Author Topic: Can UA741 op-amp do impedance matching for 75-Ohm coaxial cable?  (Read 1100 times)

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Offline rajhlinux

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Hello,

I am wondering if I can use some common op-amps that I have around to be used for impedance matching.
The transmitter is 75-Ohms and receiver is also 75-Ohms.

I need impedance matching because I would like to use an 100-Ohm CAT5e ethernet cable to transfer the signal between the 75-Ohm transmitter and receiver.

Here are some Op-Amps that I have:
LM386
NE5532P
UA741
OP07

I read that Op-Amp impedance matching can do the job but not sure if this is true.
Also would like to know if there are any circuit diagrams or tutorials on how to build such circuit.

The transmitter analog signal is a CCTV HD camera and the receiver converts converts analog signal to digital HDMI.

Here is some info about the analog signal (I am using HD-CVI):
https://shopdelta.eu/video-standards-in-cctv_l2_aid752.html

Thanks for any advice.
 

Offline Colt45

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Re: Can UA741 op-amp do impedance matching for 75-Ohm coaxial cable?
« Reply #1 on: August 13, 2022, 04:06:18 am »
The impedance matching is in the resistors, not really the opamp.

But that's besides the point, old standard definition TV still needs 6MHz bandwidth, and 741 is slow as molasses

Probably want to run the part over CAT5 as differential, too, which is a little more complicated.

see, eg
https://www.analog.com/media/en/technical-documentation/design-notes/send-color-video-1000-feet.pdf

 
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Offline rajhlinux

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Re: Can UA741 op-amp do impedance matching for 75-Ohm coaxial cable?
« Reply #2 on: August 13, 2022, 04:17:02 am »
Hello,

Thanks for your reply.

Will check the link you provided.

I know many people in the CCTV industry uses some kind of passive video balun to do impedance matching for using LAN cables instead of coaxial cables:
https://www.cctvforum.com/topic/26305-inside-a-balun/

Wondering how I can make one myself so I can order some ferrite toroids, but theres tons of them and do not know which toroid are the correct one to use for this application.
 

Offline Colt45

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Re: Can UA741 op-amp do impedance matching for 75-Ohm coaxial cable?
« Reply #3 on: August 13, 2022, 04:31:32 am »
I'm no good with magnetics, but if the things in the link really are $2.xx a pair... you can't get parts for that kind of money, just buy them.
 

Offline rajhlinux

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Re: Can UA741 op-amp do impedance matching for 75-Ohm coaxial cable?
« Reply #4 on: August 13, 2022, 04:40:26 am »
That is true, price is already low enough.

But I would like to design a custom PCB which has this functionality for another project.
 

Offline Whales

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Re: Can UA741 op-amp do impedance matching for 75-Ohm coaxial cable?
« Reply #5 on: August 13, 2022, 04:41:01 am »
I've seen lots of those ethernet<->BNC baluns for the analog video market.  If you want to know how it works then buy some and cut them open :)

Given that it's low power: the toroids are probably quite small.  From my (very basic and limited) toroid education: anything that looks like it's made out of a dark grey powder should do in this application as its only a few MHz?   The number of turns of wire on each side would vary a little bit and there would probably be 3 windings per toroid.  Probably one toroid total (ie only using one twisted pair).

EDIT: Putting in on a PCB will be hard as you'll still have to hand-wind and hand solder the windings.  Maybe some off-the-shelf pre-wound signal transformer might already exist with the right winding ratio?

Offline BigBoss

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Re: Can UA741 op-amp do impedance matching for 75-Ohm coaxial cable?
« Reply #6 on: August 13, 2022, 05:22:52 am »
https://www.minicircuits.com/WebStore/dashboard.html?model=TC1.33-1T-75X%2B

Or similar ( I don't know the frequency) Balun Transformer will match 75 Ohm Single Ended to 100 Ohm Differential.
There are others wider band or narrower.
 


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