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Recently, I encounter a signal quality issue about Analog RGB signal.
I measure the VESA spec. The spec of rise/fall time is 1.66ns if resolution is 1080P 60Hz.
If you want to achieve the 1.66ns of rise/fall time, it means the bandwidth of THS7360 must be large than 210MHz (0.35/Tr)
But, DM8168EVM set the filter of THS7360 to 70MHz as below.
So, it cause the rise/fall time on our board is impossible to meet the spec.
The table shown below is my test result.
Can anyone help to explain this setting is reasonable and the DM8168EVM have any test result on VGA signal for VESA spec? Thanks in advance.
That amplifier is not designed specifically for high resolution VGA signals. It is designed for Y Pb Pr video for component video for TV displays. The SD spec is for 480i, the enhanced is 480p. Hi def is 720p and 1080i. The 1080p format never got a component video standard because of copyright law, so the future hi def standard was never implemented.
Can you try the tests with the filters bypassed?
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In reply to Loren Siebert 1:
If 1080p60 YpbPr is desired, then using the Full-HD filters is applicable. These filters were targeted to pass the SMPTE274 1080i/1080p standard supporting 60MHz passband for Y channel. The Pb and Pr channel filters are also kept at 60MHz instead of 30MHz due to potential issues with delay differences between filters (causing skew).
If 1080p RGB is desired, then the filters must be bypassed. The filters will dictate the rise/fall times and associated bandwidth and not the amplifiers inside. RGB does not follow SMPTE standards but rather VESA standards. The bandwidths/signal definitions are significantly different from each other. So if 1080p RGB is desired, then simply bypass the filters.
As Loren points out, the main target for the THS7360 was for Component YPbPr up to 1080p60. The bypass mode is pretty fast (280MHz -3dB and 800V/us slewrate Typ in bypass mode), but it was not explicitly designed to support every VESA RGB standard. So there are limitations.
In reply to Randy S:
I measured the rise/fall time with bypass mode. The rise/fall is about 1.51ns to 1.76 ns. Is the capability of DM8168 DAC ?
Besides, there is big overshoot/undershoot with bypass mode or filter mode. The overshoot is 15% to 19%. The VESA spec should be lower than 12%.
I try to change R599,R600 and R601 to 10ohm. The overshoot is reduced to 8%, but it will change the rise/fall time to 1.9 to 2.2ns.
Is there other way to improve and don't reduce the rise/fall time ?
Besids, is there otehr model can support VESA spec ?
In reply to Chiajung Hsu:
When it comes to the DAC on the DM part, it is recommended to post onto the Davinci forum as that is a separate business unit than the amplifier business unit. It is plausible that the DAC output could be a factor in this. If so, then the THS7360 amplifier only amplifies what it "sees".
As for the THS7360, as stated previously, the design focus was for component YPbPr signals which has significantly different demands than VESA specifications. Reducing overshoot is best accomplished by placing a capacitor in parallel with R609-611 (a very small cap of maybe 1pF or so), but this will impact rise/fall times for obvious reasons. So it may require a trade-off in performance.
An alternative recommendation is to use a very fast amplifier such as the OPA3695 to perform the gain function. However, very fast amplifiers like this are not RRO and the input common-mode voltage would require the use of +/-5V supplies to easily interface to the DAC. But, the slewrate and bandwidth is significantly faster than the THS7360 in bypass mode. Be careful of using voltage feedback amps as the gain bandwidth product will be an important factor with a gain of 4.5V/V needed for this application.
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