[oopic] Re: Oopic and the LM335 Temperature Sensor

--- In oopic@yahoogroups.com, "dickydodds" <dicky@...> wrote:
>
> Guys,
>
> I am having a bit of trouble interfacing an LM335 to the Ooopic ADC
> lines and wondered if someone has already been here and got a circuit
> which works.
>
> I cant seem to get a reliable reading or a precise enough measurement.
> Even if I try to use the extvref line with a voltage as low as 4 volts
> (which deosnt seem to make a difference) - I just cant seem to get a
> correct reading. It deosnt help that the oopic cant do floating point
> math.
>
> Deos anyone have a suitable circuit that would allow me to read up to
> 5 lm335 temp chips but with some sort of op amp subtractor, say, to
> remove 2 volts so I can more or less read the true temperature
> directly, or at least make a eeprom lookup table to get the correct
> values?
>
> Any help, thought appreciated.
>
> I have tried both firmware Version 1 and 2.2+ on Oopic 2's.
>
> Cheers,
> dicky
>

I don't know what value of R1 you selected but the input impedance of
the A/D converter is going to be a problem. I might try 2k or so
because most of the specs are at 1 mA for Ir (the (5V - Sensor Output)
/ 0.001). There is the possibility that the input impedance of the
A/D converter is pulling enough current to 'starve' the sensor. This
will certainly occur if the resistor is much higher than 2k. An op
amp between the sensor and A/D pin will solve this.

Next, hook everything up and then measure the signal going into the
A/D pin with a digital voltmeter. See if it is constant. Better yet,
look at it with a scope. If there is any noise or wiggling, the A/D
conversion won't improve it. You might need to add a small capacitor
to ground at the pin. Try 0.01 ufd or 0.1 ufd.

You can use oEEPROM to read/write from EEPROM memory. It would be
better to put the table at a high address in the hopes that
reprogramming the OOPic won't overwrite it. On the S board, use the
E1 socket and a separate EEPROM. Maybe write a program to set up the
EEPROM and then your A/D program can just use it without the code
overhead of creating it. It depends on how much program space you
have left with your application.

If you need 5 A/D inputs, you better be using the S board.

You can learn everything you need to know about offset and scaling in
Chapter 4 of "Op Amps For Everyone"
http://www.ti.com/litv/pdf/slod006b

One advantage of using an op amp
for offset and scaling is lowering the output impedance to drive the
A/D converter.

Richard

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