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Old 06-18-2012, 05:57 PM   #11
kladue
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Having worked with equipment able to withstand 120VAC on an analog input, that design feature will be simple to implement with these devices. If you do not plan for things like that, it will inevetibaly bite you on the hind parts. During the 38 years doing control equipment installation, calibration, and comissioning, there are not many gotcha's I have not seen yet, not to say that someone might excel in their creative incompetence and suprise me. We have had sensors self destruct in various manners, with varying results with PLC's, which points out lack of preperation on certain manufacturers part.

 
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Old 06-19-2012, 12:12 PM   #12
carlisle_bob
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Hi

So what's the plan for 50/60 Hz rejection? The chip doesn't seem to have filtering in it. CIC's maybe? Full blown FIR's? Ignore it?

Bob

 
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Old 06-19-2012, 07:35 PM   #13
kladue
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With signal levels in the 0 -5V range the greater concern was overvoltage and rf noise pickup, 12 bit resolution makes 60 hz noise a minor if not insignificant problem. Plan for the outputs is to use a diode bridge and polymer fuse to eliminate hookup polarity and short circuit malfunctions.

 
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Old 06-19-2012, 09:56 PM   #14
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Hi

If it's a voltage output sensor, a simple R/C filter should take care of the RF pretty well. Just be sure there's a good local ground plane to tie it into.

Bob

 
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Old 06-21-2012, 03:44 AM   #15
kladue
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I have decided to consolidate the digital functions into one IC, a PCA9955 that combines on/off with PWM and is short circuit resistant and reduces IC count to 3 devices. With built in monitoring you can check outputs for opens/shorts/ over temperature conditions and respond with point shutdown.

 
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Old 06-23-2012, 07:10 PM   #16
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Hi

If you will be running high voltage on the outputs - looks like a good heat sink connection to that back pad is a really good idea...

Bob

 
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Old 06-23-2012, 09:34 PM   #17
kladue
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I was expecting to use the current limit feature to limit total TDP from the package, and still operate 24V relays. It is looking like multiple boards are going to be a better approach, control and signal on one, interface conditioning on another.

 
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Old 06-24-2012, 06:47 PM   #18
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Hi

If you run enough layers on the pc board and solid enough ground plane, then you can get power out through those heat pads. Your solder paste process has to be up to the job, but it can be done.

That said, yes I would go with a second board or at least a second set of parts. It might get a bit crazy, but you could run the chip through cascade buffers. That would take care of the heat issue and might not mess up the rest of it to bad. I've never done it with that chip, so its only a guess.

Bob

 
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Old 06-25-2012, 04:01 AM   #19
kladue
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Here is rev 0 of the I2C IO board, finally got all the things placed and wired so it can be networked up to 9 boards by adjusting addressing jumpers. Still need to build the input/output conditioning board. This will give you 16 0-5V analog inputs, 16 0-10V analog outputs, 4 thermocouple inputs, and 16 current controlled PWM - on/off outputs.

 
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Old 06-25-2012, 04:05 AM   #20
passedpawn
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How many different PWM rates can this part support simultaneously? I.e., these parts usually have a programmable "slow" and "fast" rate. While you can change the duty cycles on those, there are only 2 rates at any given time.

Also, no clock on that part? Does it have an internal oscillator?
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