Generating a PAL video signal — Summing opamp greately distorts the output - Page 1
Forum section: electronics / projects

author: DoubtfullyCompetent
author_rank: Newbie
timestamp: June 10, 2025, 11:26:08 am
content: Hi there,I am working on a little project that involves outputting a black and white PAL signal from a microcontroller (I am using a pi pico 2 board). I wanted to have full 8 bit output range, so I built a three-opamp (NE5532 - two buffers, one summing) circuit that takes a roughly 0-700mv resistor ladder video signal and a 300mv offset (called NOT-SYNC) and sums them into the final output.The circuit does work, however there's significant distortion (noise?) added by the final summing amplifier, which is visible on the screen. I am wondering how I could go about reducing that. Everything is running on a perfboard, which does make it a little difficult to make modifications, however the opamps are placed in DIP-8 sockets, so changing them shouldn't be an issue. The opamps are running from a 5V positive and -2.1V negative supply.The yellow (CH1) signal is the output signal, the blue (CH2) is the video signal, while the purple (CH3) is the NOT-SYNC signal. I choose to trigger on a line that should be uniform light grey. As you can see, the output signal is anything, but uniform.I am wondering what my options are for fixing the issue. Would a better opamp help? An invering summing configuration (I wonder whether a negative signal would work, though that's a separate question)? My education is in software engineering, so my knowledge of electronics is rather limited - apologies if the whole approach is fundamentally idiotic.Thanks!

author: tggzzz
author_rank: Super Contributor
timestamp: June 10, 2025, 11:51:06 am
content: Without looking at your circuit in detail...Are any of the voltages indicating the opamp outputs are saturating?Is the "noise" random or ringing at a specific frequency? (Zoom in on the transition and aftermath)Do you have decoupling capacitors for each opamp? Are they properly positioned with short leads?What's the 0V reference voltage connections between the different parts of the circuit? (A ground plane is ideal, but approximations are probably acceptable)What's the probing technique?A picture of the implementation (construction plus probes) might be beneficial.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: June 10, 2025, 12:09:37 pm
content: I don’t think the NE5532 would be the best option for a video amplifier.Many moons ago I employed the NE592 successfully in a video processor.

author: DoubtfullyCompetent
author_rank: Newbie
timestamp: June 10, 2025, 12:18:15 pm
content: The right probe is CH3, the left one is CH2. the CH1 probe is positioned off the board, attached to the output cable (the tick red cable running off frame). Both opamps have a 0.1uF capacitor hidden inside the DIP-8 socket (underneath them). Ground runs through wires to the ground pins of the pico board for some elements, which then connect to the ground of the power input, except for the ground on the summing amplifier, which runs directly to the power input.The noise seems to correspond to noise on the inputs, which likely comes from the pico's power regulator (you can compare it to CH3 noise), but it's greatly amplified.

author: DoubtfullyCompetent
author_rank: Newbie
timestamp: June 10, 2025, 12:24:13 pm
content: Quite possibly, however the NE592 has a different pinout from a regular opamp and would require some some cumbersome modification to the circuit. Is there another opamp that could do the job while maintaining the standard opamp pinout? I can probably make a soic-8 one work. I also intend to use the remaining opamp for audio, but that doesn't have to be particularly high quality.

author: tggzzz
author_rank: Super Contributor
timestamp: June 10, 2025, 12:41:01 pm
content: Improve probing technique: connect the probe shields near the analogue ICs.Improve decoupling; add capacitors for each IC.Unclear where the analogue power supplies are originating.Keep noise off the power lines by avoiding long wires.Filter the analogue power supplies where they enter the analogue section.Ensure a good grid for the analogue ground wires; have "redundant" ground wires directly between different parts of the analogue circuit.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: June 10, 2025, 01:08:01 pm
content: I re-read your post. Your supply voltage is too low. The 5532 was never designed as a low voltage opamp. With only -2.1 volts, you could be exceeding the valid common mode input voltage range.Use a ICL7660 to generate the required-5 volt.

author: DoubtfullyCompetent
author_rank: Newbie
timestamp: June 10, 2025, 02:17:29 pm
content: I think I may have found the source of the problem or at least a major contributing factor. The -2V (-2.1, really) is generated by an MC34063 with several LC filters. The filters appear to be insufficient (I wonder whether I've made a mistake while moving the circuit from the breadboard to the perfboard, as it was much less pronounced on the breadboard) and the noise matches pretty well with the output noise, though it's amplified. I suspected the remaining noise to be caused by the circuit running from a breadboard and it did indeed induce some of it.I've ordered a pair of OPA1612s that seem to be much better in every day and more importantly rail-to-rail, so I'll try them out when they arrive.Here's a quick comparison (with CH3 on the negative supply rail) switching between supplying the negative voltage output from the MC34063 and a cheap signal generator (my only other source of negative voltage). The noise is still visible on the screen with the better power supply, but I think that's just because the signal generator is of rather poor quality and not really meant to handle even the small currents required by this circuit.I did try adjusting the MC34063 to generate -5V, which made the issue worse, causing me to check the negative power rail.I am not sure why I failed to spot it earlier. I probably should've included details about it, I imagine it would've been obvious to someone more experienced.I've ran another test with an MCP6002 (negative rail to ground) as the summing amplifier and it does reduce the noise significantly, however it's unable to keep up with the signal. Guess I'll have to wait for the better opamps to arrive.Thank you for the help, I'll update this and maybe post the whole project at some point. I'll also get some ICL7660s for future projects, as they seem much better suited than the MC34063s that I had at hand.

author: Benta
author_rank: Super Contributor
timestamp: June 10, 2025, 02:34:49 pm
content: IMO, the NE5532 is too slow for video. It's not only about GBW, but also phase performance.Try with LM6171/6172 instead, they're perfect for the purpose.