requesting schematic review (newbie warning) - Page 1
Forum section: electronics / beginners

author: ottovon
author_rank: Newbie
timestamp: February 15, 2025, 05:44:50 am
content: Hello all, I am currently in the process of designing my first board as a beginner to electronics and I need advice and direction on how to navigate this project. I’m really new to design and I would love to learn, I am looking for support and guidance!The current schematic is incomplete (amplification for example) but the core function is laid out, I would like to know if my design is flawed. I have left my concerns below and I would greatly appreciate any more concerns.The outputs are not meant to be complementary digital signals, but route the original analog signals to corresponding high and low.Purpose:This comparator circuit is designed to analyze and dynamically route analog voltage signals generated by APDs into separate high and low analog outputs. These outputs will interface directly with an FPGA, reducing the need for complex logic processing and enabling simpler signal handling.Logic:The current design relies on the use of a comparator circuit to drive transistor logic:Two APD analog voltage readings V1 and V2 are input into a comparator as non-inverted and inverted inputs respectively.The comparator outputs two complementary signals, the raw output and an inverted output which serve to drive the bases of the transistorsThe analog voltage signals are then routed into the collector of these transistorAlso note, above transistor q2a there is a mistake there should be no junction. sorry about that!Concerns:Simplification:Is this design an over complication of what can be done more reliably and efficiently by an available off-the-shelf integrated circuit?Analog SwitchMultiplexerFunctionality:trans-impedance amplification of APDspreventing oscillation of the comparator (hysteresis)preventing noise and signal losshigh and low voltage signals of the comparator** voltage regulation (resistance, zener diodes)switching to mosfet transistors to prevent significant voltage drops rather than bjtgate activation of transistors** pull up/down resistorshow can this design be interfaced directly with the FPGA?** converting analog signals using ADCsI think the most likely solution to making a sound design is using analog switch IC, are there any recommended resources one can offer to learn how these function with minimal prior experience?Thank you for taking your time to help me out, it means a lot!I think the most likely solution to making a sound design is using analog switch IC, are there any recommended resources one can offer to learn how these function with minimal prior experience? If it would be possible to stick to my current design I would love to but for the sake of learning and developing my skills I am open to anything!Thank you for taking your time to help me out, it means a lot!

author: Whales
author_rank: Super Contributor
timestamp: February 15, 2025, 06:35:06 am
content: APD = avalanche photodiode?I do not quite understand what the existing problem is that you're trying to solve. You have some APDs, you have an FPGA, are you trying to count discrete photon events or measure stable analog readings (constant light)?

author: ottovon
author_rank: Newbie
timestamp: February 15, 2025, 07:22:23 am
content: yes, I'm not completely well-versed on the function but my school is launching a cubesat and one of the things they want to do is light-based communication. the apd's detect light and the stable analog reading is what we want sent to the FPGA.thank you for the response by the way, I would appreciate any advice

author: Terry Bites
author_rank: Super Contributor
timestamp: February 15, 2025, 03:46:10 pm
content: Is this a laser down link receiver for a satellite for testing on the bench. Not orbiting, I mean. That needs a ground station with some serious optics and super low noise fibre amplifiers and a large telescope no?Why are there two photodiodes and why do they need separate processing? How does the receiver hardware reject ambient visible light and IR? Filters will only get you so far.So, the APD amplifier with the higher output gets routed to an output or if not, ignored? Why does the FPGA need to know about the analog level from the APD amplifier other than as signal strength monitor. Check out logarithmic PD amplifiers. They give you huge range compression making that comparator function simpler to implement. Your ADCs will thank you too.My guess is that you'd get a lot of jitter that will mess with the analog switching and the FPGA inputs. Small noise voltages and tiny glitches at the inputs will flip the thresholds unpredictably. Some “latching” action inside of the comparator output state is a good idea as is some "hysteresis" see for example a TLV3603. A latch function lets you decide when to output test of the input voltage using an external clock (aka strobe) rather than instantaneously and chaotically. External logic is likely the simpler route and probably far less design effort. A couple of comparators and some "glue logic" Why do you think that would be overly complicated.I think all that is needed is a very basic "state machine". Synchronous logic can save your day.Can you digitise both APD amplifier signals separately and do the comparator action numerically inside the FPGA. Once acquired you can massage the numbers any way you like. That sounds like some trivial code to me. I'm still not understanding the need for signal routing of the APDs.The comparator in your circuit is not tied to any reference Zener or not? If you do need a precise reference, use a TL431 or similar. If your power supply is properly regulated, a simple voltage divider may be good enough.With bit rates up in the 10-100s Mb/s, transistor switches may be far too slow. You can get wideband analog switches but the time taken to turn them on or off is not usually that fast either. So, a switch might boast 100MHz (even GHz) of signal bandwidth but only be able to switch on or off at maximum speed of a few uS. Still, have a look at an OPA1S2385IDRCR ic. Pd amplifier and analog switch combo.

author: ottovon
author_rank: Newbie
timestamp: February 15, 2025, 05:47:11 pm
content: thank you for the comprehensive response, I really appreciate all of your help!! Thank you for all of the references, especially the TLV3603 and OPA1S2385IDRCR which seem very useful.Yes it is quite a dedicated ground station and I do believe bit-rates are in the range you mentioned, with regard to why these two APDs are being compared in the first place I'm not entirely sure. I will discuss this with the nerds who know way more than me and get back to you.