a/d inputs

guest · Guest

Hello,
I have a requirement to input 2 0-5VDC signals from a heavy vehicle. I have used the A/D on PIC18's before with no problem. My concern here is the signals are originating in an electrically dangerous (for microntrollers)
environment. Its a bulldozer. Any tips or tricks for isolating/buffering these signals?

Thanks

asmallri · Joined: 12 Aug 2004 Posts: 1635 Location: Perth, Australia

You could use a voltage follower implemented with an opamp capable of rail-to-rail operation on its inputs.
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MikeValencia · Joined: 04 Aug 2004 Posts: 238 Location: Chicago

Look at the "PICDEM MC" at Microchip.com

They use the LOC111, which is a linear optocoupler. You can thus preserve galvanic isolation.

Another method we use is to feed the analog voltage to a voltage-to-frequency converter such as the LM331, and send the square wave over an optoisolator, such as the H11L1. You can now either directly read this frequency with the CCP, or you can send this frequency thru another LM331, which now acts as a frequency-to-voltage converter, and feed that directly to the PIC's ADC input.

sseidman · Joined: 14 Mar 2005 Posts: 159

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Do you need galvanic isolation, or would just some robust clamping and EMI filters do the job? Is the power and other I/O for the PIC coming from the bulldozer too? I don't see what you are galvanically isolating between.
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newguy · Joined: 24 Jun 2004 Posts: 1909

I have a device working in a vehicle, and it has been working perfectly for about a year now. I was in the same boat - had to measure multiple 0-5V analog inputs.

The first thing is the power supply. I assume that you're drawing power directly from the vehicle's 12 or 24V (since this is a bulldozer) supply. You'll need a reverse polarity protection diode across your supply input just in case someone decides to reverse the leads. Of couse, it goes without saying that you need a fuse too.

I also place a TVS (transient voltage suppressor) across the input too. The supply itself is simply a 5V linear regulator. Just be careful to get one that can tolerate 30V+ input, as there are no guarantees about the actual voltage of the vehicle. [What always spurs me on is the memory of getting a "boost" from a tow truck many years ago. He boosted me at 24V (this was a 12V car) for extra starting "oomph."]

The analog input lines first encounter an ESD suppressor. From there, the signal goes into a rail/rail input and output op-amp, configured as a voltage follower. The ones I use are the TI TLV237x (x=1,2,4). Great op-amps, and quite inexpensive too.

The reason for the op-amps is for a "sacrifice" if that makes any sense. It's much cheaper to replace an op-amp, should something bad happen, than a pic. Particularly if the pic is 44+ pins, and surface mount.

If you're interested, let me know and I'll dig out the actual part #'s for the TVS and the ESD suppressors.

Guest · Posted: Wed Aug 31, 2005 11:11 am

thanks new guy,
I would love those patr#'s if you get the chance.

Thanks

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

newguy · Joined: 24 Jun 2004 Posts: 1909

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

I don't believe that a TVS placed between input power and ground
is a good idea for voltage spike protection in an automotive-type
environment.

I used a 33v-36v, 500w TVS device (similar to p/n SA30A from Vishay).
http://www.vishay.com/docs/88378/sa.pdf

This is was in a light aircraft with I believe, a 12v system, though it
might have been 24v. Anyway, the mechanical voltage regulator/
alternator system put out a large over-voltage for several seconds.
This caused the TVS device to burn up. It became a permanent short.
Then the main fuse in the device blew. (3AG type, 1 Amp, fast blow)

A bulldozer's electrical system might well have the same problem.
So I would recommend caution. Make sure that the protection
device you use can withstand all potential failure modes of the electrical
system. This should probably all be tested in the lab, ahead of time.

newguy · Joined: 24 Jun 2004 Posts: 1909

PCM,

Thanks for info! What would you recommend instead of the TVS then?

treitmey · Joined: 23 Jan 2004 Posts: 1094 Location: Appleton,WI USA

PCM programmer: Could you have done a switching regulator, then a TVS, then a DC/DC ?
-Regulator will clean out the surge
-TVS cleans up esd/lightning/power poll short
-DC/DC provide the isolation.

ps I use semtech TVS's and C&D NTE0505M

sseidman: is the NTE0505M doing galvonic isolation as I thought?
I don't see the word "galvonic" in the spec!!! ?
http://www.cd4power.com/data/power/ncl/ndc_nte_5.pdf

sseidman · Joined: 14 Mar 2005 Posts: 159