>Thanks guys for all the help.
>I found the app note that David mentioned :
>http://dbserv.maxim-ic.com/appnotes.cfm?appnote_number'0
>
>However, looking at it (fig 5, right?) I also see that MAX chip
>supplied the reference voltage to the circuit.
That's coz it has a hand reference output that's a nice "digital
voltage" of
4.096V. However, anything constant voltage would do, since it's used to
generate a constant loop current through the 3K3 resistor. (4.096V / 3K3
gives slightly over 1mA loop current).
being a really lousy
>electric engineer, I humbly submit my questions: in
this circuit I
>see two parts: one - constant current source for the RTD and another
>one is doing something funny with the voltage across the RTD.
Amplifying it! 10 times in fact.
>I
>understand why I neeed constant current source - if I know that the
>current is constant, then measuring the voltage accross the RTD would
>give me the resistance of the RTD, case closed.
Yes. Except the change in resistance for a given temperature change is very
small. Therefore the change in voltage is also correspondingly small. To
increase the "swing" of voltage for the same temperature change, the
second
half of the op-amp (the one with the 47K and 470K resistors) multiplies the
voltage change by 10. (470K / 47K).
>But why do I need that other part? Does it
conditions the voltage
>swing to more easily measurable for the ADC?
You could say it that way, yes. :-)
OH, btw, VERY IMPORTANT POINT....
When I was talking about swapping the + and - inputs of the opamp, I was
actually lying my a**e off. You need to swap the "sense" pins of that
circuit (leaving the resistor arrangement around the opamp exactly as it
is!).
That is, where they show two straight lines going left from the little
circles called "sense -" and "sense +", they should actually
cross over
before they get to the RTD. The "bottom" (closest to ground) of the
RTD
should go to the 47K resistor connected to the "-" of the opamp, and
the
"top" of the RTD should go to the 47K resistor connected to the
"+" of the
opamp.
>Do I need to change any
>R values to make it better adjusted to BX24 ADC?
No. The input range of the MAX197 in this circuit is 0-5V. This matches the
BX24.
>And what about the current source? Are there
simpler solutions than
>MAX 197 +opamp? Are there any ICs that can give me the same current
>source?
Probably the easiest way to get a current source is to replace the MAX197
with a 78L05 (or similar). Note, it's NOT a pin-for-pin replacement ;-)
Since you can get dual opamp chips very easily (that is, two separate
op-amps inside one chip package), using the same circuit as Figure 5, but
creating your own "reference" voltage is pretty easy. Make sure your
reference voltage isn't higher than 5V though, and I wouldn't go any
lower
than 4V. Where the second (half of the) opamp connects to "CH.x" of
the Max
chip, replace that with whatever ADC input you want on the BX.
What isn't mentioned in figure 5 is the power supply for the op-amp. The
circuit that I know worked (using the MAX197 as it happens, but that's not
relevant) used a "dual supply",.. .that is a positive AND negative
supply
and ground. Even though the output of the opamp should never go negative
(since the current flow is always "down" the RTD) having the split
rails
makes the biassing nicer, and keeps the circuit more stable. If I remember
rightly, we used plus and minus 5V rails to the opamp (the 5V rail was the
same as the one that supplied the MAX197 and the microprocessor that the guy
was using). The minus 5V rail doesn't have to be very strong at all (hardly
any current required), and could be derived from the plus 5V rail using one
of the Maxim / Dallas / Intersil style of low-current DC inverters (I think
the ICL7660 is one).
>Since I will not be using MAX197 for ADC, I think
its a waste
>to have it just as a voltage source for op-amp current source.
Most definitely! It was the rest of the circuit I was suggesting, since it
makes the job of reading an RTC circuit quite a bit simpler.
>Thank you so much for your help.
You're welcome. Good luck with the project!
Regards,
David.
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