Re: LM34 to A/D

On Fri, 31 Aug 2007 13:59:12 GMT, Gary Peek
<mylastname@xxxxxxxxxxxxxxxxx> wrote:

John Larkin wrote:
Don't run the LM34 from more than +5... they misbehave if you do.

John, just for everyone's benefit, what have you seen the LM34
do at higher voltages?

Latch up. Several things can make an LM34/35 latch up, one of them
being to pull the output down, as the data*** suggests to allow
reading negative temperatures; sequencing matters. Latchup is less
likely if Vcc is below about 6 volts, but it can still happen,
especially if the output is pulled low statically or by a spike, like
from an ADC mux.

These gadgets also like to oscillate with the improper load. And the
internal pulldown is very weak, so mux spikes can pull it up and then
it'll settle back down slowly.

Bob Pease told me he'd fix it, about 15 years ago. I'm still waiting.



(BTW, all that care that must be taken regarding this device's
sensitivity to capacitive loading and the fact that you need
to included a negative supply or other roundabout methods for
reading temperatures below zero has made me try the LM50 if
I need an analog type sensor.)

Yup. Lately I've been using the SPI parts, which work great once you
write the (expletive deleted) code, or a surface-mount platinum RTD.

John



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