The LM34 is driven by a circuit that converts the LM34's voltage to a
frequency that is read by a remote computer. The computer supplies the
circuit with a ground and +12V and receives a frequency on a third wire. The
circuit in turn supplies the LM34 with the same ground and 12v supply and
monitors the output voltage of the LM34.
---
OK. So what you have is a voltage-to-frequency converter and an
LM34 wired like this:
CPU+12>---------+-------------------+
| |
+-----+-----+ +-----+-----+
| +12 | | +12 |
CPU fIN<--|fOUT Vin|<------|Vout TEMP| <~~~
| GND | | GND |
+-----+-----+ +-----+-----+
| VFC | LM34
CPU GND>--------+-------------------+
Where the LM34 outputs 10mV/F° starting with 0V at 0F, and what you
want is to offset the output of the LM34 to start at -1V for 0F and
rise at a rate of 10mV/F°?
If that's the situation, then you can either lower the ground of the
LM34 to -1V or raise the VFC's ground to +1V. Raising the VFC's
ground to 1V would be easier, but since its output would also be
referenced to the 1V "ground" it would probably be offset as well,
so that may not be a good idea. Can you tell us what you're using
for a VFC or post its spec's?
If you have -5V or -12V available at your CPU and you've got an
extra wire in the cable, you could generate the -1V like this:
CPU+12>---------+-------------------------+
| |
+-----+-----+ +-----+-----+
| +12 | | +12 |
CPU fIN<--|fOUT Vin|<------------|Vout TEMP| <~~~
| GND | | GND |
+-----+-----+ +-----+-----+
| VFC | LM34
| |
CPU GND>--+-----+------+-------+-[0.1µF]--+
| | | |
| [0.1µF] [R1] |
| | | |
[0.33µF] +-------+ |
| | | |
| | [R2] |
| +---+---+ | |
CPU-12>---+--------|IN OUT|---+----------+
+-------+
79L05
If you don't, then you could use something like a 7555 to build a
charge pump followed up with a regulator, like this:
CPU+12>------------+------------+------------+--+12
| | |
+-----+-----+ | +-----+-----+
| +12 | | | +12 |
CPU fIN<-----|fOUT Vin|<-----|------|Vout TEMP| <~~~
| GND | | | GND |
+-----+-----+ | +-----+-----+
| VFC | | LM34
CPU GND------------+ | |
| | |
GND | |
| |
+---[C1]---+------|------+ |
|K | | | |
[1N4001] [Rt] +---+---+ | |
| | | Vcc | | |
+----+ +--|TH OUT|--+ |
|- | | |___ _| |
[C2] | +-O|DIS R|O--+12 |
| | | | GND | |
GND | [Ct] +---+---+ |
| | |7555 |
| GND GND |
| |
| |
| 79L05 |
| +-------+ |
+-----|IN OUT|---+-------+
| +---+---+ | |
| | [R1] |
| | | |
[0.33µF] +-------+ [0.1µF]
| | | |
| [0.1µF] [R2] |
| | | |
+---------+-------+-------+
|
GND
If you're interested in either of these approaches I'll be happy to
post the component values.
Hmmm... After looking at the spec's for the 79L05 I think the lowest
output available from it is 1.2V, and I couldn't find any 1V shunt
regulators or adjustable negative regulators with outputs which went
below 1.2V, (In all fairness, I didn't look that hard
) so in
order to keep from having to use a Zener and a voltage divider
(which might be OK depending on how tight your -1V spec is) it looks
like the charge pump and an opamp with a reference and its output
forced to -1V with proper biasing will work. Depending on the
current requirement for the LM34's output it might be possible to
use a chip with an opamp, a comparator, and a reference in it to
build the whole thing.
How much more can you tell us about your application?
In particular, what are you using for your voltage-to-frequency
converter?