Confused by switching level of M74VHC1GT04 - Page 1
Forum section: electronics / projects

author: perdrix
author_rank: Frequent Contributor
timestamp: April 07, 2018, 02:08:42 pm
content: I bought some M74VHC1GT04 inverters (from Mouser) which I intend to use to square a 1.6Vpp10MHz sine wave. With a 5V supply the datasheet tells me that VILis 0.8V and VIHis 2V.Datasheet here:https://www.onsemi.com/pub/Collateral/MC74VHC1GT04-D.PDFTo me that meant I should centre the input sine wave signal on 1.4V or so and all should be well. Oh dear me no, not at all.To get a nice 50% duty factor square wave out, I had to centre the sine wave on 2.6V. I'd understand if this were regular 5V CMOS, but it's supposed to be TTL input levels.Can someone explain this seemingly odd behaviour?Thanks David

author: fourtytwo42
author_rank: Super Contributor
timestamp: April 07, 2018, 03:02:07 pm
content: The reason is you have read the data sheet through rose tinted glasses maybe. The maximium VIL where all devices are gaurenteed to see a logic 0 is 0.8V but devices may see a logic 0 at any voltage above this up to the minimium VIH. Simeraly All devices are gaurenteed to see logic 1 above 2V BUT may in fact see logic 1 at any voltage above VIL max.If you want better defined input voltages use a schmitt trigger with defined thresholds or use a voltage comparitor with you defining the thresholds through circuit components.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 07, 2018, 03:20:34 pm
content: Use a 2G part and bias the input of one with a resistor from the other wired as an oscillator.Tim

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 07, 2018, 03:36:54 pm
content: As fourtytwo42 said.And if your input signal is a sine wave, definitely use a gate with schmitt trigger inputs. Otherwise, you'll get into other problems as well, such as elevated power draw when the input signal is around the middle voltage.

author: iMo
author_rank: Super Contributor
timestamp: April 07, 2018, 03:41:21 pm
content: The problem is you have purchased a part with TTL "input levels". Use a part with CMOS input level and best with a Schmitt-trigger - those are the "..14" parts..

author: BrianHG
author_rank: Super Contributor
timestamp: April 07, 2018, 03:49:50 pm
content: If you want a strict restrained switched level, maybe a CMOS gate isn't the right part. Changing manufacturers may throw off your best bias point and the 74HC14 is still no guarantee you will get a 50% duty cycle at a particular voltage. Especially when you consider your power supply range and the ambient temperature of the IC. Maybe you should be thinking along the lines of a high speed comapartor with logic output, or, an amp, or something as simple as a NPN transistor like MMBTH10 with 3 resistors and a cap which should make a clean large 5v square wave which you may pass through a CMOS gate if you need rise and fall times under 2ns.

author: Simon
author_rank: Unknown
timestamp: April 07, 2018, 07:23:01 pm
content: Some more context might help with a solution that is the best fit. Does it have to be a 50% duty out ?

author: perdrix
author_rank: Frequent Contributor
timestamp: April 07, 2018, 08:12:18 pm
content: OK M74VHC1GT14 it shall be.Dave

author: BrianHG
author_rank: Super Contributor
timestamp: April 07, 2018, 08:33:05 pm
content: Quote from: perdrix on April 07, 2018, 08:12:18 pmWith a 5v supply, and only a 1.6vp-p source, just be sure you will guarantee cycling clearance so that the schmitt will actually register a high and low transition at the input under the worst case scenario.