Need help using an op-amp to invert a voltage input

Thread Starter

cristy3200

Joined Mar 3, 2020
12
Hello and I good day, sorry if this might seem a easy to do thing, but I just started diving in the the world of analog video/audio as a side hobby so please bear with me :)
What I want to do is to invert a voltage range that ranges for 0 to 1V using a Video Op Amp, the reason for this is to invert composite video signal then using the same method to invert it back, I have a vague idea that this could be achieved with just a simple Op-Amp and some resistors but I cant seem to find and useful info on it cause it seems analog video is like dinosaurs. I'm thinking of using an AD810 Op-Amp for this job, but I don't know how to go on about doing this
Can someone point me in the right direction please? Any help is appreciated
I have managed to find here on the forums a similar schematic for inverting a voltage range of 0-5V, would this be adaptable for inverting a high bandwidth signal la composite video if we use the AD810?

Thank you for helping with this
 

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crutschow

Joined Mar 14, 2008
25,413
Where do you want the zero volt value of the inverted out to be with respect to the input?
Why are you showing a 5V signal in the schematic?
 
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Thread Starter

cristy3200

Joined Mar 3, 2020
12
Hello and thank you for your response, I found the schematic on the forums its not exactly for this application, after a little bit more research it seems I need a unity gain circuit cause the only thing I need to do is invert the signal in that 0V becomes 1V, or 0.7V becomes 0.3V, then on the other end of the cable I will use the same circuit to invert the signal back so it can be displayed correctly on the screen, the AD810 seems fit for this job cause it can work with 6.5MHz that PAL with audio. I can supply any voltage to this circuit depending on the requierment cause I can use a separate power supply. But the problem is I am not experienced in designing the circuit, so any help is much apreciated
 

Thread Starter

cristy3200

Joined Mar 3, 2020
12
Thank you for the detailed reply, I want to move the sync pulses in the 0.7 to 1V range instead of the 0-0.3V range so that the signal cannot be viewed directly by anyone that has access to the cable, by reversing the signal a normal screen I guess would just roll and display garbage, I am using multiple runs of composite coax from my garage in the basement of the apartment up to 2nd floor, and I don't want people to be able to just see whats inside my garage if the unplug the cable, I know its a low budget solution but it should do the trick good enough I think for casual thief's
 

Thread Starter

cristy3200

Joined Mar 3, 2020
12
Also, would it be possible to do this without a dual output DC power supply? Cause I see that there is need for +5 G and -G.
Thank you
 

crutschow

Joined Mar 14, 2008
25,413
would it be possible to do this without a dual output DC power supply?
The problem is that the AD810 is not a rail-rail op amp so the signal voltage would need to be translated to some DC level above ground.
To get the signal back to ground level for the receiver circuitry would require AC coupling, which would then need a DC restore circuit to avoid signal level shifting with differing video content.

What supply voltages do you have available?
 

Thread Starter

cristy3200

Joined Mar 3, 2020
12
I can supply anything from DC 3.3V up to 19V, maybe replacing AD815 with a rail to rail OP-Amp? unfortunately I don't know a suitable replacement?
Would the schematic you have kindly provided above need to be modified if replacing AD815 with a rail to rail opamp?

Thank you again for your time and effort in helping me with this
 

Thread Starter

cristy3200

Joined Mar 3, 2020
12
I can supply anything from DC 3.3V up to 19V, maybe replacing AD815 with a rail to rail OP-Amp? unfortunately I don't know a suitable replacement?
Would the schematic you have kindly provided above need to be modified if replacing AD815 with a rail to rail opamp?

Thank you again for your time and effort in helping me with this
Would the AD8051 be a suitable replacement? Its marked as Rail-to-Rail video op-amp with a high bandwidth
 

crutschow

Joined Mar 14, 2008
25,413
Would the AD8051 be a suitable replacement? Its marked as Rail-to-Rail video op-amp with a high bandwidth
The problem is that a rail-rail device cannot pull the signal completely to zero volts.
How close to zero does the output signal need to go?
Can you look at the original signal with an oscilloscope set for DC input measurement?
 

ci139

Joined Jul 11, 2016
1,696
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Thread Starter

cristy3200

Joined Mar 3, 2020
12
I cant seem to find the voltage swing tolerances for NTSC but I would guess they can be 0.1V +/- out of spec on the low voltage side, so I would guess 0.1V to 1.0V, the range of 0 to 0.3V is used only for the sync pulses, no image data in that range, so when inverting there should be no data or color loss if the voltage does not go to perfect 0V.
As for the impedance, I was wondering the same thing, if the compsite signal is 75ohm should R1 be also 75ohm?
Thank you again for helping me with this
 

ericgibbs

Joined Jan 29, 2010
10,790
Sorry if its a stupid question, but in this schematic what is AC 1 and V3? I am just starting to learn about reading schematics like this, thank you
Hi cristy,
That is a simulation signal source which generates a signal that you expect as an input for your amp.
The AC1 is also a test signal used to to determine the AC response of the amplifier.
OK.?
E
 

Thread Starter

cristy3200

Joined Mar 3, 2020
12
Hi cristy,
That is a simulation signal source which generates a signal that you expect as an input for your amp.
The AC1 is also a test signal used to to determine the AC response of the amplifier.
OK.?
E
Thank you for clearing it up :)
While taking a look at the schematic for the signal for composite video, I think we can safely asume, that any signal between 0 and 0.3V will be interpreted as a sync signal, so is it safe to say that we can use a rail-to-rail op-amp to invert the signal?
Tho its strange that every schematic for this seems reversed regarding the color and white/black levels, as far as I know NTSC uses negative modulation, so the black level will be at the top and white level at the bottom.
 

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Thread Starter

cristy3200

Joined Mar 3, 2020
12
Yes, that is the one, I want to invert it then invert it back to original form on the other end of the cables, it should allow tolerances for the 0-0.3V range on the sync pulses right? So a rail-to-rail can be used?
 

crutschow

Joined Mar 14, 2008
25,413
Here's the LTspice simulation for the circuit using AD851 RR amps, showing the transmitter inversion and the receiver recovery back to the non-inverted signal.
R6 and R9 provide the termination impedances for the coax to minimize reflections.
The circuit seems to be somewhat sensitive to the bias voltages Vbias1 and Vbias2, so the resistor values may need to be tweaked in the actual circuit for proper signal bias.
Note that the recovered signal (green trace) has about a +160mV bias.

1583340429661.png
 

Thread Starter

cristy3200

Joined Mar 3, 2020
12
Thank you very much for taking your time to help me with this, I will start recovering resistors from old electronics so its not gonna take weeks for them to get here if I order them and start building
When referring to tweaking the resistor values to lower the bias are we referring to R7 and R10 resistors? If yes would replacing them with variable resistors make the job easier? Also for my learning, on the transmitting end R3 is 1k but on the receiving end R1 is 2k, except for that they seem the same circuits, why is there this difference? Is it because of R6?
And one more question if I may, is R9 mounted on the transmiting circuit or receiving circuit?
Thank you again for taking your time in helping me learn
 
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