/en/products/detail/diodes-incorporated/MMBT3904-7-F/814494 https://www.digikey.com/en/products/detail/diodes-incorporat...
- Taniwha 3y ago
  I tune the bias on the output stage (another 2N3094) - the goal is to get ~50% 1/0 when sampling - note that there's a software time domain filter in there as well - we're measuring after that. If you're making small runs buying a reel that doesn't work is a risk, I've already made the decision to switch to using a zener for the next production run
  - cushychicken 3y ago
    Damn, you could have fixed this with a lot less hassle and hardware revision by biasing that output comparator properly with a second resistor to GND in addition to the 3.3V pull-up. Happy to show you what I mean with a quick sch sample if you give me a contact email. My email is the same as my username but at Gmail. Only mention this because: - MMBTs are dirt cheap, - they’re even cheaper when you buy at qty, and your device uses three, whereas you’d only have a single zener and single MMBT otherwise. End of the day, though - your board, your design, your decision.
    - Taniwha 3y ago
      I'm actually switching to a zener for the next rev
      - cushychicken 3y ago
        Hear you. Good luck with the next run.

magicalhippo 3y ago

Ah I guess the manufacturing-dependent parasitics plays a role, given the floating base and such. Fortunately they're quite cheap tho :)

cushychicken 3y ago

A roll of 5k MMBTs from a sole source would probably cost you less than $500 if you looked around a bit. Another option: characterize a bunch of different vendors, then selectively populate resistors for proper biasing based on test results from each vendor. What are you tuning? The bias resistor? Edit: wow you can do even better than I thought. You could get 30k MMBTs for $500!! https://www.digikey.com/en/products/detail/diodes-incorporated/MMBT3904-7-F/814494 https://www.digikey.com/en/products/detail/diodes-incorporat...

Taniwha 3y ago
I tune the bias on the output stage (another 2N3094) - the goal is to get ~50% 1/0 when sampling - note that there's a software time domain filter in there as well - we're measuring after that. If you're making small runs buying a reel that doesn't work is a risk, I've already made the decision to switch to using a zener for the next production run
- cushychicken 3y ago
  Damn, you could have fixed this with a lot less hassle and hardware revision by biasing that output comparator properly with a second resistor to GND in addition to the 3.3V pull-up. Happy to show you what I mean with a quick sch sample if you give me a contact email. My email is the same as my username but at Gmail. Only mention this because: - MMBTs are dirt cheap, - they’re even cheaper when you buy at qty, and your device uses three, whereas you’d only have a single zener and single MMBT otherwise. End of the day, though - your board, your design, your decision.
  - Taniwha 3y ago
    I'm actually switching to a zener for the next rev
    - cushychicken 3y ago
      Hear you. Good luck with the next run.