Hello Miro,

Could you share a block diagram or more information about your circuit?

I have a few follow-up questions:

1. Are you driving a single video output (such as a single monitor or display)?

2. Are both channels the same style of video protocol/signal type?  For example, both may be SD video implemented as CVBS via NTSC protocol.

3. Are you looking to mux or switch between two sources, or will you ever need both video sources available simultaneously?

I would take a look at OPA875 to get an idea for how a 2:1 video multiplexer amplifier would work, if that would assist in your investigation.

I am happy to support your further on this topic.

Best,

Alec

Hello Alec

I am driving a single video output and switching between two inputs. Both channels are the same style of video. I am looking to switch between two sources, no simultaneous operation.

I think OPA875 will work, can you please confirm:

1. Two OPA875 inputs will be connected to two THS7374 outputs separately?  Then two THS7374 outputs connected to pin 6 on OPA875?  

2.  How should pin 5 and 6 on OPA875 be connected?  

** if you can please provide a simple sketch that would be awesome

3.  What should 402 ohm resistor values for OPA875 change to/stay as is?

4. Will 3.3V on Pin 8 for OPA875 be enough for normal operation?

Thanks for the support

Miro

Hello Miro,

The OPA875 does not include any internal filtering (such as THS7374 includes), but the OPA875 does include G = +2V/V to account for losses due to 75 Ohm termination.  You will need at least 6V supply for OPA875, if not a much higher single or split supply.  The minimum specified supply voltage for OPA875 is +/-3.0V.

If you need the filtering & features of the THS7374, you may need to use both devices with the OPA875 being your 'mux' to switch between the two sources.  I will work on a good test circuit & drawing for you to help illustrate.  You could consider using a single OPA875 to switch between the two inputs, with a single THS7374 as a 2nd stage to buffer & filter the output of the OPA875.

Pins 5 & 6 on the OPA875 are referenced to GND, so a logic low (0) would be GND (below 0.8V).  A logic high (1) would be the Vs+ voltage (at least 2V)

The above is accurate for dual (split) supply configuration, which is optimal for this device.  I can also support single-supply if you only have positive supply voltages & GND available in the system.

The resistor values change based on what the output loading would be; the goal is to maintain 75 Ohm matching to achieve buffer operation (+6dB from the amp, -6dB from the matching, results in 1V/V of gain overall).

Please keep me updated on questions or new information.

Best,

Alec

Hi Alec-

THS7374 powers on 5.5V max so if OPA875 requires min 6V supply then I would need two regulators...thoughts?? Currently I am using S24SE3R307NDFA at 3.3V to power on THS7374. 

I think this would be most beneficial "You could consider using a single OPA875 to switch between the two inputs, with a single THS7374 as a 2nd stage to buffer & filter the output of the OPA875."

I am not sure what the voltage of the video signal is, in other words how would I get logic hi to 2V? We will measure video IN and see what that voltage is.

We only have Single-supply voltage in the system

Schematic example would help.

Thank you 

Miro

Hello Miro,

Great, thank you for the update.  I have a better idea of what we are working with here.  Let me draw up some good details and information so we can both have the same visual for a working understanding.  Understood about the voltage rails, signals present in the system, etc.

Generally an analog standard definition video signal has a voltage range of about -300mV to about 700mV depending on the video standard (0mV being at or below black level and ~700mV being peak luminance, 100% white level).  The THS7374 datasheet has some good background information on analog video also.

Let me work on some supporting content for you, I appreciate you working with TI on this project.

Best,

Alec

Hello Alec

Great to hear additional information was helpful. We measured voltage of the signal that is passing through when disconnected of the video input we want to block; it spikes to +1V... regardless with the logic circuit we discussed it should be easily blocked. Please let me know if anything else is needed or on further schematic progress.

Thank you

Miro 

Hello Miro,

Thank you for sharing the update.  I will have time to work on this project tomorrow & early next week; I have some project work I am completing today.

I will share my next questions and schematic as I build them.

Thank you!

Best,

Alec

Hi Alec,

We obtained some additional measures that might be helpful. Were you able to power up IC's with the same fixed voltage supply, i.e.3V3 or 5V?

Thank you

Miro

Hello Miro,

I am still looking into the best circuit for operating OPA875 without needing a 2nd rail. 

Would you be able to share your additional measurements?

Best,

Alec

Hi Alec

I was able to locate https://www.digikey.com/en/products/detail/texas-instruments/LMH6570MA-NOPB/804718  that can operate on single or split supply configuration (Page 12  section 7.1), can you please help configure that circuit with THS7374?   Provide Rin, Rt, Rg etc. values

If OPA875 is better for our application; I did a quick search and OPA875 can operate with a single supply - see below. They are saying one of the power supplies (negative voltage) becomes ground does, that help?  We are approaching preliminary design rough idea and would like to have something on hand. 

https://www.google.com/search?q=can+opa875+operate+with+singlevoltage+supply&sca_esv=d01356799376b2bc&source=hp&ei=OcZKaIesOr6S0PEPi6rn6QE&iflsig=AOw8s4IAAAAAaErUSafGPU5X1EnD8P7AwJ-RqKn0Wl3G&ved=0ahUKEwjHrMe57-uNAxU-CTQIHQvVOR0Q4dUDCA8&uact=5&oq=can+opa875+operate+with+singlevoltage+supply&gs_lp=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&sclient=gws-wiz

OPA875 in a single power supply mode

e2e.ti.com/.../opa875-opa875-single-supply-circuit-application

Thank you

Miro 

Hello Miro,

Fantastic, I will look this all over for you.  Thank you again for being considerate and sharing good details.

Best,

Alec

Hi Alec

As we spoke earlier
You could consider using a single OPA875 to switch between the two inputs, with a single THS7374 as a 2nd stage to buffer & filter the output of the OPA875.

Since OPA875 has one output, how would I connect it to CH1 and CH2 that are coming in the THS7374 amplifier? Are they tied together?
When one input is selected at OPA875 how would signal know to which channel to go to if they are tied together?

Should I use two OPA875? Can you please share how to connect them with THS7374?

Thank you for your prompt response

Miro 

Hello Miro,

If you only need a single-ended (one channel video signal) at the output of your overall signal chain, you could use a single output of the OPA875 to drive the single CVBS input (pin 1) of the THS7374.  If this feels odd to only use one channel of the THS7374, you could drive the THS7374 into the OPA857 with proper 75 Ohm termination and matching.

The single THS7374 IC would use two of its four input channels for your two video signals, the OPA875 would help mux these signals.

It could also be possible to use the OPA875 without the THS7374, but you may need to add external filtering to filter out high-frequencies.

I am back in-office after some travel, so I can provide more support later this week.

Best,

Alec

Hi Alec

Glad you are back. This is what I would need: "The single THS7374 IC would use two of its four input channels for your two video signals, the OPA875 would help mux these signals". I have two signals that I needs to control with 2 mux chips, correct? Looking at the OPA857 datasheet  A logic high (1) would be the Vs+ voltage (at least 2V). What is the Max voltage that can be applied at logic high without damaging the chip? Also can you please provide a working email for better communication and what are your office hours so we can have quicker responses? Thank you Miro

Hello Miro,

We aligned offline already; I am responding here also.

If you only have two input signals and only need one output signal, you could use a single video mux amplifier.  I am under the impression the video signals are single-ended (not differential) and only need a single conductor for each signal.

Let me confirm the chip absolute maximum values for you.

Best,

Alec