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Old 2nd November 2012, 08:46 AM   #11
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Quote:
Originally Posted by davidsrsb View Post
And load driving, video is usually 75R load impedance, which requires high currents.
In the video world, the common specs are differential gain and phase, not harmonic distortion
Differential gain is the video analog of audio IM distortion. AFAIK there is no audio equivalent to differential phase.

Sofaspud mentioned the Analog Devices AD811 which I've used LOTS of for HD video. They are outstanding for this task but you need to know how to deal with CFB amps. They're not difficult but you don't go changing gain by altering the feedback resistor as it also changes the bandwidth.

Your audio amps won't do well - they'll do some but it's up to you how much bandwidth you can live without.

High speed amps do better on 4 layer boards with a ground plane so they don't turn into oscillators.

For video you don't want high impedance systems as they get unstable and noisy. Low impedance will run a little higher power but that's a small price for good performance.

G
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Old 2nd November 2012, 08:50 AM   #12
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Are you trying to build a video line driver?
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Old 2nd November 2012, 10:27 AM   #13
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Originally Posted by sofaspud View Post
Are you trying to build a video line driver?
RGB to ypbpr...

the Amiga has a 15Khz sync signal which I'm yet to find a tv that can sync it on the VGA input... however, they appear to be able to sync it on the ypbpr input...
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Old 2nd November 2012, 12:56 PM   #14
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It's the LT1254, as I was wanting a quad opamp, that could be run off +-12volt rails, and was suitable for video
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Old 2nd November 2012, 01:07 PM   #15
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Why don't you use a part that is made for this application? take a look at the MAXIM site for example: Amplifiers - Maxim

But there are many manufacturers that offer these kind of amps..
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Old 2nd November 2012, 11:06 PM   #16
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The LT1254 is a quad video opamp? what were you suggesting I buy?
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Old 3rd November 2012, 04:36 PM   #17
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Quote:
Originally Posted by ThyDntWntMusic View Post
The LT1254 is a quad video opamp? what were you suggesting I buy?
If you're looking at the LT chip consider the AD8024. It's not 'better' but the data sheet offers more guidance.

With Voltage feedback opamps you will not require input buffers as the gain of the amps can be set with the feedback resistors. With CFB amps you'll want unity gain input buffers so you can change gains via input resistors of stage 2.

Question: Is your RGB with / without sync? Sync on Green? Separate sync? More difficult yet, separate H and V sync.

G

Last edited by stratus46; 3rd November 2012 at 04:46 PM.
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Old 3rd November 2012, 11:41 PM   #18
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Originally Posted by stratus46 View Post

Question: Is your RGB with / without sync? Sync on Green? Separate sync? More difficult yet, separate H and V sync.

G
composite sync, but it's 15Khz...
which I'm yet to find a TV or monitor that can accept this on the VGA input... SCART should, but very few TVs here in Australia have SCART anymore, however, I've found every TV in the house to happily sync the 15Khz fed directly in to the ypbpr input, it's just a matter of getting the colours right
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Old 24th November 2012, 03:15 AM   #19
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It's not working right at all, I have no green?

I found the "Colour Wheel" thing in workbench... which confused me, as I was fairly sure there was meant to be some green in there?

This is with the LT1254CN Opamp:
http://www.datasheetcatalog.org/data...ogy/lt1253.pdf
Pinout on page 2!

Click the image to open in full size.

So I plugged my LCD monitor back in, which has a GBS8200 built in to the base... ah yes, green... so my circuit is displaying green as red, blue, black, and purple! as well as combining multiple shades of every other colour into one colour!

Click the image to open in full size.

So, I then plugged the TL074CN opamp in to the circuit, which lacks the bandwidth required to display a proper image, data sheet found here, I believe the pinouts are THE same, and I've checked it four times now!
http://ampslab.com/PDF/tl074cn.pdf

and was greeted by this:
Click the image to open in full size.

yes, it does display black as bright green... but at least it displays green?!?!
Faults LT1254? I've tried two different ICs, and they give identical results....

I think I'll order/find some different opamps, and see what happens... but can anyone explain whats going on?!?
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Old 24th November 2012, 05:29 AM   #20
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Quote:
Originally Posted by stratus46 View Post
If you're looking at the LT chip consider the AD8024. It's not 'better' but the data sheet offers more guidance.

With Voltage feedback opamps you will not require input buffers as the gain of the amps can be set with the feedback resistors. With CFB amps you'll want unity gain input buffers so you can change gains via input resistors of stage 2.

Question: Is your RGB with / without sync? Sync on Green? Separate sync? More difficult yet, separate H and V sync.

G
That pretty much answered the issue...

The EL2045 opamps originally used in this circuit are VOLTAGE FEEDBACK... while the TL1253 (and even the LMH6722 suggested earlier on) are CURRENT FEEDBACK!

they're completely different! and it explains why the TL074 gives correct colours, as it's also voltage feedback...

sorry, I haven't posted the schematic, it was a design by someone else, on the Amiga forum

Amiga RGB to YPrPb convertor - English Amiga Board
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