Reactance is like resistance in DC circuits: it opposes current flow.
Yes but the vital difference between a reactance and resistance is the phase between voltage and current. So you can not simply say that the more current current flows. When you add the resistance and reactance together it is a vector addition with a vector product. Here it is not the individual voltage and current at any instance but you have to consider the real power and the phase angle of the power.
I am not sure what point you are trying to make because current flow here is so small it is not an issue.
Grumpy_Mike:
Yes but the vital difference between a reactance and resistance is the phase between voltage and current. So you can not simply say that the more current current flows. When you add the resistance and reactance together it is a vector addition with a vector product. Here it is not the individual voltage and current at any instance but you have to consider the real power and the phase angle of the power.
I know that all, even if I'm unable to to calculate the multiple interactions between the different components of the circuit.
Grumpy_Mike:
I am not sure what point you are trying to make because current flow here is so small it is not an issue.
Nothing specific, just saying that I increased the voltage divider resistance to reduce the current on the arduino side of the circuit hoping that would increase battery duration in a battery powered circuit. At the moment the arduino is USB powered so this is not an issue.
I've noticed in a simulator I should change C1 to 10uF and C4 to 4,7uF to have a decent circuit response at very low frequencies (the 50-200Hz range I keep mentioning): with the capacitor sizes in the original circuit the simulator has a very variable response with just a little frequency change, I believe partly as a consequence to that high pass filter side effect I mentioned before.
Because I understand so little about AC circuits, will those capacitor capacitance changes have any side effect I should be aware of?
I can share both the simulator software and the circuit I replicated in it if needed. I know it's just a simulator....
Another question for you Mike: why are we biasing opamp input to 6V (R1 and R2 voltage divider) to then drop the DC voltage component at the op amp output (C4)?
I mean, I thought that if we were connecting the arduino ground to the opamp power supply virtual ground we would have had a +/-5V output rather than a 1-11V... the former wouldn't require the removal of the DC component. I believe there's a reason if you didn't go for that, just wondering which that reason is...
Because I understand so little about AC circuits, will those capacitor capacitance changes have any side effect I should be aware of?
That will be fine.
I mean, I thought that if we were connecting the arduino ground to the opamp power supply virtual ground we would have had a +/-5V output rather than a 1-11V...
Well for a start you had a 12V supply so it is best to set the vertuial ground in the middle of your power supply. You will never get a +/- 5V output because this is not a rail to rail op amp and the closest you will get to the rail is within a volt, if that. You then need to remove the DC component from the opamp's output because you then need to put a 2.5V DC bias on the signal for the arduino that only uses 0 to 5V. It is much simpler to do this than trying to adjust what bias you have. You would still have to do this if you used a 10V supply and had the vertuial ground at +5V because that is the rail voltage of the arduino.
Yes, the op-amp is biased at 1/2 of supply.
The output deviates from that 1/2 Vcc.
With a 12V supply and no signal, the op-amp's output is a 6V DC level.
A 2V p-p signal's crest would be at 7V and its trough would be at 5V.
[quote author=Runaway Pancake link=topic=193115.msg1455698#msg1455698 date=1383613077]
Yes, the op-amp is biased at 1/2 of supply.
The output deviates from that 1/2 Vcc.
With a 12V supply and no signal, the op-amp's output is a 6V DC level.
A 2V p-p signal's crest would be at 7V and its trough would be at 5V.[/quote]
I understand that. What I don't understand is why we bias the input at 6V and then drop that 6V bias at the output instead of using a 0V bias and get an output swinging above and below 0.
In other words, wouldn't be simpler to connect the arduino ground to the virtual ground and remove C4? This should improve stability of circuit response at different frequencies as the op amp output impedance will have a null imaginary component (no reactance, just resistance).
rlogiacco:
What I don't understand is why we bias the input at 6V and then drop that 6V bias at the output instead of using a 0V bias and get an output swinging above and below 0.
An output "swing above and below 0" is exactly what you would have - if you used a dual-ended supply!
You cannot have an output with a negative voltage without there being a negative supply!
rlogiacco:
In other words, wouldn't be simpler to connect the arduino ground to the virtual ground and remove C4? This should improve stability of circuit response at different frequencies as the op amp output impedance will have a null imaginary component (no reactance, just resistance).
I made the point before: two 1K resistors between +5 and Gnd, their junction is "virtual ground". With that as "reference" your DVM tells you that there is +2.5V above and -2.5V below. Then connect "virtual ground" to circuit ground and what's the result?
