Cold spray, where do you buy?

The system already has a clock, and the ring oscillator/counter are buried somewhere deep inside the FPGA fabric. The incremental EMI will be trivial.

A simple diode path is all

Plus an external ADC, which may or may not be available

In just about any chip design review I suggest this

The best solution would be to include an SPI temperature sensor, available internally and externally. Core voltage *can* be tweaked to offset prop delay TC. That's actually done in some ASICS.

But most users don't need to flatten prop delay over temperature; they only need to be assured that their logic will work worst-case.

John

It made cool air, but nothing as cold as freeze spray. We ran a hose from our compressor upstairs to the lab, so maybe we didn't have enough air. But it was already unbearably noisy. Running a bigger hose, and putting the vortex tube in a padded box, was too much work compared to ordering a case of freeze spray.

John

You can use digital inputs to convert analog values. More interesting ideas are using LVDS inputs. This is an example from Lattice Semiconductor:

but I think this would be possible with Xilinx and Altera parts as well.

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Frank Buss, fb@frank-buss.de
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Cool. Fig 4 is a sort of delta-sigma ADC. It's not widely advertised, but most LVDS receivers are pretty good, and very fast, rail-to-rail comparators.

One could also do a single-slope multichannel ADC with one linear ramp and a bunch of LVDS inputs, each with a counter or a latch driven by a counter.

John

eas

I seem to remember that some (xilinx?) guy has a patent on using the ldvs input as a comparator for an ADC

-Lasse

A simple digital input would do it, too. E.g. with two pins of a microcontroller you can use a standard LED for light emitting and light sensing:

Maybe this could be used with a FPGA, too, for temperature measuring with a

1N4148. But I don't know, if it needs a schmitt trigger input, which is not available for all FPGAs (e.g. only some configuration pins on Cyclone II, but Xilinx FPGAs have schmitt trigger inputs with 100 mV to 200 mV hyteresis).

Of course, on page 34 of this journal:

there is a simple schematic with only two resistors and two FPGA pins for a user defined schmitt trigger :-)

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Frank Buss, fb@frank-buss.de
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Forward bias one of the protection diodes?

--

John Devereux

Yup, I have used substrate diode for stuff like this but those solutions are often met with outbursts of disgust at design reviews ;-)

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Regards, Joerg

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That's real engineering, where competitors look at the circuit and all they can say is "What the hell are they doing here, and how?"

[...]

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Regards, Joerg

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If you have the space on the board, I would use the SE95, because no calibration is needed. I've used it with high sampling rate and digital lowpass filter for a resolution of 0.01°C. It's fairly stable and intersting to see the temperature falling, when slightly blowing it. I think this could be used for something like this:

:-)

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Frank Buss, fb@frank-buss.de
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Problem is, that series is no-stock too often and that's somewhat of a red flag for me. I2C stuff sometimes goes lalaland after a while. Mostly I prefer a simple analog method and then either into an ADC, or dual-slope when it has to be cheap.

Oh man, electric candles with matches. That reminds me of the 2nd generation walkman that my classmates had in school. Those had a "talk-through" button which allowed a conversation with other kids. Conversation, what a concept :-)

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I know a product, which uses it for 6 years and I think the sensor is a standard chip, which will be in production for some more years. But the MAX6633 looks very similar, could be replaced with no hardware changes.

How do you avoid (or implement) calibration for analog and ADC? You are right, if you just need the information "it is hot" or "it is cold", then you don't need complicated chips.

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Frank Buss, fb@frank-buss.de
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That can get tricky, as can adding external hysteresis to any comparator. If you get a spike that's faster than the internal loop-back delay of the device, the positive feedback arrives too late to do much good. So a rising edge with noise, but less noise than the theoretical hysteresis, can still double-clock things.

Better is to use two LVDS inputs as comparators. Apply the signal to both, and set up the other sides with the desired thresholds. Drive an RS flipflop internally.

John