Monday, June 24, 2019

Example 1 -

Jim Williams and Guy Hoover designed a low distortion oscillator some time back [1]. This application note made use of a LED controlled Photo Resistor that was used to adjust the gain of the oscillator. This part has since gone obsolete and the replacement part provides a curve of LED Current Vs. Resistance that looks like this,

Technically the above curve is correct, but, the interesting part of the curve is in the range of 0 to 5 mA, that is where all the useful resistance change happens. If the curve is plotted on a log chart then it becomes much more customer friendly or useful as shown below,

Same data, simply plotted with the X-Axis set to logarithmic. Now the curve is useful to the end customer, who incidentally is a circuit designer who needs to know how the resistance changes in the useful control range of LED current. For extra credit: Also plot a curves of the response over temperature as this information is needed for a complete circuit design.

Example 2 -

A certain well respected manufacturer of IC’s sells a low noise OPAMP and provides a curve like this one to document the Voltage Noise Vs Frequency. As in the last example the interesting part of the
curve to any designer is the very first data point of this chart, not the 99.9% shown.

This curve is better than most manufacturers in that it seems to be a 'Real' instrument trace instead of the 'Artists Conception' that we normally see on data sheets, but the useful noise detail in the low frequency 1/f region is obscured. This could have been easily solved by switching the instrument to measure ‘Log Frequency' mode. I have yet to see a low frequency FFT analyzer that doe not include this setting.

Usually Voltage Noise versus Frequency of an OPAMP is presented more usefully as this one above that I made for a LT6910-1 at a gain of 100. The interesting portion of the curve is in the 1/f region. Again, Log Scaling for frequency fixes the problem.

Example 3 -

It is important to have some 'domain knowledge' of your customers needs and what you are measuring. Here is a plot for an wide band receiving antenna that is similar to one I once was sent by another engineer. He measured this antenna and sent me the plot as a Smith Chart because that’s what he figured RF engineers wanted to see – a Smith Chart.

The above Smith Chart is a valid plot, but about all you can determine is that the antenna does seem to have several minor resonances going on and at one point, low in frequency it is actually pretty close to 50 Ohms (Where the marker is). Anything else, really is impossible to determine from this presentation.

Better to plot the antenna’s SWR [2] over the same frequency range as above, now you can see that the antenna is resonant at about 300 MHz and has a decent SWR (< 5:1) for receiving applications in the frequency range of 800 – 1200 MHz. Further I can determine what the reactance is at any frequency by the common SWR formula,

Where Zo is the characteristic system impedance which is usually 50 Ohms for RF work and ZL is the load impedance, in this case the antenna impedance at a given frequency [2].

Conclusion:

It is really easy to actually help your customers, even if you don’t have the specific domain knowledge that they have. You can (and should) always look at competitors data sheets and use the best available format for each curve or table. As a sanity check, take your data sheet to a few customers and ask them what they think, this will provide immediate and valuable feedback.

As a final note: It is difficult for any manufacturer to keep everything up to date, as silly errors invariably creep in. Hence feedback is essential. Most datasheets that I now see have a HTML link to a help, feedback support site or email printed right on them, usually in the data sheet footer. There is no better way to get feedback than to ask for it and really no better feedback than from your customers right then when they are reading your data sheet and trying to use it.

Hall Of Shame:

Well, we have all done it, the ink isn't even dry on the page and we find a glaring error, or how about when you look at something you did years before and see an obvious fault you never saw before? Yes, so much to do, so little time for error checking! Here I present the 'Hall of Shame' of funny or wrong data sheet data....

[Edit 12Apr20]

Remember that first time your College Professor reminded you to "Think about your numbers" and use the digits responsibly? I still remember it. He told us to stop writing down 1% accurate numbers to 10 decimal places. Check out this actual data sheet curve above. Can I return the part if I only measure 40.163155 MHz @ - 3db???
Now with software we do this in "secret", by making all our PC software floating point calculations using: 64 bit / doubles, by default whether her we need to or not! (And I almost spelled: 'Whether' as 'Weather'. How embarrassing would that have been?)

Everything is "Guaranteed by design" when you don't spec anything! The Row for "Charge Injection" of an Analog Switch had no numbers in it, but it did reference this handy note. I think I will always refer to this condition as "Note 9" from now on.

The chart on the LEFT was from a FET Bus Switch data sheet, it shows absolutely nothing of value and did not even specify the test (load) conditions, it's a analog switch - naturally it will have a 1:1 slope with no load! Much more useful would have been to show the channel resistance versus input voltage as shown on the RIGHT that another manufacturer had on their data sheet. So again: LEFT Not useful at all, RIGHT very useful!

--- More to come ---

References:

[1] Linear Technology Application Note 132, “Fidelity Testing for A to D Converters”

[2] Wikipedia article on SWR.
https://en.wikipedia.org/wiki/Standing_wave_ratio

Article By: Steve Hageman / www.AnalogHome.com

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