DG303BDY Dual SPDT analog switch - Page 1
Forum section: electronics / projects

author: NikkiC
author_rank: Contributor
timestamp: December 17, 2019, 11:50:00 pm
content: I have an older project that is implementing a pair of the DG303BDY Dual SPDT Analog Switches to switch 2 different voltage references to a 12 bit DAC, and a 5V microcontroller that controls them. I am now upgrading the project using a 3.3V microcontroller. The problem is the DG303BDY needs a >=4v logic level for high and my microcontroller just isn't and won't work with the DG303. Since I am going to have to redesign the pcb, this means I am looking for an equivalent part to replace the DG303 with that can be controlled by a 3.3v microcontroller. Needs to have equivalent or better performance than the DG303. My supplies to those switches are +/- 15v. I am having trouble trying to find a suitable replacement, one that has the same performance and doesn't require a huge amount of pcb rework.Yeah. I can probably use two transistors per chip to accomplish the logic shift (since there are 2 control lines per IC) or some other method, but I'd like to keep the parts count down. This is a personal project btw.Is there a part that someone can recommend? It has to be SOIC as my pcb is SMD. I've attempted to find a suitable replacement, but my god it's almost been like searching for a needle in a haystack for me, I am sure some of you more experienced guys could probably come right up with a suitable part, so I was hoping someone could help me with this.My appreciation in advance for any advice, help ETC, it's a bit overwhelming. There are a lot of analog switch IC's out there!Cheers,Nikki

author: exmadscientist
author_rank: Frequent Contributor
timestamp: December 18, 2019, 06:46:04 am
content: I see lots of options out there... so many that it's hard to know which one to recommend because I don't know what other factors are important to you. (Cost? On resistance? Leakage? Large package pitch? Small package area? Thermal performance? Availability? Second source? Not being from Maxim, because screw Maxim? And so on....)I wonder if you're getting caught up looking for a dual SPDT switch when instead you could be looking for a quad SPST or dual 2:1 analog mux. The quad SPST switches I'm thinking of have 2 NO switches and 2 NC switches; you can then gang one of each together to make a SPDT switch. (In fact, the DG303B actually appears to be this way internally; it's not a true SPDT since you have to externally short the two drains.) This is a pretty common configuration because it's very flexible and can be used in a few more configurations than a true SPDT, at the trivial cost of two more pins on the package.In general analog switches are a real pain to search for because there are multiple ways to describe the same switch... and even within a single manufacturer or single product line, two functionally identical switches might be labelled differently.

author: NikkiC
author_rank: Contributor
timestamp: December 18, 2019, 01:53:47 pm
content: Thank you for your reply. Sigh. It's a mine field actually. Yes, I did notice that trying to find comparable products from the same mfg is akin to pulling teeth. Just when It looks like I may have found a good replacement, there is always one thing about it that breaks the euphoria. Voltage too low, or way way too much R(on) resistance compared to the original, et al.I am to the point where I am thinking that it is just less painful to wack in my own level shifting, this component equiv search is proving itself as being exceedingly annoying, exceedingly time consuming and exceedingly unproductive. Thank you for your search query suggestion, I will definitely try that.The DG303 is a rather expensive component (I use two of them) and the more I think about it, the more compelled I am to just do my own level shifting to make the existing chips happy. This has become so annoying that I am almost to the point of just dead bugging some switching transistors onto them, cutting some traces and just be done with it already. But to bodge such a beautiful laid out pcb that I spent a significant amount of time designing and am proud of seems to be such a shame.So the search continues at least until I can't take it anymore. Just 4 sot-23's and a few resistors, but even laying out a new pcb with those still feels like a bodge to me.Cheers,Nikki

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 18, 2019, 03:47:43 pm
content: What about the ADG5421? It's not available in SOIC but in MSOP-10 (smaller but still easy to deal with), so it shouldn't be a problem integrating this in your PCB without having to move things around.https://www.analog.com/en/products/adg5421.html

author: NikkiC
author_rank: Contributor
timestamp: December 18, 2019, 06:46:11 pm
content: Thank you for your reply.The deal is that the DG303 has 2 banks of 2 switches with one bank defaulting to on and other bank defaulting to off. I would have to use two of the ADG5421's to replace the single DG303, and probably some code changes as well. And all that is well and fine, not that big of a deal actually, but the cost of the ADG5421 is. So not only do I double my parts count, but since the cost of both chips are comparable, I'd be spending double since I would now need two chips for each DG303.But other than that, it would fit the bill perfectly, but this is an example of the big wall I run across, with the parameters matching or exceeding the DG303 but falling short (which in this case is just dual switches instead of quad).Sigh. This is just nasty, but at this point, it looks like I have no choice but to level shift it. Not that there is anything wrong with it, but I guess I'll have to snag a hand full of 2N7000 mosfets and do something like this:

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 18, 2019, 07:06:51 pm
content: Quote from: NikkiC on December 18, 2019, 06:46:11 pmDo you mean "defaulting" as the state for a '0' logic level? If so, the ADG5423 does exactly this. It's mentioned in the datasheet that is common for the two parts. But you would have to use two indeed (no code change needed though.) Cost is in the same range as the DG303 currently, but yes it would then double it.The ADG436 seems to be what you're after, but it's more expensive that a DG303 (less than two ADG5423 though!)

author: exmadscientist
author_rank: Frequent Contributor
timestamp: December 19, 2019, 05:32:09 am
content: If cost is a concern and you're already using two of these, consider the DG333A or ADG333A: it's actually a quad SPDT for around the same price as one of your existing DG303B dual SPDTs. ADG5434 is similar but appears more expensive, or ADG5433 is a triple instead of a quad.Otherwise, it looks like DG405B, DG456, ADG5413, ADG5413F, ADG5436, or ADG5436F all seem to meet your basic requirements. If you don't like any of those, ADI has several other parts that would do, but those are their preferred ("recommended for new designs") options.

author: NikkiC
author_rank: Contributor
timestamp: December 20, 2019, 04:06:35 am
content: QuoteThank you for your reply. From what I can tell, the DG333A looks to be pretty much 2X the DG303 in one package. Actually, I wish I had known about this part before choosing the two DG303's in the older project to begin with. At first glance the 333A appears to simplify the pcb layout, is actually half the price of the DG303 (due to the fact of needing 2x of the DG303) and has a little bit lower On-Resistance. All in an SOIC-20 package.I will take a glance at the other recommendations, but it looks like this is even better than what I was looking for, reducing the package count, simplifying the layout, it must be too good to be true! Thank you!

author: NikkiC
author_rank: Contributor
timestamp: December 20, 2019, 04:19:31 am
content: QuoteTake a look see at the DG333A. This looks to be 2x DG303's in one SOIC-20 package at roughly the same cost of a single DG303. Seems to have pretty much the same specs except maybe 25 ns slower switch speed, which is not a big deal. It is a little bit different, but at first glance looks to be more what should have been used in the first place. So I will be looking at this part a lot more closer. Appreciate your input.