Hey! I was wondering if anyone had any tips on op amps unusual in video applications - just had a chance to rummage through a sale at a place that built boat radar and got some im unfamiliar with but seemed promising - for instance lt1078. Doing some experiments now and getting some results but I’m seriously fumbling my way through any op amp schematics and just guessing at redesigns for video applications - any tips would be great
Thanks!
Update - not amazing but it’s something!
Hey! Great you got some video coming out already 
I didn’t know LT1078 before so I’ve checked the datasheet, it is a precision amplifier, which means they’re made to have better specs than “standard” op amps in terms of offset, internal noise etc… which is great. Though for video, what you need to check is Gain Bandwidth Product (GBP), which will tell the bandwidth of the op-amp and is also dependent of gain.
LT1078 GBP is around 200kHz, which is rather low in this application, as a composite video signal contains luma ranging from DC to around 5MHz with a color subcarrier around 3/4MHz depending on the standard.
As a reference, TL072, which is often used in audio project, has a 3MHz GBP, and it’s bit low to let the colors/high details get through.
LM6172, as used in various DIY video projects, has a GBP of 70MHz, which is more suitable. Also AD8072 as discussed here
Then for op-amp design, I would say the first choice might be single supply vs dual supply.
For single supply design, I’d recommend checking Rob Schafer’s work
Another good reference for single supply design is Chroma Cauldron/Russell Krammer Wave Comber
Since composite video signals are often AC coupled at the output, a single supply design will ask for an offset of the signal at the input of your circuit, so it get into the usable range of the op-amp powered by the single supply. That’s also why rail-to-rail op amps are preferred in this case. An advantage of single supply is that it is easy to get from a DC wallwart.
For dual supply design, you can check LZX Cadet/Castle, ReverseLandfill and Visible Signals schematics. One of the advantage of dual over single supply is that the op-amp can process negative going signals (AC coupled video for example). Then the dual supply itself needs to be generated either from an AC wallwart with diode bridge/filtering/regulation (aka linear power supply) or from a DC supply using a switching design (as flyback topology).
thanks so much!!!really appreciate your generosity here
The LM6172 has been the go-to op amp in all of the through hole LZX DIY modules, but Lars from LZX mentioned in a post on the LZX forums a while back that the ADA4851 is something they are considering for future DIY modules since it is so much cheaper than the LM6172 (and has twice as many op amps per package). The TSSOP-14 package is about as forgiving as surface mount packages can get. People in the DIY space are usually pretty scared of surface mount, but anyone who feels comfortable soldering through hole stuff can handle this too Look up drag soldering tutorials on YouTube, EEVBlog has some good ones.
If you want to use this in a breadboard for experimenting, you should pick up some SOP14 to PDIP adapter PCBs.
I’ve been looking at various cheap op-amps with a view to using them in a low-cost RGB / VGA system I’m designing.
I’m a bit confused as to the difference between Gain Bandwidth Product (GBP) and slew rate. Which is better for video use? A high(er) GBP or a high(er) slew rate?
Two examples I’ve found:
MC33079: GBP=15MHz, slew rate=7V/µs
LF347 : GBP=4MHz, slew rate=16V/µs
Thanks!
Both are important but I think you’ll need a much higher slew rate than either of those chips provide. You want something that is able to handle a dramatic shift from your lowest amplitude to your highest amplitude without slewing it.
Common op amps for video have these slew rates for comparison:
ADA4851-4 - 375V/μs slew rate
LM6172 - 3000V/μs slew rate
The key question is just how many op-amps are in your design and to compare the price per op-amp between the lower cost ICs you’ve found & the quad op-amp ADA4851-4YRUZ. If the price difference was 20p per opamp & there’s 12 in total, we’re talking £2.40 which isn’t a lot when compared to the cost of 10 good quality pots or switches.
You could also look into having the SMT ICs & other SMT components pre-soldered onto the PCBs at the point of fabrication.
the lm6172 is starting to get too expensive (+4 euro a piece), so I will start using the ADA4851
Lars from LZx made a info sheet, search for LZX Interface Examples RevA.pdf