YPbPr to RGB Color Conversion Circuit Issues

Thread Starter

Savestate

Joined Feb 17, 2021
2
Hey there, I'm just getting into circuit design for the first time and have gotten a bit stuck on a problem. I looked up different types of OpAmps and how they work and I thought I was able to put together a circuit that converted YPbPr to RGB but I'm not getting the expected output for Green given the example inputs.

Since I am working with a single power source, I had to make a Virtual Ground, which I believe I did correctly.
Also the resistor values are mostly arbitrary, I don't know what's typical for these types of applications.

The summing amplifiers used for Red and Blue work great, so no problems there (I think), the issue lies somewhere in the pipeline for Green, which uses two voltage dividers and two differential amplifiers.

I simulated the circuit in LTspice, which is how I was able to determine the values reported here.

(sync separation not implemented yet, trying to get color conversion working first)

Anyone have any ideas at a first glance where the problem might lie? The formulas used are on the screenshot, I can make clarifications as well. Again, I'm very new to this sorta thing so there may be some really obvious stuff that I'm not aware of.

Thank you!

 

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crutschow

Joined Mar 14, 2008
27,210
You cannot ignore the effect of the 330Ω op amp input impedance when using resistive dividers on the input.
Not only does the 330Ω load down the resistive divider (since it goes to the op amp virtual ground input), changing its attenuation, but it also changes the gain of the positive input (which depends upon all resistances being equal).

One option for the voltage dividers in the GREEN channel is to remove the 330Ω input resistors and generate a voltage divider with the equivalent impedance of 330Ω to maintain the proper gain of 1 for the positive inputs to the differential stages.

For example a value of 3320Ω for R11 and 365Ω for R12 will give a 0.1 voltage divider with an equivalent output impedance of 328Ω.
 
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Thread Starter

Savestate

Joined Feb 17, 2021
2
That did it, thank you! I talked to someone else as well and they noted that if budget wasn't an issue, I could add another opamp in follower configuration. Considering the bandwidth of the opamps though, it would be an unnecessary cost, but good to know both solutions to this problem.
 
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