my question is that: i read that opAmps should be fed by "+V and -V". is that the same thing to feed with "2V and ground". i think voltage difference is important. but i also read somewhere that if we do not use -V and use ground instead, negative part of the signal will be clipped during amplification. is that true?
to be clear my question is: is "2.5V ,-2,5V" connection to power lines of the opAmp same with "5V, Ground" connection?
in terms of power supply, there are broadly 2 kinds of opamps -
single supply (lm358 and similar) - these are generally low power also, they take a +Vcc and GND as power. If what you are amplifying is a symmetrical signal (eg. audio signal), then you will need to capacitively couple and bias it through a resitor voltage divider to somewhere midway in the opamps swing range or the negative portion of your signal will clip... google "biasing"..... and do the same for output. For other uses, this is not necessary.
dual supply - these take symmetrical +/- Vcc power inputs and your audio signal can be wrt GND directly can be plugged in through a capacitive coupling and the opamp output will swing both positige and negative. however it is a pain to create symmetrical supplies and these opamps often operate at higher voltages
net-net, which opamp u pick depends on use... but for most simple uses, its much easier to work with single supply low power opamps that you can feed power to from arduino's voltage regulator.
On your +/-2.5v question, biasing an input signal will put it at conceptually at gnd... however, i don't really know if there are symmetrical opamps which operate at such low voltages as +/- 2.5 - check the datasheet. But there definitely are single supply low power opamps which work GND to 5V... you simply bias your input signal
I use OPA134 (or OPA2134 for stereo) op-amps. They work single-supply or dual-supply. As you've seen, single-supply is made to run at +5 and GND. (This range is perfectly acceptable to the OPA134), whereas dual-supply is made to run at, e.g., -2.5, +2.5 so that GND is the mid-way point of a 0v AC (e.g., audio) signal.
OPA134s are probably overkill for your project, but they're not terribly expensive even still, and I know they'll work with the voltages you have at your disposal.
This "audio ground" connects to the ground of the incoming audio signal. Do NOT connect this to your Arduino circuit GND, otherwise you'll have a short. It probably won't break anything, but you would have halved your voltage potential and still aren't able to swing negative, so it won't work.
For the resistors, you can use anything from about 1K to 100K. There are compromises to every value. As I understand (subject to scrutiny!), lower values provide a more stable ground, but also eat power due to lower resistance between Vcc and GND. 10K would probably be reasonable. (I'd appreciate review of this paragraph -- I haven't done much with single-supply op-amp circuits.)
You might also look at CMOS opAmps labelled as "rail-to-rail" input and
output. They should run off single-sided 0-5V power.
+5 ---| R1 |--- + ---| R2 | --- GND
|
| <- audio GND
This "audio ground" connects to the ground of the incoming audio
signal. Do NOT connect this to your Arduino circuit GND, otherwise
you'll have a short.
I don't understand what is being said here at all. ????
I think the proper thing to do is to use a voltage-divider on the analog signal,
in order to raise the reference level up so the negative part of the AC isn't
clipped.
However, where it says "Audio GND", that should read "audio signal", on
the one side and there should be another wire going "up" to an Arduino
A/D channel. And simply connect the Arduino and audio grounds together.
Plus, you should also insert a capacitor, say 1-uF to 10-uF, to block the
2.5VDC from getting back to the audio side.
Also, if your audio signal has larger swing than +/-2.5V, you can insert
another R in series with the capacitor, so the total voltage divider effect
will become 5K / (R + 5K), where the 5K comes from 10K parallel 10K.
Eg, if R=5K, then you can apply a +/-5VAC signal.
Let me clarify my approach, with the disclaimer that I'm new at engineering and could be very wrong:
My understanding is that you can connect the audio source's ground to the +2.5v level obtained from the voltage divider. Then, the AC swings relative to that virtual ground. Of course, you still need to capacitively couple the audio signal to prevent any problems with DC offsets. I've seen a schematic actually decouple both the audio ground and audio signal inputs, but there was also way more going on that I didn't completely understand.
In contrast, the approach that you're discussing seems to be to offset the audio signal so that it's raised above (the one and only) ground even on the negative swing. It seems to me that either way would accomplish the same end result. (Assuming, of course, "my way" is actually a valid approach.)
Again, all of my audio circuits thus far have been built with center-tapped transformers for the PSU, so I've always run at +/-5 or 15v with a real ground that is naturally halfway between. I've never tried a single-supply AC circuit, so I could be completely full of it.
conceptually yes, it amounts to a similar result as you say. The gotcha in LM358 which is the opamp indicated in the pic and some other similar workhorse single supply opamps is that they don't swing rail to rail... at least in single supply operation... it swings between GND and V+ - 1.5V, so powered with an Arduino, the output can swing GND to 3.5V... so you need to bias the audio input signal with resistor voltage divider to around 3.5/2 ~ 1.7V.
So the usage is -
Audio signal line wire 1 -> capacitive coupling -> voltage divider -> amplifier circuit input-> amplifier output -> capacitive couple for speakers or direct connection to Arduino analog pin and do analogRead() (you need to subtract bias voltage value)
Audio signal line wire 2 - connect with GND
My understanding is that you can connect the audio source's ground to the +2.5v level obtained from the voltage divider.
Probably not a good idea, as "both" Arduino and audio circuit are likely grounded
to the same point eventually - ie, mains ground. ARduino will get it via plugging
in the USB port, and audio amps are just made that way.
Even if someone says they're only gonna use the ckt with their battery operated
dowhatjamacallit, one day they or someone else will plug their AC powered audio
amp into the ckt.
I've been told that's not such a big deal. Anything that connects to mains has isolation. After all, it's not uncommon that line and neutral are swapped in your outlet -- no big deal to the AC circuit, but if you interconnect two devices that assume their neutrals are the same, and they're not, that would be a direct short. So, there will be a transformer in-line, which means the circuit ground is floating relative to the house wiring. As I understand it, this is part of why the ground lead is necessary between two devices -- otherwise, there's no guarantee they'll be at the same potential, and the reference will accordingly be offset from whatever it's expected to be.
Have said that, I still believe your point is valid and it probably is better to tie grounds directly and offset the audio signal instead.
Maybe the circuit I saw once (I'm thinking it may have been an automotive audio schematic) where the signal ground was also capacitively coupled ensures that only AC will ever flow through either conductor. Interesting stuff -- thanks for the discussion. I'm still a bit too new to electronics design to foresee all the implications of some of these decisions.
Do it his way. As I mentioned, it'll work ok [maybe] until you plug in an audio
source whose ground is wired to the same ground potential as the Arduino
ground.
I'm done here. Try it "yourself" and see what happens. That's the only way
to really learn.
