LMH1980: LMH1980

Joshua Ron

Part Number: LMH1980

Hello,

The LMH1980 has a recommended low-pass filter (on page #12 in the datasheet) for the video input, but it appears that its cutoff frequency (2.79MHz) harms the video signal itself (PAL/NTSC in my case).

Is this correct ? If yes, what filter should I take, in order not to harm the video signal ?

Thanks,

Joshua Ron

over 2 years ago

0 Noel Fung over 2 years ago

TI__Guru* 82240 points

Hi Joshua,

The suggested cutoff frequency is correct. The RC filter will not hurt the chroma component as the roll off of this simple 1st order filter is 20dB per decade. You may adjust the cutoff frequency a little bit up but it should be less than 4MHz.

0 Joshua Ron over 2 years ago in reply to Noel Fung

Prodigy 10 points

Hi Noel,

Thank you for your reply.

I probably miss something about the PAL/NTSC - what is the bandwidth required for it (provided the system is just a camera and a monitor) ?

0 Noel Fung over 2 years ago in reply to Joshua Ron

TI__Guru* 82240 points

Hi Joshua,

the spectrum will look like below, so the filter cutoff was chosen as < 3MHz.

0 Joshua Ron over 2 years ago in reply to Noel Fung

Prodigy 10 points

Thank you very much for the diagram.

So I understand that low-pass filter with cutoff <3 MHz , does harm the video signal - it cuts the Chrominance and part of the "Y" as well.

I think this filter is good for Sync extraction only.

Joshua

0 Noel Fung over 2 years ago in reply to Joshua Ron

TI__Guru* 82240 points

Hi Joshua,

It will hurt chroma signal but the degree is mind. As I have mentioned before, the roll-off of a simple RC filter is 20dB/decade. The filter will not eliminate the chroma component completely.

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LMH1980: LMH1980