I am performing an RGB modding of an NTSC CRT TV. By performing tests I found out that the chroma chip will accept 2-2.5V Vpp RGB signal to get the proper colors. So I need to amplify and shift my input signal (0-0.7Vpp RGB signal).
The PCB has only a +5V supply available, so I chose to follow the SINGLE-SUPPLY OP AMP FORMS NONINVERTING LEVEL SHIFTER configuration as indicated on Application note 4836 from Maxim integrated, with some slight modifications
To simulate 0.7Vpp I used a 1.5V battery and applied a voltage divider to which I added a 75-ohm line resistor and replaced the 75-ohm terminal resistor with a 43 ohm one so to lower the voltage to 0.5V
To simplify the problem I decided to shift the signal by the full 3.3V of the voltage regulator (gain near unity)
Rf is 100 ohm and Rg is 4700 ohm to obtain a gain near 1
Here is the wiring diagram
I tested the circuit on a breadboard the circuit. I used a MAx4383 (4 channel version of 4380)
I only get about 2V on the output instead of 3.8 V (3.3V + 0.5V). I can't figure out why.
I measure the voltage on some points: I only got 2V on the IN+ pin of the op-amp. I believe the problem is because the current flows back from the 3.3V to the 43-ohm ground resistor, but on the other hand I don't understand how different my summing arrangement is from what is indicated on the application note? I would appreciate some help on that.
As a second step, I tried to isolate the 0.5V signal from the 3.3V supply by inserting a voltage follower with another MAx4383 op-amp. I get a clean 0.5V output signal from the voltage follower. Yet again when I sum up this clean 0.5V signal with the 3.3V (no ground resistor this time on my input line), instead of getting 3.8V I still get 2V.
If I disconnect the 0.5V input, I get on the output of the second op-amp about 3.3V, which is what I expect.
here below is the diagram
What did I miss and what is the best solution to this problem?