40MHZ voltage compactor help - Page 1
Forum section: electronics / projects

author: BiOzZ
author_rank: Regular Contributor
timestamp: July 19, 2013, 10:15:34 pm
content: Ok because this is a personal project i have no problem saying i know nothing about working in these frequenciesbut i am using a 1hz-40mhz DDS chip that outputs a sine wave ... im going to compare it to a pot to get a rough duty cycle square wave controlled DDS func ... i dont need anything close to the 40mhz infact thats more of a luxery as im using 2 of the DDS chips one up to 40mhz with 1hz resolution and another up to 4mhz with 0.05hz resolution ... i have the 4mhz down and it works all nice ... i would probably just use the 40mhz as a clock gen but anywhofor the 40mhz i planned on using a high speed 300mhz+ operational amp that can drive like 500ma or so and use that as the output with an adjustable input voltage and offset ... but would a compactor be better to use? im not looking for fucking insane switching time but im looking to atleast at 30khz to have it look like a square wave atleast ...i will be driving it with + and - 12v and the input is 1.5v-3v so i plan on using the same opamp to amplify and handle the offset ...any ideas for what chip to use? should i use a highspeed comparator and a pullup? i never had an issue using opamps as compactors if im not doubling up any outputs but i only work on SMPSs at work ....

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 19, 2013, 10:51:59 pm
content: Hi, sounds interesting.Are you talking about comparators not compactors? I only know of compactors in that they exist on garbage trucks.Otherwise I dont have a recommendation but will be interested to hear what gets suggested.

author: Rerouter
author_rank: Super Contributor
timestamp: July 19, 2013, 11:18:54 pm
content: i believe i know the analog devices chips your using, if you just want a plain hard square wave out with no amplitude control, all you need is to feed in your set point to the other input of the op amp / comparator and as it crosses the threshold will slam from one rail to the other,comparators are better suited than op amps for this as they are made to slam rail to rail, where op amps are more made for a wide control range sometimes saturating at the rails,as for what to use, as you are wanting to pull and push a decent whack of current i would recommend a fast but lower current op amp followed by a simple push - pull amplifier (pnp and npn) yes you may loose 1 odd volt off each rail but it makes dissipating the heat easier, and you can equally use the base resistances as a rather nice form of current limiting,---------------------------------------------------------------------------------------------------------------now that i furthur read it, you want to control the offset.. well now if it was just a simple comparator it would be slamming rail to rail when one input becomes higher than the other which leaves little to offset,however if you both want to control the amplitude of the output, as-well as its offset, (e.g. 3V p-p square wave centered at -2V) it is still possible, but requires an op amp,for this you begin by summing in a offset voltage, in your case likely input of 3V fed in through 10K, and your offset voltage of -1.5V (for zero centered waveform) summed in through another 10K, this can either be off another pot or buffered through another op amp, and i would recommend buffering it if you wanted a linear response in offset, with a range depending on how far above zero you want it,next up would be the gain, this can be as simple as a potentiometer between the output and the inverting input with another resistor between the inverting input and ground, you size this likely for a gain of 8 (+-1.5V into +-12V) allowing you to go from full rail to rail or decrease and offset as you please,-------------------------------------for your final point, use a push pull comparator, only some are open collector type that need a pull up resistor,

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: July 20, 2013, 12:09:30 am
content: You forgot to mention what you intend to clock?My initial suggestion would have been 1/6th of a 74HC14, a pair of 100K resistors for a voltage divider, and a capacitor for DC blocking. Add source of 3.3-5V DC for the 'HC and voilá, instant variable HCMOS clock source.Then I noticed the 12V @ 500mA and had my doubts.

author: Marco
author_rank: Super Contributor
timestamp: July 20, 2013, 12:14:45 am
content: Opamps don't generally like being used as comparators, they are not designed for high voltage differences between the two inputs ... the transistors go into saturation and become slow (or worse, input protection simply blows up). You could try to design around that, but you could also just get something like a LT1719 comparator.