Instructables

Step 18: Forming incoming signal

Picture of Forming incoming signal
Let us get to the hardware now. The circuit may look complicated but it is really simple.
  • There is a 1 MΩ resistor at the input, to give a mass reference to the signal and have a high impedance input. A high impedance "simulates" an open circuit if you connect it to a lower impedance one, so the presence of the Girino does not mess too much with the circuit you want to measure.
  • After the resistor there is an emitter follower to decouple the signal and protect the following electronics.
  • There is a simple offset that generates a 2.5 V level with a voltage divider. It is attached to a capacitor to stabilize it.
  • There is a non-inverting sum-amplifier that sums the incoming signal and the offset. I used this technique because I wanted to be able to see also negative signals, as the Arduino ADC could see signals only between 0 V to 5 V.
  • After the sum-amp there is another emitter follower.
  • A jumper lets us decide if we want to feed the signal with a offset or not.
The Operational Amplifier that I intended to use was a LM324 that is able to work between 0 V to 5 V but also between, say, -12 V to 12 V. This gives us more possibilities with the power supplies. I also tried a TL084 that is way faster than the LM324 but requires a dual power supply. They both have the same pinout so can be changed without any modification of the circuit.
 
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womai2 years ago
One thing to be careful about: If you supply the op-amp with +/-12V it can drive almost as much into your Arduino - which will most likely kill the latter! Would be a good idea to add a series resistor after the last op-amp stage, and behind that two clamping diodes (Schottky type) going to 0V and 5V, respectively.
womai2 years ago
To the circuit itself:

The third op-amp stage isn't really needed. Did you try to run without it? (make sure the unused op-amps inputs are tied to VCC and VSS, respectively, to avoid oscillations).

Second, to protect the input stage against overvoltage I'd add clamping diodes to VCC and VSS, respectively, right at the place where the 1 MOhm resistor is. Good, cheap, fast, low-capacitance diodes would be e.g. 1N914.
womai2 years ago
Just a general comment - and it doesn't affect the content of what you say, you did a very nice Instructable! - what you have in your circuit is called an op-amp "follower" or a "buffer", not an "emitter follower". An emitter follower would be a single transistor (with suitable resistors), not a full-blown op-amp. In fact, your op-amp could have FETs in the output stage (not BJT = bipolar junction transistors), and FETs have source, drain, and gate, but no emitter whatsoever.