[quote author=Runaway Pancake link=topic=193115.msg1457190#msg1457190 date=1383702224]
I made the point before: two 1K resistors between +5 and Gnd, their junction is "virtual ground". With that as "reference" your DVM tells you that there is +2.5V above and -2.5V below. Then connect "virtual ground" to circuit ground and what's the result?[/quote]
I believe we didn't agree on what I'm referring to as virtual ground, possibly due to the presence of two voltage dividers in the circuit.
Please consider the two attached circuits: the first represents the current design, the second what I'm trying to describe with words definitely unsuccessfully.
I believe the second is producing a +6/-6V power supply by splitting the 12V supply and using the Arduino ground as 0V reference for the op amp bias (non inverting input) and the AC ground. This should produce the positive and negative voltage supply we need to amplify the AC 0V biased input: as a consequence C4 would not be necessary as the output should already have no DC component.
Where am I wrong?
UPDATE
I've fixed the 6V bias diagram
I can't see the difference between those two circuits. You have a ground symbol in a different place on both circuits but to what end. Both supplies, 12V and 5V, are floating so what you connect to the physical ground makes no odds.
Are you mixing up this symbol with a signal ground or signal common? ( both the same thing )
Grumpy_Mike:
I can't see the difference between those two circuits. You have a ground symbol in a different place on both circuits but to what end. Both supplies, 12V and 5V, are floating so what you connect to the physical ground makes no odds.
In my mind the first has:
- 6V into opamp non inverting input
- 6V +/-0,25 into opamp inverting input
- 1/11V opamp output
while the second has:
- 0V into opamp non inverting input
- +/-0,25 into opamp inverting input
- +/-5V opamp output
that just as a consequence of the different ground connection between the power supply and the arduino, considering arduino gnd as 0V as well as the virtual ground created on the voltage supply and the AC input ground: they get all connected together to have a 0V reference.
Grumpy_Mike:
Are you mixing up this symbol with a signal ground or signal common? ( both the same thing )
very possible as I don't know what those are. :~
Attached is the simplest, reduced form for this subject.
Dicker with the values as you will, but this is how it must be.
I don't know why someone else placed the input between the inverting and non-inverting inputs.
The ac (capacitively) coupled input, as I see it, should be between non-inverting input and Gnd.
The ac coupled output must be retained, along with, for this application, the dc restoration diode.
I sure hope you get your bag of parts there - PDQ..
After playing a bit with the circuit in the simulator I've finally understood (sorry for my dumbness) why the second version of the circuit doesn't have any difference.
Nevertheless, I would suggest to anybody willing to play with AC circuits two links I found useful to understand this:
http://www.falstad.com/circuit/
The simulator doesn't replace a real oscilloscope, but it can help a lot when trying to figure out what's happening in the circuit as AC is not immediate as DC (at least for me 
)
I'll update this post with the simulator file I've used to describe the above opamp.
UPDATE: the simulator file attached
opamp_v1.1.txt (911 Bytes)
I'm back here, again struggling with this simple circuit: apologies.
I'm going to attach once again the reference circuit to simplify everybody's life.
Please remember my frequency range of interest is very low, between 50Hz and 150Hz.
The circuit is using capacitive coupling which represents an issue with low frequencies acting as an high pass filter. One example of such limitation is C1 which with 100nF presents a corresponding reactance at 50Hz over 3kΩ causing an impressive voltage drop of my input signal: I'm going to replace it with a 10μF capacitor (32Ω at 50Hz => 80mV drop), but I'm seeking advice for a better solution on this side. The same issue is presented by C4 on the other side.
With a bigger capacitor I'm able to get some decent signal into my amplifier, but such capacitors are correspondigly big in terms of size (I'm using polyester capacitors).
I believe my only choice is to avoid the DC decoupling by using direct coupling, but I think to move further along this route I'll have to abandon the TL082 opamp and move into one of the suggested ones.
I've got a LM386 and a LM358 to play with while I wait for the MCP602 to reach this corner of the world: by reading the datasheet it seems the LM358 seems a better fit with its single low voltage supply.
believe my only choice is to avoid the DC decoupling by using direct coupling,
You are getting mixed up here I think you meant to say:-
believe my only choice is to avoid the AC decoupling by using direct (DC) coupling,
Anyway whatever words you are wrong. Quite simply you do not DC couple audio signals and you have to avoid getting DC into a microphone to avoid distortion and damage.
