Biasing a Schmitt trigger input pin? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 30, 2018, 12:11:45 pm
content: 74HC14 Schmitt trigger IC that I want to use to convert a slow rising edge, noisy 460kHz waveform of about 4V P to P that feeds a divider IC unreliably to a square wave. I attach the circuit i want to mod below. I want to take the output from Q2 via say a 0.1uF ceramic to the input of the 74HC14. The output of the Schmitt trigger then goes to the input of the divider IC U2. So I think I should bias it mid way between the ground and Vcc? How do you choose divider resistor values though? Two 1K or two 100k resistors would divide equally, but how do you decide the best values please??

author: Zero999
author_rank: Super Contributor
timestamp: January 30, 2018, 01:43:04 pm
content: How about using the 74LVC1404 for the oscillator, rather than discrete transistors?http://www.ti.com/lit/ds/symlink/sn74lvc1404.pdf

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 30, 2018, 02:03:33 pm
content: I would certainly use that, but for now I want to get the PCB I have going without any butchery beyond necessary, plus I can't find a seller in the UK save from Digi-Key with £12 shippingGood call, looks ideal for a second gen build, thanks

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 30, 2018, 02:11:01 pm
content: Hero999 gave you the right solution. What I don't understand is how you can shoehorn a 74HC14 into your PCB layout but cannot replace the discrete transistors.

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 30, 2018, 02:46:47 pm
content: Daughter board, just wanted to get it going properly as near to the OE as possible first! Thanks.

author: Audioguru
author_rank: Super Contributor
timestamp: January 30, 2018, 03:58:01 pm
content: Why add biasing and a coupling capacitor? Feed the output from the oscillator directly into the input of the Schmitt trigger.

author: Chris Wilson
author_rank: Super Contributor
timestamp: January 30, 2018, 04:51:29 pm
content: I don't know, the guy who suggested the Schmitt trigger IC said to set bias 50/50 and AC couple to the output at Q2..... The waveform shown won't reliably trigger the divider, although others who have built this have no issues. it *MAY* be the replacement for the divider I fitted as the original suddenly died may have been a faster one, the markings on the original divider defied Google.Thanks for the reply.

author: David Hess
author_rank: Super Contributor
timestamp: January 30, 2018, 06:59:39 pm
content: It is usually not very critical but the best DC bias point with an AC coupled input will depend on the duty cycle. If the duty cycle is 50% or close, then two equal value resistors will work fine. A pair of 100k resistors is about as high as I would use due to leakage considerations.

author: Damianos
author_rank: Frequent Contributor
timestamp: January 30, 2018, 08:07:29 pm
content: Try a higher value for R17 (for example 1kOhm) and test again the oscillator signal and circuit behavior.This may solve the problem, without any other changes.

author: Zero999
author_rank: Super Contributor
timestamp: January 30, 2018, 08:17:06 pm
content: That waveform looks like it will trigger the 74HC14 directly. If it doesn't, try the 74HCT14, which certainly will be triggered at that voltage range.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 06, 2018, 12:47:21 pm
content: Quote from: Hero999 on January 30, 2018, 01:43:04 pmI am going to try this but there are two different versions of these available to me from Mouser, would you mind taking a look and telling me which I need please? I am hoping it will be the one that comes in a SSOP package as the other one is VSSOP and *REALLY* tiny for hand soldering! The spec sheets seems to just suggest the two part numbers are just different package sizes, but the Mouser stuff suggests they are electrically different...One is here:https://www.mouser.co.uk/ProductDetail/595-SN74LVC1404DCURand the other here:https://www.mouser.co.uk/ProductDetail/595-SN74LVC1404DCTRThanks very much for your help over the years much appreciated

author: danadak
author_rank: Super Contributor
timestamp: February 06, 2018, 02:08:25 pm
content: Biasing the Schmitt, or standard CMOS gate, will subject the gate to noise, andpickup to noise unless you scale the divider to lower Z to avoid pickup. Thatrequirement loads oscillator unnecessarily, and burns power.The tolerance of thresholds on CMOS Schmitt awful as well, so no guaranteewhat duty cycle you will wind up with.Best to create a schmitt of your own with reference and fast amp and precisionR's.Regards, Dana.

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2018, 09:21:52 am
content: Quote from: Chris Wilson on February 06, 2018, 12:47:21 pmThe data sheet is correct. The only difference between the two is the package. They are electrically identical.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 07, 2018, 01:56:48 pm
content: Thanks for the reply, I have got a couple of VSSOP oscillator drivers, and a couple of SSOP ones, luckily, because the SSOP adapter boards that came have the lands for the IC pins too far apart. Despite thinking it impossible for an aging human who likes a drink to solder the VSSOP one to the VSSOP adapter I managed it. Probably luck or a fluke... Geee, these things are SMALLI have it oscillating and feeding its own Schmitt output section and see a nice square wave. I want to drive the 74AC74 flip flop with it, all on the same 5V supply. I measure (on my scope) an output of 2.48V RMS and about 6.3V peak to peak. Is that going to be OK for the 74AC74 please?Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: February 07, 2018, 06:33:27 pm
content: Quote from: Chris Wilson on February 07, 2018, 01:56:48 pmThat should be fine. The peak voltage is higher due to ringing, possibly due to the inductance of the probe's 0V lead. It shouldn't be a problem. Worst case is the ESD protection diodes will kick-in and absorb the spike.

author: Chris Wilson
author_rank: Super Contributor
timestamp: February 08, 2018, 11:25:46 am
content: Quote from: Hero999 on February 07, 2018, 06:33:27 pmThanks Hero999, sounds like your idea is going to do the job with any luck, thanks to you and all who have assisted here, appreciated!