Capacitive coupling in to logic gates, does it work for weak signals? - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: July 26, 2023, 08:15:18 pm
content: The 74HC4046's phase comparator section has a feature where a relatively weak signal can be fed to one end of a capacitor, the other end of the cap goes to the signal input pin, and the chip is able to interpret this signal as if it were high enough in voltage to be a normal logical input to be compared against a full voltage logic signal on the reference input.Can similar techniques by applied to typical 74** logic chips? AND/OR/NAND/NOR/XOR? Particularly in the use case of using a XOR chip to perform phase comparison between a low voltage signal and a reference 0V-5V to compare it to, that is to say doing the same thing as the type 1 phase comparator in the 4046, but having one chip do 4 channels rather than having to have as many 74HC4046's as there are channels (and each of those 4046's having far more pins present and taking up much more board space than needing just for XORing a logic signal with a weak signal)?Is there a trivially easy way, like capacitative coupling on a 4046, to get weak analog signals to register as if* they were full voltage signals for 5V logic?*anytimes when the instantaneous nalog input was below the time averaged mean voltage being a 0, above being a 1, or vice versaThanks

author: Xena E
author_rank: Frequent Contributor
timestamp: July 26, 2023, 08:25:48 pm
content: You can use hex inverter gates as a poor man's op amp, one of the early contactless line voltage testers used a technique similar to the one I understand you to be asking about, there's a recent teardown video by Clive Mitchell of one such unit.That may give you some inspiration.

author: Benta
author_rank: Super Contributor
timestamp: July 26, 2023, 08:26:55 pm
content: Try using 74HCU04 with feedback (1 Mohm) as linear amplifiers. It's the same thing the HC4046 does.Note: the "U" is important!

author: MrAl
author_rank: Super Contributor
timestamp: July 26, 2023, 08:33:50 pm
content: Quote from: Infraviolet on July 26, 2023, 08:15:18 pmAre you talking 7400 series?They have lower input impedance than CMOS, so maybe higher value capacitor. You do have to be careful you don't overdrive the input though or the device might get damaged.

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2023, 08:55:22 pm
content: To answer the original question: no. The 4046 is unique in that it's just as much an analogue, as a digital IC. The input consists of a un-buffered inverter, with a built-in feedback resistor. It probably uses smaller transistors to limit the supply current and parasitic capacitances.The ON Semiconductor data sheet shows the inputs in more detail. Refer to page 7.https://www.onsemi.com/pdf/datasheet/mc74hc4046b-d.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 26, 2023, 09:01:30 pm
content: Yes but.Notice the input structure. It's not shown in the datasheet in detail, but the appnote provides some hints:https://www.ti.com/lit/an/scha003b/scha003b.pdf?ts=1690403372452Each inverter is probably a pair of transistors (CMOS). This is different from an inverter gate (like a 74HC04), which is typically buffered, meaning it's actually three inverters chained. Which gives much more voltage gain, and phase shift, than a single gate would. Which means, when feedback is applied, it's as likely to break into some undesired oscillation mode (ring oscillator) as the amplify the input signal.You can, however, get these in the 74HCU04. Which is the preferred way to make things like oscillators and amplifiers, in lieu of an integrated one, or a standalone oscillator as such.There are no other unbuffered functions in 74HC, as far as I know. Possibly in CD4000 (but possibly those are mostly archaic now?). I do see CD4011UBE (unbuffered NAND) for example available.So the trick is, they're biasing the input with a large-value resistor internally, which makes it supremely sensitive to AC input; and works differently from just any old gate, because you can't get that feedback from the first internal gate node.You may also find the HC7014 of interest: a buffer with more precise thresholds. It has hysteresis, so won't amplify anything, but with the well-defined thresholds, it's capable of receiving much smaller signals than average CMOS does.The more general component(s) you're otherwise looking for, is either a comparator (<mV threshold)., or some amplifiers, maybe with AVC, maybe with an adaptive threshold or "slicer" (think: setting threshold to halfway between +/- peak to recover DC bias of an AC-coupled signal). Which starts to look like much of a radio system, which is no accident!Tim

author: Infraviolet
author_rank: Super Contributor
timestamp: July 26, 2023, 09:07:10 pm
content: 74HCU04 as an amplifier, I've been meaning to look in to that some more for other uses cases too. Thanks.

author: Benta
author_rank: Super Contributor
timestamp: July 26, 2023, 10:31:22 pm
content: Quote from: Infraviolet on July 26, 2023, 09:07:10 pmDon't expect great voltage gain, you'll need to daisy-chain a couple. But the first one with the feedback resistor sets the DC bias.

author: Zero999
author_rank: Super Contributor
timestamp: July 27, 2023, 09:00:17 pm
content: The 74HCU4 uses around 25mA per gate, at 6V, when the output is biased half way. It's very non-linear. The applications are limited to a squarewave oscillator, or to amplify a signal, so its output saturates.https://assets.nexperia.com/documents/data-sheet/74HCU04.pdfI presume the 74HC4046 doesn't use as much power, when its input is capacitively coupled? The data sheets I've seen only specify it, when the input is either at VDDor VSS. I would have thought it has transistors with a higher on resistance than the 784HCU04 to save power.

author: Benta
author_rank: Super Contributor
timestamp: July 27, 2023, 09:51:00 pm
content: Quote from: Zero999 on July 27, 2023, 09:00:17 pmInteresting.I've used both the 4046 and the 74HC4046 several times, but never with the capacitive input coupling, only with logic-level inputs.Can't say how it behaves here.