Analog pulse discriminator and digital converter - Page 1
Forum section: electronics / projects

author: cinbody
author_rank: Newbie
timestamp: March 28, 2016, 09:54:27 pm
content: Hello All,I don't have a lot of analog experience and would like opinions on this circuit.General parameters and goals:Take a gaussian-ish shaped input pulse on the order of 100-200 ns wide at 50-200 mVreceived randomly and at rates "as high as possible", maybe high kHz to low mHz range.Bring this pulse up to a sharply defined digital levels for input to a uC for the purpose of counting.I have a plan on the uC side, but I am in unknown territory on the preamp side, so attached is whatI have scrounged up and simulated. I used a PWL file as input to the simulation.R1 and R4 are adjusted to set the discrimination level any where from accepting 5-6 mV input pulsesup to ignoring everything but 130 mV input pulses. The result is a 3.3 V digital output with a minimumoff time of 60 ns which I think the uC will handle.My thinking is that multiple low gain stages of LMH6612 (vs only a couple at high gain) willgive me the best discrimination range while maintaining good separation between final digitalpulses created by the comparator at the end.Am I way off base? Gross conceptual errors? Better ways of doing this?I looked at analog front ends, delay lines and a handful of other conceptual paths to takeand none seemed to work as well and as cheap as this.

author: moffy
author_rank: Super Contributor
timestamp: March 29, 2016, 12:28:55 am
content: For the comparator put a little hysteresis around it(google the term if you don't know it). If you need gain then do it in as few stages as possible, simplicity, cost and noise reduction. You might want to put a pulse stretcher/monostable after the comparator to extend the pulse so the micro can see it.

author: cinbody
author_rank: Newbie
timestamp: March 29, 2016, 03:11:01 am
content: Ok, thanks. I'll go back to an earlier version and play with it some more.Quote from: moffy on March 29, 2016, 12:28:55 am

author: exmadscientist
author_rank: Frequent Contributor
timestamp: March 29, 2016, 05:55:05 am
content: Let me guess... PMT pulse discriminator?Seconding that you generally want as few gain stages as you can get away with.A retriggerable monostable ('123 or similar) is virtually a must on the logic output. You need the retriggering if you want to be able to deal with pulse pile-up properly; even then, careful design is important.R14 and R15 should be replaced with a single potentiometer wired as a potentiometer, not a rheostat. (Analog SEEKrets, Chapter 5 has quite a few things to say about pots and how not to wire them up.)You probably want to rig things up so that you need only adjust asinglepotentiometer to set the discriminator level. It's tricky enough to setoneknob to the correct level for your signal, much less four. I'd keep the gain fixed and adjust only the comparator's threshold, if that would meet your other goals for the circuit.

author: tggzzz
author_rank: Super Contributor
timestamp: March 29, 2016, 07:17:37 am
content: Choose an amplifier that behaves as you expect when it is overloaded, i.e. the output is as high or as low as it will go. Some suffer "phase reversal" or can take a long time to exit the overloaded state.Read "The Art of Electronics" by Horowitz and Hill for many practical tips.

author: cinbody
author_rank: Newbie
timestamp: March 29, 2016, 04:40:21 pm
content: Quote from: exmadscientist on March 29, 2016, 05:55:05 amYes,.. thanks to all of you for this great advice. It really helps to narrow down the massive field of possibilitiesthat one gets presented with when taking on something new. I have plenty of things to study now

author: danadak
author_rank: Super Contributor
timestamp: March 29, 2016, 08:26:35 pm
content: This part does not CM inputs to either rail, so be careful what you feed it.Regarding phase reversal, contact TI FAE to get an answer to that. Most modernOpAmps no longer have this problem, but if not stated in datasheet a check withvendor in order.Why not a single fast comparator ? With DAC out of UP to control threshold.....http://www.ti.com/lsds/ti/amplifiers-linear/comparator-products.page#p116=Push-Pull&p78=In&p2192=&o4=ACTIVEThere are 4.5 nS parts here with R-R inputs, like this parthttp://www.ti.com/lit/ds/symlink/tlv3502.pdfRegards, Dana.