LTC6905IS5-80 Oscillator - Drive DIV pin - Page 1
Forum section: electronics / beginners

author: 2X
author_rank: Frequent Contributor
timestamp: August 12, 2025, 10:32:47 am
content: Hi,on the datasheet of the LTC6905IS5-80 Oscillator shows at page 8 how to select the output frequency of the oscillator without Three-State buffer but with two NOT gates.My query is how to achieve the Hi-Z/open switch with this circuit at page 8 and select 40MHz output?Datasheet:https://www.analog.com/media/en/technical-documentation/data-sheets/6905fd.pdf

author: hli
author_rank: Frequent Contributor
timestamp: August 12, 2025, 11:11:28 am
content: From the manual (in my version on page 5 - look at the pin descriptions):QuoteThis is exactly what this circuit provides.

author: 2X
author_rank: Frequent Contributor
timestamp: August 12, 2025, 11:29:26 am
content: Quote from: hli on August 12, 2025, 11:11:28 amThanks for the reply.When we applied:0 0 -> DIV = 5V (80MHz)0 1 -> DIV = 2.5V (40MHz)1 0 -> DIV = 2.5V (40MHz)1 1 -> DIV = 0V (20MHz)From the below (page 5 on datasheet):To detect a fl oating DIV pin, the LTC6905 attempts to pull the pin toward midsupply.This is realized with two internal current sources, one tied to V+ and Pin 4 and the other one tied to ground and Pin 4.Therefore, driving the DIV pin high requires sourcing approximately 15μA.Likewise, driving DIV low requires sinking 11μA.When Pin 4 is floated, it should be bypassed by a 1nF capacitor to ground or it should be surrounded by a ground shield to prevent excessive coupling from other PCB traces.Could you explain me how this floating detection is achieved/related with the two current sources and 2.5V? Also, at the block diagram there two current sources of 15uA but on the text says 15uA for sourcing and 11uA for sinking.Why 11uA for sinking?Why not to use a Tree-State buffer where is just a component and there is no need of the two resistors (one pull-up and one pull-down)? The driving with the two NOT gates has any advantage?Datasheet:https://www.analog.com/media/en/technical-documentation/data-sheets/6905fd.pdf

author: AnalogTodd
author_rank: Regular Contributor
timestamp: August 12, 2025, 02:04:52 pm
content: Quote from: 2X on August 12, 2025, 11:29:26 amThe driving with the two NOT gates allows you to switch frequency with logic inputs. If you are concerned about leakage on your board, then using two 10k resistors to create a divider to the DIV pin will accomplish the same thing.The potential difference between the sourcing and sinking currents required may be there is an input impedance of the three-state detect circuit. Rather than show detailed internal circuitry, simplification in a block diagram makes it easier for the end user.

author: Andree Henkel
author_rank: Regular Contributor
timestamp: August 12, 2025, 03:27:47 pm
content: Quote from: 2X on August 12, 2025, 11:29:26 amsure, you could just use a tristate buffer.there you have two logic signals too, the "inhibit" that puts it into tristate or buffer mode and the inputProblems may arise, if you have longer traces from your buffer, and crosstalk / leakage into them, they are susceptible as the Net is high impedance. The dual Buffer configuration provides lower impedance and thus the net is less susceptible.Of course there are proper layout techniques to prevent crosstalk and leakage.I used the LTC6905 with DIV at V+, version with external resistor to set frequency, because we had three versions with different frequencies, other than the preconfigured ones.There is a potential problem which may or may not concern you.LTC6905 produces a spur frequency at its output in addition to the desired frequency, depending of DIV setting this spur isDIV at V+: Fout/64; DIV floating it should be Fout/32 and GND Fout/16actually the operation is, there is an oscillator operating at this Frequency equal to the spur, it is fed into a PLL (Phase Locked Loop) which generates the actually desired frequency. But the base oscillator frequency leaks to the output.We needed to implement filters that filtered the (unwanted) spur.