You either use bigger capacitors or larger resistors or both. The frequency response is governed by the time constant of the resistor and capacitor. That is simply the product of the two values is equal to a time called the time constant. The fact that this might give you bigger physical capacitors that you want is just tough.
Grumpy_Mike:
Anyway whatever words you are wrong. Quite simply you do not DC couple audio signals and you have to avoid getting DC into a microphone to avoid distortion and damage.
I understand that, but can't I directly couple on the Arduino side?
Grumpy_Mike:
You either use bigger capacitors or larger resistors or both. The frequency response is governed by the time constant of the resistor and capacitor. That is simply the product of the two values is equal to a time called the time constant. The fact that this might give you bigger physical capacitors that you want is just tough.
You suggested to avoid bigger resistors as it would make the circuit more susceptible to noise...
I'm going to share here what I'm getting out on the Arduino side...
You suggested to avoid bigger resistors as it would make the circuit more susceptible to noise...
Yes it will but all engineering is about striking the best compromise, there is no one best answer otherwise it would all be a lot simpler than it is.
I understand that, but can't I directly couple on the Arduino side?
You can but you need to put the correct bias on the output, that is not easy.
Grumpy_Mike:
You suggested to avoid bigger resistors as it would make the circuit more susceptible to noise...
Yes it will but all engineering is about striking the best compromise, there is no one best answer otherwise it would all be a lot simpler than it is.
I understand that, but can't I directly couple on the Arduino side?
You can but you need to put the correct bias on the output, that is not easy.
Ok, it seems clear you believe it will be easier to find a compromise with RC values. I'll do some calcs, tuna couple of simulations and post here the values I believe are going to be necessary and seek your advice in a few hours.
As usual, thanks a million!
Grumpy_Mike:
Yes it will but all engineering is about striking the best compromise, there is no one best answer otherwise it would all be a lot simpler than it is.
After a few calculations and some simulations I believe I've reached the following values representing an acceptable compromise, on paper at least:
- R3, R5 and R6 raised to 2.2k?
- C1 and C4 raised to 47?F (6,8? at 50Hz)
The high pass filter cut off frequency will be around 1Hz, low enough to have practically no impact on my frequency range.
Can I replace C1 or C4 (or both) with electrolytic capacitors? Non polarized capacitors of such capacitance are not commonly available...
Another possible solution that just comes to my mind is to use a corresponding low pass filter to lower the higher frequency boundary, but that could be tricky: what do you think?
Can I replace C1 or C4 (or both) with electrolytic capacitors?
You can if the voltages / currents are small. Strictly speaking the AC will break down the dielectric when the capacitor becomes reverse biased but I have seen it used in these situations.
Another possible solution that just comes to my mind is to use a corresponding low pass filter to lower the higher frequency boundary,
Not sure I understand that.
Grumpy_Mike:
Can I replace C1 or C4 (or both) with electrolytic capacitors?
You can if the voltages / currents are small. Strictly speaking the AC will break down the dielectric when the capacitor becomes reverse biased but I have seen it used in these situations.
I believe I should find a compromise here: either I go for those RC values by using polarized capacitors or I use the following:
- C1 1?F (non polarized)
- R3 10k? (with the pot going into the 100k range)
- R5 and R6 1k? (unchanged from initial suggestion)
- C4 100?F (polarized, positive plate on the Arduino side: this should be safe enough as the Arduino side voltage divider should be always positive compared to the opamp output)
This, in conjuction with a slight shift of the frequency range to 100Hz - 200Hz should give me (again on paper) a decent constant amplitude: the simulator reports a 22mV difference in peak voltage (44mV difference in amplitude is an 0,8% error)
Another possible solution that just comes to my mind is to use a corresponding low pass filter to lower the higher frequency boundary,
Not sure I understand that.
another of my sick ideas: because the RC is configured as an high pass filter I was suggesting to level the situation by adding a low pass filter to balance the loss: the outcome would be a much more attenuated output but with a more constant amplitude.
As I said, a sick idea.
because the RC is configured as an high pass filter I was suggesting to level the situation by adding a low pass filter to balance the loss: the outcome would be a much more attenuated output but with a more constant amplitude.
I get it. You end up with a band pass filter and depending on the frequencies you will get more or less of a variation in the pass band.
Filter designing is a complex matter and is best done by applying the appropriate theory rather than an add-hock guess. It involves using the complex frequency plane and poles and zeros, to name but a few of the maths you have to do.
