I need to create an analog output that varies from 0-5Vdc, based on a value that varies from 1x10^-6 to 1x10^-3

[DanNatCorning]

OK, so here I am again.  I am almost done with this application.  I measure a helium concentration, which usually varies from approximately 1 x 10^-6 to as much as 1 x 10^-3.  I need to create an analog output that varies from 0 to 5 Vdc, based on this concentration.  The problem is that I don't know the exact minimum or maximum that I might ever read.  So I have two questions.  One, how do I determine a value that I should use for min/max, to ensure the best resolution possible?  Second, how do I implement the scaling (0 volts = 1 x 10^-6 and 5 volts = 1 x 10^-3).  I know this is not a complex thing, but I'm burned out after 18 days' straight programming and debugging this application.  Any help is ALWAYS greatly appreciated.

[gwd]

It depends on what you want to DO with the signal. If you want to display the value to a user then a log scale would be a good idea.

You need to realize that if you convert to linear values you will have much poorer resolution at the low end of the scale.

[DanNatCorning]

I need to send the voltage to another system, which will in turn be scaled to display for an operator the helium concentration (in ppm) based on the voltage received (0-5V).  would it be possible to split into mantissa and exponent?  If so, how do I do the scaling?

[johnsold]

If I understand correctly you want a 1 ppm to 1000 ppm scale that corresponds to your 1x10^-6 to 1x10^-3 values. If X is the original value, Y is the ppm value, and V is the voltage, then Y = 10^6*X (ppm) and V = 5*Y/1000 (volts). Or V = 5000*X.

If you want to allow some overrange, you could set 0.5 volts to be 1 ppm and 4.5 volts = 1000 ppm. Change the formulas above to adapt.

Lynn

[DanNatCorning]

feeling sheepishly dumb right now...I came up with the same thing 5 minutes ago. Sometimes I can't see the forest for the trees   thanks for the help.

[johnsold]

I think your "...burned out after 18 days' straight..." comment in the original post explains it all. Sometimes you just have to take a deep breath and get a little nudge in a new direction. Glad you figured it out.

lynn