I guess there is a good reason for this - for (say) LED dropping resistors or pull-up resistors (which covers a good deal of what resistors are used for theses days) the exact value is not critical at all, so by sticking to E12 values
There may be a bit more. A 681R 1% resistor is the same as a 680R 5% resistor if you up the wattage rating. As an example, in the same package the 1% is rated at 1/4W and the 5% as 1/2W. As a boss of mine used to say, when it comes to power dissipation there is no magic so why the power rating difference (actually there is a bit more to that as well- maybe the subject of a future blog)? The real difference is that if you allow the 1% to warm up the resistance will drift out beyond the 1% limit. When it comes to LEDs there is a good probability that the current you are driving them with will warm up the resistor, but for an LED as you point out, who cares?
@Antedeluvian - as you know I am a confirmed scrounger of bits from old boards. While I very rarely remove resistors these days unless they are very easy, I do see a lot of them and can confirm that (for thru-hole components anyway) the vast majority of them, even though they are 1% resistors, confirm to E12 values (not even E24, though you do see some of those). The same seems to apply to SMD resistors though I cannot speak with the same experience on those - they are usually so small it's a pain to check the values.
I guess there is a good reason for this - for (say) LED dropping resistors or pull-up resistors (which covers a good deal of what resistors are used for theses days) the exact value is not critical at all, so by sticking to E12 values, manufacturers don't need large (and expensive) inventories.
The color code for a 68Ω resistor (the three colored bands on the resistor) is blue-gray-black. You can find all sorts of information about resistor color codes on the Internet, including this rather nice resistor color code converter. Another useful tool is this LED resistor calculator, which rounds the result to the closest resistance value that is actually available.
Both these tools work with, at best, 5% resistor values, the E24 range. I haven't looked at hobby distributors recently but we are finding it increasingly difficult to find 5% resistors (that were originally used to pinch some pennies), without a minimum order. It seems the technology has improved to the point where 1% reistsors are easy enough to make without sufficient rejects to justify a 5% range. It is easy enough to change the values to the E96 range- it just means tons of ECNs.
At one point, I did read that 0.1% resistors were becoming cheaper, but they were going to revert back to the E24 series rather than the E96. I am not sure where that is right now.
On a self-serving note I should mention that it is easy enough to do these calculations with any computer based tool- I have used and written about using Excel for electronics extensively. Using some variation of the LOOKUP function and a table of resistor values, it is easy enough to find the nearest standard resistor value. My latest blog on Planet Analog goes a step further and is about finding the best match for a resistor pair using standard values (as in the gain of an op-amp or the voltage of an LM317). Excel-Optimizing Resistor RatiosPart 1 and Part 2. You can see all my blogs on Excel interspersed amongst some others here.
@antedeluvian: I must say that in this day and age of word processing and electronic publishing that their data sheets are decidedly "retro" to say the very least.
They aren't alone. Some companies do a really good job at this. Others ... not so much. The datasheets from Mixed Signal Integration are distinguished as much by the useful information they leave out as by the information they actually decide to include.
For example, as David Ashton points out, from the data sheet we don't know if the spectrum values generated by the chip are linear or logarithmic. This really is a key piece of information (I'm going to determine it experimentally thsi coming weekend).
@David: I notice the minimum strobe pulse width is less (18us) than the settling time (36us) - which implies you can read the data even if the strobe has gone back high? (ie you could actually read data during the purple times in your diagram?)
David -- I decided this point was important enough that I've gone back and added some stuff to the main column (page 1) -- take a look and see what you think.