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Old 5th July 2009, 05:56 AM   #16
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Quote:
Originally Posted by ericgibbs View Post
hi,
Look at this simplified drawing.

Can you follow it OK.?
Yes, your comments really helped!

So in a sense the amplification for this OP AMP is quite similar to how the LM386 works, it is set by the ratio of R5 and R1+SPAN. Am I right?

And as for the offset, if I understand this right, it is a simple resistor (pot in this case) reducing the voltage by 0.453?

Other than, I just have a few more questions (sorry for all the questions):
  • What is the purpose of R3?
  • What is the purpose of everything before R4? (the diode, capacitors, extra resistors)

You said the LM386 is not suitable, but why? It seems to be doing the same thing as the CA3140.

Many thank!
slepax is offline  
Old 5th July 2009, 08:53 AM   #17
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Quote:
Originally Posted by slepax View Post
Yes, your comments really helped!

So in a sense the amplification for this OP AMP is quite similar to how the LM386 works, it is set by the ratio of R5 and R1+SPAN. Am I right?
The Gain [span] is set by R3[Rin] , R1 and the 20Kpot [Rf]

And as for the offset, if I understand this right, it is a simple resistor (pot in this case) reducing the voltage by 0.453?
R4 is the Zero pot, connected to the 4.7V ref voltage

Other than, I just have a few more questions (sorry for all the questions):
  • What is the purpose of R3?
    As above, its Rin
  • What is the purpose of everything before R4? (the diode, capacitors, extra resistors)
    The caps are for power rail decoupling. The 4.7V zener diode provides the Vref voltage for Zeroing

You said the LM386 is not suitable, but why? It seems to be doing the same thing as the CA3140.
The voltage swing range of the LM386 output is not suitable for the application.

Many thank!
Hope this answers your questions.
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PT100 input to ADC-aaaimage01.gif  
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PIC tutorials: Nigel's www.winpicprog.co.uk/ Bill's: www.blueroomelectronics.com/
Last edited by ericgibbs; 5th July 2009 at 08:59 AM.
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