Dears,,
Please support of customer question.
Question :
>PIn 6 #, 9# : More detail function.
>Customer want to reduce output ECAP(470uF). Pls send me your recommend.
>Is this part only 3 output HD system? IF 3 input SD signal is available operation?
Because customer selection NCS2564(Onsemi) is SD/HD available selection.
Thank you.
From Avnet korea
Eddy.lee@avnet.com
Eddy,
1) See p. 7 of the THS7373 datasheet for detail on the function of Pin #6 and #9. Pulling pin #6 high disables the part; pulling it low enables the part. When pin #9 is low, the internal 36MHz HD filters are used. When pin #9 is high, the filters are bypassed so that 1080p60 signals can be passed.
2) Reducing the DC-blocking capacitor will lead to sag/tilt in the video signal that will degrade the video quality. If a smaller capacitor is desired, it will be necessary to use a part with SAG correction. Capacitors on the order of 47uF can be used with a driver that has SAG correction. I suggest looking at the THS7303, which is a 3-ch device with SAG correction and selectable filters for SD/HD/bypass. Note that the THS7303 is a 2:1 MUX device. If the MUX functionality will not be used, the unused inputs can just be tied to GND.
We do not have any 4-ch devices with 1 CVBS channel and 3 SD/HD selectable filter channels that also has SAG correction. If a 4th, dedicated CVBS channel is required, it will be necessary to use a second single channel op amp, for example, the OPA360.
3) For the THS7373, the filters are set for HD (36MHz cut off). It will pass the SD signals, but the DAC images will not be filtered, which could lead to visible image interference.
For #2, this is why many designs have moved to DC coupling on the output. This eliminates any AC coupling issues such as tilt, is lower cost, smaller PCB area, and meets most industry standards except for Japan EIAJ spec which could only be a potential issue within Japan. The minimum capacitor size I would recommend is 330uF, but I have seen 220uF used in some situations. I personally do not recommend this value just to account for tolerances and temperature effects.
If AC coupled, the recommendation is to pass at least 10X lower frequencies than the refresh rate. If we assume 50Hz is the lowest refresh rate, then the system would need to have the corner frequency at most 5Hz. Using F(-3dB) = 1/(2 Pi R C) where R = 150ohms (double terminated 75 ohms), and F = 5Hz, then C is at least 212uF (=> 220uF).
For #3, many SOC's, but not all, do upsample the signal - especially the SD video signal. If the system does indeed upsample (ex: 81MHz or 108MHz or higher) then using the fixed HD filter would not present any problems to a SD signal (480i/576i).
Hi Kristoffer
Thank you for your Answer.
One more question about 2).
If use output CAP is MLCC CAP replace of ECAP?
I think it will be difficult to find cost-effective ceramic capacitors at the required capacitor values (330uF, 470uF for THS7373). If going with the THS7303 with SAG correction, the capacitor values will be reduced to the order of 47uF, but you will still have to consider the cost/size tradeoff between the electrolytic and ceramic capacitors.
Randy may have more to add, too.
Hi Randy
I always thank you for your kind support.
One of my settop customer request is schematic review and receive THS7373 front and back part value from TI comment.
I already send customer "Typical AC Input System Driving AC-Coupled Video Lines" schematic guide.
She'll process design THS7373 and 7374 about others model.
Please support your simply comment. 7801.Question.doc
from Eddy Lee (eddy.lee@avnet.com)
The only change I would recommend would be to eliminate all the input capacitors on the THS7373 input pins (C72, C105, C200, and C201). The signal coming out of the Video DAC is ground referenced and this matches up perfectly with the input to the THS7373.
Everything else looks good to me.
Note that this an External Forum and the schematic you posted will be shared with anyone who comes to this site. So you may want to edit your post to remove this if it is sensitive information.
-Randy