LTC timerblox higher frequency hack possibility? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: August 26, 2018, 03:06:10 pm
content: http://www.analog.com/media/en/technical-documentation/data-sheets/6990fc.pdfSo this thing does not recommend you go above 2MHz because I guess it has some kind of internal problem of crosstalk on the ADC (page 19), it says the 'divide' input ADC will 'suffer'. I am guessing there will be some kind of noise problem that eventually leads to the DIV code being changed while its operating leading to very erratic operation.I want more then 2MHz.I was thinking, maybe its possible to fry the ADC front end a little bit so it works at a lower impedance then decrease the impedance of the voltage divider, that should increase its noise tolerance right? It's a really dirty solution but I wanted to get more for less.Other then, any ideas? Or do you think the problem lies with internal power distribution? Could you see any kind of trick you can do with current levels or impedance to make it adjust wider with some kind of degraded parameter that can be fixed with external parts or that I just don't care about?I also thought it might just like, end up shifting the bins of the ADC (just how it ends up being) so you can maybe tune the voltage divider at high frequencies to put it in some kind of stable region? I mean its a 4 bit ADC. Is it possible you can have the 0-1.25 volt range turn into a 0.33-1.5 range under extreme internal EMI (maybe due to RFI rectification), so you can shift the most stable location to the midpoint of 1.5/.33 rather then 0/1.25 like by design to maintain noise margin? Or feed noise into it from an oscillator to perhaps cancel something out internally (i see this as winning the lottery though, and it would be serious PCB litter)I want like 0-7MHz resistor controlled oscillator out of it.And add some filtering, shielding and heavier supply decoupling to help it run on shitty noise marginsAlso on page 4 it says to limit DIV pin capcitnace so it settles quickly. Why would it matter on a fuckin 4 pin mode set ADC?Are they just saying that to ensure stable start up? What if you don't care about stable start up.. I mean is their control system that bad that its going to latch up or even burn out if the ADC codes cycle on startup? I don't care about a hiccup. It says its 'glitchless' anyway, I can wait 5 minutes for it to settle down, my application does not care at all.

author: senso
author_rank: Frequent Contributor
timestamp: August 26, 2018, 03:51:30 pm
content: The LTC6990 is built around a master oscillator with a 1MHz maximum frequency.You cant divide for less than one, so how do you expect to get more than 1Mhz frequency?Stick a PLL on the ouput and multiply the value, or start with a PLL from the get go..

author: coppercone2
author_rank: Super Contributor
timestamp: August 26, 2018, 04:18:14 pm
content: that's not what the data sheet says, read page 19Pushing ISEToutside of the recommended 1.25μA to 20μArange forces the master oscillator to operate outside of the62.5kHz to 1MHz range in which it is most accurate.Theoscillator will still function with reduced accuracy in itsextended range (see the Electrical Characteristics section)they even say in the first page:is a precision silicon oscillator with a pro-grammable frequencyrange of 488Hz to2MHz.I am not using it in a control loop so I don't care if its monotonous or the set voltage begins to get nonlinearif you read it further (PG 19) you will get to my point where they say the DIVcode is the crux of the system.I just wanna know about the chip I know there are 500 ways to get a higher frequency.At least read what I wrote, I referenced the datasheet and everything........... but I got a dodo responseI also called it a hack in the name of the thread.

author: coppercone2
author_rank: Super Contributor
timestamp: August 26, 2018, 04:27:51 pm
content: It's a recommendation. I don't know enough about IC's to form a good opinion.