My question is about the operational amplifier of the schematic: **broken link removed**
What is the function of this amplifier ? It amplify the signal ok, but the goal is to work with saturation or not ? What are the roles of the capacitors C1 and C2 ?
I have an other question, the Resistor R4 and capacitor C2 wich are in serie after the lm386 are for adapte impedance, isn't it ? How did he choose these valors for adapting the PIC ?
Seriously speaking, the ADC input is effectively open circuit for the amplifier. There's no understanddable reason why they used an audio power amplifier instead of a simple OP.
Seriously speaking, the ADC input is effectively open circuit for the amplifier. There's no understanddable reason why they used an audio power amplifier instead of a simple OP.
'In order to correctly sample the signal we have to do two things. Firstly we need to amplify the signal to ensure we can use as much of the 0-5V range as possible. Secondly we have to move the signal's ground (of 0 volts) to a 'virtual ground' of 2.5Vs. This will allow the PIC to sample both the positive and the negative sides of the input signal. To do this the demonstration board uses a simple amplifier IC (the LM386-1). Since the IC is powered from a 0V and 5V power supply it has the handy side-effect of also moving the signal into the middle of our required power range. The LM386-1 was used because it is cheap and simple, however you could use a rail-to-rail opamp to achieve the same thing with a few more external components.'
An opamp that is biased at +2.5V with 2 resistors can be used with an input coupling capacitor to replace the LM386 power amplifier. The LM386 has internal biasing and has internal feedback resistors for a voltage gain of 20. Two feedback resistors added to an opamp can make its voltage gain 20.
The output of an LM386 with a 5V supply and feeding a high resistance has an output swing of from about +0.6V to about +4.4V which is not rail-to-rail. Many opamps have an output swing that is the same.