AD5941 Analog electrochemical sensor breakout board  - Page 1
Forum section: electronics / projects

author: TrotsKeeper
author_rank: Newbie
timestamp: December 13, 2022, 06:00:15 pm
content: Hello, I'm trying to make a breakout board for an electrochemical sensor [https://www.analog.com/media/en/technical-documentation/data-sheets/AD5940-5941.pdf ] in KiCAD. It's my first time making something like this and I'm not sure if its ready to be sent for PCB manufacturing. Overall idea is that this breakout board will be used to connect to an Arduino via SPI and I can collect data. I'll be only using liquids for electrochemical sensor aspect. Am I missing something ?

author: CountChocula
author_rank: Unknown
timestamp: December 13, 2022, 07:16:44 pm
content: I don't know the specifics of this chip, but a few general comments:I can't see in your PCB screenshot whether you have a board outline in your edge_cuts layer; without it, the manufacturer won't know how to size your board. Note that, in the absence of an outline, the 3D viz will simply display the smallest area that encompasses all the elements on the board, so that output can be deceiving!You shouldn't allow silkscreening to overlap pads—here, you have text over the location where the chip needs to be soldered. I'm actually not sure what would happen if you sent this in to a fab house, but… it can't be goodThere is no supporting circuitry… is this on purpose? A quick scan of the chip's datasheet reveals that, at a minimum, it requires a number of decoupling caps, which should really be on the board.

author: Abdurrahman YAMAN
author_rank: Newbie
timestamp: December 16, 2022, 11:45:16 am
content: Hi,You are making basically LFCSP to DIP adapter nothing else.If you want to use the PCB with breadboard or something, you may consider to make the PCB compatible with it.If you are trying to do a breakout board that involves every supporting circuitry and the sensor itself, you have to refer to the datasheet for it.Regards.

author: TrotsKeeper
author_rank: Newbie
timestamp: December 16, 2022, 03:41:49 pm
content: Hello,- regarding the edge_cuts; I realized I don't actually have board outline. I just have the measurements for the chip itself from the Analog Devices library.- I've moved the text on chip down - is it better ?- This is the hard part for me; I'm not sure where in datasheet I should look for regarding the decoupling caps and what values I would need. My main idea is that once I have this board with the chip I can connect it directly to arduino nano ? Does that make sense ?

author: TrotsKeeper
author_rank: Newbie
timestamp: December 16, 2022, 03:44:57 pm
content: I want this PCB to connect with arduino nano - if I give it the required power and connect it to arduino via SPI will it be able to talk with the arduino - Basically, whats the minimum I need on PCB to get it started to connect with arduino ? I know CountChocula mentioned about at minimum having decoupling caps but I'm not sure where in the datasheet

author: CountChocula
author_rank: Unknown
timestamp: December 16, 2022, 06:56:59 pm
content: Quote from: TrotsKeeper on December 16, 2022, 03:41:49 pmHowdy! The measures aren't going to help—the manufacturer needs an outline in the edge.cuts layer that they can use to size, position, and route your board. This is as simple as adding a rectangle that encloses all your traces, parts, and mechanical elements to that layer; if you want to post your KiCAD project, I'm sure one of us can help you add it.QuoteI understand what you want to do, but it's notquiteright. Bypass caps should be as close to the chip as possible in order to be effective, so mounting them outside the board may not achieve the results you want. (In all fairness, the chip may work perfectly fine regardless, but then again it might not, at which point you won't know why.)There is no shortcut to reading through the datasheet to figure out what support circuitry you may need; I know it can be a slog, but, without it, the risk of malfunction and/or failure are pretty highA cursory look at pages 20 and 21, which show the pin assignments, tells me that you need capacitors between these pins and ground:A 4.7µF cap on VREF_1V82A 470nF cap each on VBIAS_CAP, VREF_2V5, and DVDD_REG_1V8There is an additional AVDD_REG pin that could use a cap for the internal LDO regulator; it doesn't specify a value, so maybe I would throw anything between, oh, say 1µF and 10µF (but I'm just guessing, someone with more experience might be able to give you a better idea).Note also that the chip operates between 2.8 and 3.6V, whereas most Arduinos operate at 5V; if that's the case for you and your Arduino doesn't provide 3.3V outputs, you will need to add some preregulation from 5V to 3.3V so that you can power the device from the Arduino's VCC pin, and then level shifting at least in the direction from Arduino to the AD5941 so that you can safely interface the Arduino's output pins to the '5941's input pins (this won't be necessary in the other direction). Again, you may get away without the level shifting (but not the 5V->3V3 regulation!), but you'd be taking the chip's life in your hands every time you put power to itNote that this last bit of support circuitry doesn'thaveto be on your board—you could just power the chip from an external 3.3V supply and use a commercially available level shifting breakout (likethese ones,which are very cheap and readily available), for example, just as long as you don't forget.

author: KT88
author_rank: Frequent Contributor
timestamp: December 16, 2022, 11:51:18 pm
content: Something to keep in mind is that in some use cases very high impedances are required. This means as an example for the RE of an electrochemical cell guarding is required. The TIA input also requires special attention. Routing these signals through a connector is a challenge in its own right...