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Robert Finch

VGA DAC Bandwidth on Nexys4ddr

Question

Hi, I'm hoping to PWM an extra bit(s) out of the LSB of the VGA DAC on the Neyx4ddr board so I'd like to know what kind of bandwidth the VGA DAC has. On the schematic it has a 4k resistor, but what's the capacitance ?

Edited by KaitlynFranz

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I asked the engineer that would know and he said this:

I couldn't find the capacitance specified for the 4.02K, 1%, 0402, Panasonic resistors that are called out in the parts list for R111, R116, and R122. However, when searching google I did find an technical note written by Vishay that discusses the frequency response of thin film resistors. Since this is a thin film resistor and they are all very similar my guess is that the TN applies to the resistors on the Nexys 4 DDR. 

1. In the TN they claim that an 0402 resistor has a capacitance of 0.0262 pFand an inductance of 1.89 pH. 

2. Based on Figure 5 I'd guess that in absence of any other external capacitance or resistance the analog bandwidth of a 4.02K resistor would be > 200 MHz.However, the connector, VGA cable, and the receiving end are going to add additional impedance. I'm not sure how much impedance any of these additional items adds so its hard for me to guess what the analog bandwidth actually is. 

Sorry I couldn't find a concrete answer but hope this helps.  If you do test this, I would love to hear about it.

Marshall

Edited by JColvin

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That is a really cool idea!  If I understand correctly, you want to PWM some of the signals to get higher than the standard color depth?  Is that even supported in the VGA specs or is it for some other application?

I don't know if anyone has tried the max bandwidth of the DAC circuit on the VGA port.  I will ask around and see if I can 

I will also see if I can get the capacitance values off of the BOM.

Marshall

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I personally haven't had any issues running the VGA ports on our boards at 1080p, which has a pixel clock of ~150 MHz. A test pattern with vertical black/white stripes still seems like it has crisp blacks and whites, even at the line edges.

Interesting idea PWMing the LSB for additional resolution... I think how well it works will have a lot to do with the particular ADC in the LCD monitor. I don't really know enough about the internals of a modern monitor to predicate how these VGA signals are sampled. A good way to test if your PWM works would be to just keep the upper bits all fixed to a particular color value for the entire frame, then do a test pattern with bars that have the LSB PWMed to different duty cycles.

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