You have forgotten a pull down resistor. Both inputs MUST have a resistive path to someplace within the power supply voltage and in your it will be ground. The non-inverting input is floating.p2mine said:still, for me is OK....
it's exactly the same....
I don't know what could be wrong.....
can you please explain?
It might be sensible to have input buffers, especially if you want a good isolation betweeen signal source, use rather low impedance values (low noise) and also a good HF filter.carlosfm said:The more I zoom the image the less I see.
Where's the problem there, P-A?
In my oppinion, the first row of NI op-amps is not needed...
There's a second buffer after that, it should be enough.
Indeed! I didn't see that. Where have you connected V+ and V- ?carlosfm said:Oh!!!
Check the PSU on the summing op-amp!
where is the problem???
sorry for the resolution, but this is a big picture and i cannot make it to big....
now, I can't find any problem here.
carlosfm, the first row of inverters is nedeed, because otherwise i'll have too much crosstalk, if the input is connected to a high impedance line.
the PSU of the right-most and left-most OA is ok. it's only the thing that the circuit it's upside-down....
for those of you who have protel99, i have shared the project here :
mydesign.zip
so, the question remains....
Where is the problem????
sorry for the resolution, but this is a big picture and i cannot make it to big....
now, I can't find any problem here.
carlosfm, the first row of inverters is nedeed, because otherwise i'll have too much crosstalk, if the input is connected to a high impedance line.
the PSU of the right-most and left-most OA is ok. it's only the thing that the circuit it's upside-down....
for those of you who have protel99, i have shared the project here :
mydesign.zip
so, the question remains....
Where is the problem????
OK, P-A, I need to have an explanation for this.... 
I have found thsi link:
http://www.phys.ualberta.ca/~gingrich/phys395/notes/node102.html#ch6feed
they say that it's ok the way it is...
I need a unity gain, so i will not have the resistive divider.
I could have a resistive divider with unity division, but i don't want to add 16 resistors only for that.
if you think i need an resistor connecting to ground, can you please make a drawing, or show me somthing similar?
I have found thsi link:
http://www.phys.ualberta.ca/~gingrich/phys395/notes/node102.html#ch6feed
they say that it's ok the way it is...
I need a unity gain, so i will not have the resistive divider.
I could have a resistive divider with unity division, but i don't want to add 16 resistors only for that.
if you think i need an resistor connecting to ground, can you please make a drawing, or show me somthing similar?
uaaaaaaaaaaaaaaaaaaaaaaaaaa again!
thank you carlosfm....
I haven't seen that either(probably very soon i would need some glases)
about the second stage of buffers, that's part of the filter circuit.....
it's actually something like:
30kHz filter ----->OA------>filter for satelites
. . . . . . . . . . . . . . . . . . |
. . . . . . . . . . . . . . . . . . |
. . . . . . . . . . . . . . . . . . +----->filter for subwoofer
thank you carlosfm....
I haven't seen that either(probably very soon i would need some glases)
about the second stage of buffers, that's part of the filter circuit.....
it's actually something like:
30kHz filter ----->OA------>filter for satelites
. . . . . . . . . . . . . . . . . . |
. . . . . . . . . . . . . . . . . . |
. . . . . . . . . . . . . . . . . . +----->filter for subwoofer
All opamps has "input bias current" and if you place a cap in series with either or both inputs there will will no DC currents into the opamp. If you _any_ functioning opamp design a DC current cal always flow into or out from the inputs.
The document you are refering to is about an _ideal_ opamp, not found in real life.
If you don't understand this. Take a classical input stage of an opamp. If you have bipolar transistors, from where do you get base current if you have a cap i series with the base? You must always provide a DC current path from the inputs. That's a law, unless the input stage is special (happens sometimes but it's very rare).
The question comes, what happens if you have forgotten this current path? You design will not work at all if you bipolar transistors but may work weirdly if you have JFET of MOSFET input. The last types requires only pA or nA of currents and it may flow in the surface of th pcb, in the dirt, flux etc.
The document you are refering to is about an _ideal_ opamp, not found in real life.
If you don't understand this. Take a classical input stage of an opamp. If you have bipolar transistors, from where do you get base current if you have a cap i series with the base? You must always provide a DC current path from the inputs. That's a law, unless the input stage is special (happens sometimes but it's very rare).
The question comes, what happens if you have forgotten this current path? You design will not work at all if you bipolar transistors but may work weirdly if you have JFET of MOSFET input. The last types requires only pA or nA of currents and it may flow in the surface of th pcb, in the dirt, flux etc.
I don't see what P-A talks about.
We have DC bias current from OP amp output through feedback to Vin-. And Vin+ is connected to ground or output of previous
OP-amp through a resistor. DC Bias current will be provided this way.
For active cross-over i believe you should check this page out.
http://www.woodartistry.com/linkwitzlab/filters.htm
It's my absolute reference for this.
Hope this helps.
Kindest Regards from Alexander
We have DC bias current from OP amp output through feedback to Vin-. And Vin+ is connected to ground or output of previous
OP-amp through a resistor. DC Bias current will be provided this way.
For active cross-over i believe you should check this page out.
http://www.woodartistry.com/linkwitzlab/filters.htm
It's my absolute reference for this.
Hope this helps.
Kindest Regards from Alexander
well, i've made some improvements, as P-A sugested.
I did made a point-wiring of this filter, but it did worked weird.
Ths low pass-filter worked fine.
the problem was with the high-pass filter. it did oscilate on high frequencies.
all the IC's were decoupled with 0.1u capacitors and the ground for all the circuits was taken from a single point.
i didn'd know why it did oscilate even today.
some reasons could be:
1. the gain was supraunitary.
2. I used ca324 and probably it is not a good OpAmp
3. As P-A said, i didn't have any resistors on the positive input, which was coupled with an condensator.
i've revised the schematic, and now it looks like this:
I did made a point-wiring of this filter, but it did worked weird.
Ths low pass-filter worked fine.
the problem was with the high-pass filter. it did oscilate on high frequencies.
all the IC's were decoupled with 0.1u capacitors and the ground for all the circuits was taken from a single point.
i didn'd know why it did oscilate even today.
some reasons could be:
1. the gain was supraunitary.
2. I used ca324 and probably it is not a good OpAmp
3. As P-A said, i didn't have any resistors on the positive input, which was coupled with an condensator.
i've revised the schematic, and now it looks like this:
An externally hosted image should be here but it was not working when we last tested it.
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