Analog switch interference DG413 - Page 1
Forum section: electronics / beginners

author: bonzer
author_rank: Regular Contributor
timestamp: May 26, 2020, 09:08:27 pm
content: Hello everyone. Please help me to decide on my next step.I'm working on a project where I connect together two terminals of two analog switches controlled by same input but with inverse logic so I have some sort of a 2 to 1 AMUX. The voltages that change are two TL413 regulators: one generates 5V and the second -5V. I work at a maximum frequency of 20kHz. Break before make in datasheet of DG413 is 25nS.Should I worry anyway about possible short circuit between 5V and -5V?I don't want spikes or big currents entering my DG413 because I'm afraid of possibly damaging it with higher currents than it can accept.I would place two resistors like in the schematic to ensure low current in this istants but I can't place them there just "because I want to stay safe" as it's a university work so I need to prove their necessary. I can't try it yet practically but my simulator gives me some spikes for those moments when it works if I leave it without resistors, even though the model doesn't look that reliable and it takes a lot easier for it to simulate when resistor are placed.Anyway you can understand me better if you look to the files that I attached.Should I leave it without resistors or it'd be better to keep them?

author: RoGeorge
author_rank: Super Contributor
timestamp: May 26, 2020, 10:00:57 pm
content: Would be even better to place R15 and R16 on the left side of the switch, between the + of each source and the switch input.The reason for R15, R16 is to protect the +5V and -5V sources against a short circuit in the case of an internal failure of the switch (also protect against an accidental make before brake).

author: Kleinstein
author_rank: Super Contributor
timestamp: May 27, 2020, 12:10:08 pm
content: The switches do produce some current spikes from charge injection, even if there is no cross conduction. A few pC of charge injection over a switching time in the 10-100 ns range would be a few 10-100 µA. Depending on how the +-5 V references are made the extra resistors to the left side of the switches can in deed help, but is probably not needed. Some resistance can help to limit the speed the input to the LM311 changes. To fast a slew rate there could in theory cause some feed through - not sure about the LM311 as this is usually not specified.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 27, 2020, 12:19:39 pm
content: Why not use a single reference and an op-amp wired as non/inverting with one (SPST) switch?Note that the switch resistance acts in series with the resistors, so if you're using these as part of a voltage divider, remember to adjust for that somehow. (Switch resistance has a tempco, among other things, so can be compensated out but with some effort; it may be worthwhile using a spare switch as a reference so the added resistances can be nulled.)Who cares about current limiting, if the sources are shunt regulators (I assume TL413 was a typo -- nice coincidence with the DG413 around, heh -- and TL431 is meant?) -- they're fine. The worst that can happen is a few us extra recovery time, which may or may not be important.Going to an LM311 (comparator) seems rather trivial: I wonder if the switch is even needed at all? Why not for example invert the signal being compared to, and use two comparators? Or two wired as a window comparator? What does the clock do, can it be replaced by keying or gating the comparator outputs instead?Tim

author: bonzer
author_rank: Regular Contributor
timestamp: May 27, 2020, 01:15:50 pm
content: Thank you for your answers.Because I have no op amps left, this is why I use two regulators, otherwise I could use another op amp and invert it.Yes it is a TL431, sorry.After your questions I want to share the whole circuit.It is part of a VCO project, to generate +-10V triangle wave, from 200Hz to 20kHz. It's a famous solution so it's easy to understand how it works. 20kHz is quite low so I don't have to bother about settling time of voltages at non inverting input of LM311 as these time costants are pretty fast and I need my voltage ready only at the end of every semiperiod.So you would ask me then why am I afraid of fast spikes near my regulator? Because I use these regulators also as references for another part of the circuit (a triangle to sine shaper) where spikes should be avoided as it ruins THD.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 27, 2020, 05:23:37 pm
content: Since you have 1k\$\Omega\$ R37, R38, the current is already limited by those, so maybe R5 and R6 are not needed at all.What's the worst that could happen if the switch is shorted by an internal meltdown, or shorted between U4 and U5 by an accidental make before brake? Not much, mA range only.In the end it's all up to the required performance and specs, but for now I see no reason for the existence of R5 and R6, because the maximum current will be limited anyway by the already existing R37 and R38.

author: bonzer
author_rank: Regular Contributor
timestamp: May 27, 2020, 06:40:16 pm
content: Thanks for your answers!Yes, actually I shouldn't be afraid too much about currents. I didn't mentioned what values are +-Vcc but they are +15V and -15V.Maximum current I could have circulating through that switch is about 30V/2k = 15mA. DG413 should support a maximum of 30mA.I could even put those resistors 1.5k to have 10mA.But what I'm more afraid of by leaving it without resistors is the zero voltage that is generated on regulators because of that short circuit. In fact it generates "spikes" going to zero, and they wouldn't bother me if I used regulators for threshold voltage only for LM311. But I use them also as references for my shaping circuit that I didn't put here. But I placed there some 1nF capacitors as you see that should keep that voltage constant when they combine with those 10k R5 and R6 resistors but not without them.Because of the functionality of TL431 I can't chose whatever capacitance value I want, so I should go for low capacitance or large capacitance to avoid instability (see attachment). This forces me to have something like 4.7uF or higher capacitors. At these values I have electrolytic capacitors but I wanted to avoid because they work bad at higher frequencies. But now I noticed that I could care less about the other higher harmonics around my circuit as long as I have a constant reference voltage at its terminals.So I think about removing those R5,R6 and changing C3,C2 to 4.7uF

author: gerry
author_rank: Newbie
timestamp: July 17, 2021, 02:05:05 am
content: hi binzer can you share the vco circuit that makes triangle waveform from 200-20khz??