Video Amplifier (MAX4315) - suggestion needed

[howie]

What seems to be the problem if the output from a video amplifier, MAX4315 to be specific, fades out as time increases. Meaning, video output from MAX4315 is clear upon Video input but as time elapses the pictures diminished.

I measured the waveform from the oscilloscope and it appeared that as video display diminishes, the amplitude of the waveform decreases. In connection with voltage reading, as display diminishes the voltage increases. I am quite confused regarding this.

Does anyone have an idea on what seems to be the problem?
I attached schematic and data sheet for reference.

Regards.

 

[betwixt]

It should work, I use a similar device without problems.

Things to look out for are:

1. the output stage is designed to drive a 150Ω resistive load, normally 75 +75Ω it may misbehave with lower resistance or a capacitive load.

2. maximum input voltage is VCC-2.8 so be sure the signal doesn't peak above 2.2V.

3. don't feed it signals below ground level. If the signal has excursions below zero you should give the device a VEE below ground too.


Brian.

 

[howie]

It should work, I use a similar device without problems.

Things to look out for are:

1. the output stage is designed to drive a 150Ω resistive load, normally 75 +75Ω it may misbehave with lower resistance or a capacitive load.

2. maximum input voltage is VCC-2.8 so be sure the signal doesn't peak above 2.2V.

3. don't feed it signals below ground level. If the signal has excursions below zero you should give the device a VEE below ground too.


Brian.

_______

Thanks for your inputs..

Regarding suggestion "2", you indicated that the maximum input voltage should be "VCC-2.8". Does this mean 5.0 - 2.8 = 2.2V, So 2.2V must be the maximum VCC? Can you further expound on this? Where did you get 2.8VDC? Thanks..

Regards.

 

[keith1200rs]

See page 2 of data sheet - second item in table:

Input voltage range 0.035V to Vcc-2.8

So if you run the chip from 5V, the signal input must not go above 2.2V.

Keith.

 

[jpmanicdao]

I also encountered that same problem in my design wherein Video fades after 10-15 seconds. My application is for DVR SPOT OUT. in which until I cannot solve that part wherein Video is fading.

 

[howie]

See page 2 of data sheet - second item in table:

Input voltage range 0.035V to Vcc-2.8

So if you run the chip from 5V, the signal input must not go above 2.2V.

Keith.

Thanks for your suggestion Kieth..

Let me explain the circuit attached above, Video inputs 1 to 8 (VIDEO_INPUT_CH1-8) are inputs from NTSC cameras and output is performed through SPOTOUT.

I already measured voltage from video input (pins 5 to 12) and it is within the of 0.035V - 2.2V, 1.05V to be exact, BTW, the voltage input you are telling are the video input channels right? We tried the suggestions given above which had a 150-ohm driving load. The video appears but it fades through time.

Have you seen the circuit attached above? Any wrong values from the circuit? Or do you have any suggested circuit s?

Regards

 

[keith1200rs]

It isn't a chip I have used although I have designed a lot of video circuits in the past. What happens to the output when it "fades"? Does the DC output level shift or does the peak=peak amplitude just reduce? I assume you have it configured so SHDN is to VCC? Have you checked the VCC is OK when it fades? When you say the input is 1.05V - video is 1V peak-peak including sync pulses so what is the max/min voltage you are putting in? Does your video include sync pulses?

My suspicion would be overheating, given it is all DC coupled. Does anything get hot?

Keith.

 

[betwixt]

I wonder if the problem is shutdown pin. It should be ground to disable the outputs or VCC to enable them, your schematic has it at half VCC, perhaps it's hovering at shutdown point.

Brian.

 

[howie]

Hello guys,

Thank you for all your suggestions. I also believe that the problem is on the SHUTDOWN pin (SHDN, pin 14).

The following changes were performed (Refer to the attached schematic):

Case 1: I've tried Brian's idea to drive a 150-ohms load (R1731 from the attached schematic was replaced with 150 ohms R1854 remains the same at 0 ohm), the input voltage does not peak above 2.2V as suggested. The output fades.

Case 2: SHDN pin was directly connected to VCC-5.0V (R1858 from the attached schematic was connected with 0 ohm.). Output still fades.

Case 3: Output video fades after 15secs. But when video input is re-inserted, the video output becomes clear but fades again after 15secs. (I suspected that this is from the BNC cable but or BNC connector but I aready replaced the following)

I attached a video clip and schematic for your reference.

Regards to all,
Howie

 

[keith1200rs]

Hmmm. I was more interested in the oscilloscope signal than the display image and that doesn't fade - it disappears suddenly. The LCD fades because it loses sync, I think. I think you should concentrate on probing the input and output video at the chip pins, power supply lines, the shutdown pin and address lines. Could it be selecting another input?

Keith.