Roff
Well-Known Member
I'm not entirely comfortable with your "we dont need a negative supply" decree. Perhaps you could explain why this is. I could understand it if the LM331 is going to be feeding a micro. You could just remove the frequency offset in software.Hi again guys,
My apologies, i thought someone already made it clear to the OP that there would be the need for some kind of output resistor to ground and so this would be an automatic assumption. I think i would opt for a 1k resistor myself.
As far as the 5v power supply is concerned, i would tend to want to use a 5v *regulated* supply. However, because of the ratiometric operation of the HED and of course the two 20k divider resistors are automatically ratiometric this circuit could very well work with an unregulated supply too, as long as it doesnt dip too much lower that say 4.5 volts. I want to take another look at that HED data sheet first through. Yeah ok, it works only down to 4.5v so using a 5v unregulated supply is cutting it close. If the line dips down to low line the DC could very well dip as low as 4.25v which would not really be good enough. I would suggest a regulated wall wart or a wall wart with more DC output like 9v and a good 5v regulator at least as good as the LM317L or better.
As i said *several* times in this thread however, we dont need a negative supply so dont worry about that anymore.
EDIT: I think you can remove offset at the other comparator input to the LM331.
With a 1k pulldown, you might get down to 200mV with 250uA flowing through the feedback resistor.
I already explained that the HED offset AND gain (volts per gauss) are ratiometric. This means that, while the offset at the op amp output will be constant, the signal level will change if vcc changes, even if the magnetic field strength doesn't. That would be OK if the LM331 were ratiometric, but it isn't. Hence the need for a regulated supply.
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