Reduce capacitance - Page 1
Forum section: electronics / general

author: raphaeldavidf
author_rank: Contributor
timestamp: April 30, 2015, 08:20:10 pm
content: Hey guys,The capacitance from the pins SCLK and MXSX are messing up the communication and I'm looking for a way to lower that capacitance.The capacitance now, measured from GND:SCLK: 7 nFMXSX: 7 nFI need to keep then under 70 pF.is there a way?Many thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 30, 2015, 08:44:22 pm
content: Um, measured how? Where is 7nF coming from? Have you given this measurement a basic reality check?I have no idea what the NLX90316 is, but I'm guessing it's not 7nF.What is the rest of your circuit, and how is it connecting? Layout? I would be much more concerned about signal quality and EMC than the exact magnitude of capacitance on the lines.Tim

author: raphaeldavidf
author_rank: Contributor
timestamp: April 30, 2015, 09:18:05 pm
content: Quote from: T3sl4co1l on April 30, 2015, 08:44:22 pmHi Tim, thanks for helping out.I measured it with a cheap LCR meter that I got from ebay. The 7nF is coming from the IC I suppose.The MLX90316 is a rotary position sensor an I'm using it with a TXB0108 voltage level translator so it can talk with my pic. The problem is that the TXB can't work with capacitive loads higher than 70 pF. Other stuff, that have capacitance lower than 70 pF, are working ok with the TXB, so I guess that the 7nF might be right.Attached the circuit that is trying to talk with the MLX and the MLX board layout.

author: tggzzz
author_rank: Super Contributor
timestamp: April 30, 2015, 10:10:53 pm
content: Quote from: raphaeldavidf on April 30, 2015, 09:18:05 pmThose usually work by injecting a voltage through a resistor and timing how long it takes to reach a certain voltage. There will be many non-linearities in your measurement that will "confuse" that way of measuring capacitance.Add up all the known capactitances in your circuit (look at the data sheets) and I would be very surprised if you can get within two orders of magnitude of 7nF. If there is an identifiable capacitance of that magnitude, tell us what it is and then we can suggest how to reduce it.In any case, what makes you so sure that capacitance is the cause of your problems?Are the lines open-collector or open-drain? If so they will need a pull-up resistor. Again, the data sheets will tell you.

author: raphaeldavidf
author_rank: Contributor
timestamp: April 30, 2015, 10:45:38 pm
content: Quote from: tggzzz on April 30, 2015, 10:10:53 pmHi tggzzz, thanks for the helpI don't have any other way of measuring capacitanceI looked the datasheet and it doesn't give any capacitance, so the only known capacitance are those two 100nF caps in the schematic. Is there another way to find them out?I think capacitance is the cause because of this pdf that I found when searching about the problems I'm getting:Quotehttp://www.adafruit.com/datasheets/txb0108appnote.pdfFrom the TXB datasheet typical application it doesn't look like I should put any pull-up resistor.

author: Marco
author_rank: Super Contributor
timestamp: April 30, 2015, 10:59:23 pm
content: What are you connecting these boards with?PS. this level shifting IC is expensive and unnecessary for an unidirectional protocol, just use a CD40109 to go up and a resistor divider to go down.

author: raphaeldavidf
author_rank: Contributor
timestamp: April 30, 2015, 11:03:46 pm
content: Quote from: Marco on April 30, 2015, 10:59:23 pmribbon cable with KK connector

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 30, 2015, 11:17:51 pm
content: Quote from: T3sl4co1l on April 30, 2015, 08:44:22 pm

author: raphaeldavidf
author_rank: Contributor
timestamp: April 30, 2015, 11:25:05 pm
content: Quote from: T3sl4co1l on April 30, 2015, 08:44:22 pmAttached the schematics from the TXB board and the layout from the MLX board in the reply, 3º post.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 30, 2015, 11:31:02 pm
content: Quote from: raphaeldavidf on April 30, 2015, 11:25:05 pmBut those just show off-sheet connectors, not headers, and what kind and how long the cables between..?Tim

author: raphaeldavidf
author_rank: Contributor
timestamp: April 30, 2015, 11:43:18 pm
content: Quote from: T3sl4co1l on April 30, 2015, 11:31:02 pm40 cm ribbon cable with KK connector and header, but I already tried with shorter cables, and the capacitance measure was made without the cable(direct on the MLX board header)If you like I could post some pictures

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 30, 2015, 11:54:09 pm
content: Are you using the SPI version of the MLX90316? That is what it appears from your board layout.Why do you need any kind of logic level converter at all?The MLX90316 says that it operates on 5V Vdd and the data line must be pulled up to 5V or 3.3V through a 1K resistor (R2)It also needs that funky MOSFET transistor to use the single data line as both input and output. (See the data sheet Fig 18 on page 36)From what we can see in your postings, it appears that your complete interface to the MLX90316 is fundamentally incorrect.Back to the drawing board and consult the MLX90316 data sheet more carefully.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 01, 2015, 01:43:11 am
content: Probably enough cable to be worth adding some source termination resistors, about 50-100 ohms. More if you don't need the speed.Use more pins, and ground every other wire (50 mil pitch ribbon, dual row header, one row grounded -- nice and simple). This gives a transmission line impedance of about 100 ohms, and reasonably low crosstalk.Mind that SPI will be susceptible to more than modest EMI (i.e., >3V -- enough to violate logic thresholds, despite the extra grounds). You can help that a little by adding filtering (ferrite beads and extra capacitance), but this directly affects maximum baud (just as the extra resistance does). If you only need a few hundred kHz, it can help a lot.Tim