Custom sig gen to integrate in a project - Page 1
Forum section: electronics / beginners

author: stcoso
author_rank: Regular Contributor
timestamp: February 17, 2021, 02:18:25 pm
content: Hi!I've recently designed and built a LED driver (based on a Howland current source). It works well but it (obviously) needs a signal generator to be driven.This are the requirements:Only positive square waves (and positive DC)Reasonable rise/fall times (25/30ns)Frequency up to 1MHzVariable duty cycle. Obviously at 1MHz i do not expect to be able to go to 1%0-10Vpp but i'll need an amplifing stage so this is not a big issueSo... a DAC? a DDS ic? An MCU with integated DAC? Something else???Thank you in advance

author: mvs
author_rank: Frequent Contributor
timestamp: February 17, 2021, 02:59:04 pm
content: Pulse wave is not essentially analog signal, so why to use DAC or even DDS?MCUs with timer/counter are usually able to output pulse wave. Frequency and resolution will depend on timer clock speed. Some AVR MCUs have 64 MHz timers, more modern ARM MCUs can go over 100MHz.To amplify signal to 10Vpp you can use some fast comparator.

author: stcoso
author_rank: Regular Contributor
timestamp: February 17, 2021, 03:38:15 pm
content: Because I need to some control over the amplitude

author: mvs
author_rank: Frequent Contributor
timestamp: February 17, 2021, 07:34:09 pm
content: You will need quite fast DAC to achive 25/30ns reise/fall times. They are usually not cheap.You can try to build switchable attenuator or R2R ladder DAC using MCU GPIOs and resistors. To amplify signal you will need a fast opamp with slew rate >300V/µs.But all this effort just to make a LED driver work, is a bit ridiculous...

author: stcoso
author_rank: Regular Contributor
timestamp: February 18, 2021, 07:07:25 pm
content: Before judging one should know WHY the effort... just saying.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 19, 2021, 02:28:38 pm
content: Look for high speed PGA and VGA devcies eg VCA822 to drive your current source.Feed your pulses through a high speed comparator to generate fast rise times. eg LTC6752- you may want an attenuator inbetween the comparator and the VGAMake sure you read app notes on high speed comparators- they can be fussy.Good pcb layout and decoupling is a must if you are dealing with fast edges.You'll be going no where on a breadboard. Protype it manhattan style.Loads of choices for comparator and PGA/ VGA. form AD and TI and others.

author: srb1954
author_rank: Super Contributor
timestamp: February 20, 2021, 12:14:29 am
content: Quote from: stcoso on February 17, 2021, 02:18:25 pmWhy do you need 1Mhz frequency and 25ns rise/fall times?Does your Howland current source circuit even respond to signals this fast?

author: Terry Bites
author_rank: Super Contributor
timestamp: February 21, 2021, 06:39:23 pm
content: I've worked with fast current sources for years I'd be truly amazed if any one can get submicrosecond resposes from one. The parasitics will through your caerfully selected resistors in to imbalance.What output impedance are you aiming at?I'd love to see your shematic for it.If you need a fast high current pulse in an LED, dump a capacitors charge through an inductor.https://www.sciencedirect.com/science/article/abs/pii/S1005888516600237

author: ogden
author_rank: Super Contributor
timestamp: February 21, 2021, 06:51:41 pm
content: Quote from: stcoso on February 17, 2021, 03:38:15 pmControl supply voltage of level converter. Then you don't need DAC with fast settling time, any cheap I2C DAC will do the job.

author: Terry Bites
author_rank: Super Contributor
timestamp: February 22, 2021, 04:58:21 pm
content: Lets see the schematic so we can judge if your're input speed requiremets are actually necessary.

author: stcoso
author_rank: Regular Contributor
timestamp: April 24, 2021, 05:28:57 pm
content: Quote from: srb1954 on February 20, 2021, 12:14:29 amI'm sorry for the delay in my responses, I've had too many things to manage. Hope this will not be considered necroposting.Anyway, yes, it does respond quiet well and it's nothing too exotic. It's really similar to the "CAEHCS" illustrated here:https://www.analog.com/en/analog-dialogue/articles/a-large-current-source-with-high-accuracy-and-fast-settling.html

author: stcoso
author_rank: Regular Contributor
timestamp: April 24, 2021, 05:30:55 pm
content: Quote from: Terry Bites on February 21, 2021, 06:39:23 pmCannot use this (classic) approach. I need (optical output) square wave.

author: stcoso
author_rank: Regular Contributor
timestamp: April 24, 2021, 05:53:30 pm
content: Quote from: Terry Bites on February 22, 2021, 04:58:21 pmI do not really understand this attitude... in anycase the schematic is really similar to the one posted above from analog.com . I'm not able to show right now proofs that it works but if really needed I've a video of an early prototype from 2020 driving a LED@1A (500khz duty ~25%) with already decent rise/fall times. The measure was done with a photodiode terminated directly into the scope 50Ohm. I'm attaching a screenshot...