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Doug Grant

PIC (& Software) 1, Op-Amps 0

Doug Grant
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Chuck Sampson
Chuck Sampson
9/27/2013 5:52:17 AM
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Re: The Path of Least Resistance
I think, in my humble opinion, whether or not the analog solution is difficult or easy depends on the designer's experience. If you give what is basically a digital problem to a software guru, you're going to get a software solution. 

As an analog designer, I would never think of using an op amp for a comparator. I like the lm339. It's pretty cheap and fast. The lm4030 has a 0.05% initial accuracy with 75 ppm temp drift. 0.1% resistor are pretty cheap nowadays. 

I know one thing for sure, using mcu comparators for responding to fault thresholds like over-voltage and over-current is bad idea. The power fets are blown way before the mcu has figured out the voltage is too high.

I find it somewhat weird that the software guys would want to even bother with coding up such a simple circuit. The guys I work with are always asking me to stick some "glue logic" on the design so they don't have to mess with it. 

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Brad Albing
Brad Albing
6/4/2013 8:23:05 PM
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Re: RF power detector
If it's worth talking about, it's worth talking about a lot. Sometimes, even if it's not worth talking about, it's still worth talking about a lot. E.g.: Fox News; MSNBC.

Too much is not enough.

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sonic012
sonic012
6/2/2013 4:50:32 PM
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RF power detector
Seriously guys, I've never seen so much discussion over a simple RF power threshold detector.

Regards,

Alex GB

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JeffL_2
JeffL_2
5/30/2013 1:40:02 PM
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Re: Some points to ponder
Right you are about the specific design, I was taking the opportunity to make the larger point.

There's a bit more interesting discussion to have over my second point, whether or not it's relevant to this specific design would depend on how accurately the "limits" have to be measured and implemented. This relates to an assignment I had where I was working for an engineer who prided himself on his "out of the box" thinking. He was determined to do data conversion in whatever manner got him the best accuracy for the smallest dollar, high conversion speed was not particularly relevant. At the time A/Ds over 12 bits in resolution were horribly expensive, so in order to get the most accurate measurements he fed the voltage to be measured into a highly linear V/F converter, then measured the resulting frequency more or less as I outlined in my second point. Nowadays external 16-bit converters (assume SPI interface for simplicity) are fairly inexpensive (again assume single quantity), the AD7171 is running around $1.72, whereas a V/F with .01% linearity like the KA331 is still only $0.71, true it needs a one-point calibration, it may not be a runaway winner but it's pretty interesting that the approach is still at least competitive, isn't it?

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Brad Albing
Brad Albing
5/30/2013 1:24:24 PM
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Re: PIC Solution vs Analog
That stacked op-amp approach could work. I'm not sure if it's really any better than the approach that Doug ended up with. The PIC part does provide a quick and easy way to get the design that he wanted. And I say this even tho' I'm an analog guy from way back.

Of course, my quad comparator (LM339) + single long resistor chain is good too. Especially with the clever way that I calculate the resistor values with respect the the required tap voltages.

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Brad Albing
Brad Albing
5/30/2013 12:59:58 PM
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Re: Some points to ponder
@JeffL_2 - Certainly for any number of designs, that FPGA would be good to use and would simplify the design and lower the BOM cost. I just meant in this case (Doug's design), it would not be cost effective - and by extension, in other fairly simple designs, an FPGA would not be cost effective. Altho' certainly one of the low-end PIC parts would be.

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Hughston
Hughston
5/30/2013 11:01:28 AM
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Newbie
PIC Solution vs Analog
Some ADCs let you set an interrupt when an input gets to a certain threshold. Is there a PIC that lets you do that? If so, then you could maybe just use on PIC and not two. If you are a good analog designer you could do an LTSpice simulation very quickly and check your analog design in less than an hour. You could use a tri-color LED to indicate multiple thresholds and that might also simplify the design but use more power.

Analog approach: With single colored LEDs, the LED can be turned on by the voltage difference between the an op amp output and the op amp output above it in the chain. So as an op amp switches low, the LED gets powered by the output of the op amp above it in the chain. But when the next op amp in the chain switches, then the first LED turns off then the next one gets turned on. The last LED is powered from the positive supply rail. You need a cheap reference or a regulator. The regulator might have a power good signal out to enable the whole thing.

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
5/30/2013 12:49:37 AM
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Master
Sanctity
Doug,

It is a thorough account you have given, about the whole case. I guess there is nothing blasphemous(?) about deploying a mcu in place of an analog circuit. However, the same techniques may not apply to , say, amplifying a signal in the order of micro or nano volt without a proper instrumentation amplifier. Analog is closer to the real world, agreed yet on some occasions, it is street smartness which wins the title rather than skills.

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JeffL_2
JeffL_2
5/29/2013 11:36:00 PM
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Newbie
Re: Some points to ponder
Really? A Xilinx Spartan-3A with 100 pins and 50K gates is $6.12 quantity 1 at Digikey. I'm not familiar enough with the technology to know EXACTLY how much "stuff" that translates to but it was kind of my specific intent to raise the premise that the "bottom fell out" (sort of) of the price structure while most folks who weren't thinking about it weren't looking. In many ways most of us aren't particularly well prepared to address incorporating parts like this because of the rather steep learning curve of  the tools and so forth, but be honest, THAT'S not the fault of the wizards who come up with these parts, is it now? Time to learn new ways to do things maybe? Just wondering out loud...

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Brad Albing
Brad Albing
5/29/2013 11:16:39 PM
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Blogger
Re: Some points to ponder
@JeffL_2 - just a partial response to your point #1: The use of an FPGA could be made to functionally solve this design problem (probably), but I'm guessing even the cheapest FPGA would wreak the budget of this particular product's design.

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