Help: Identify this Diode (schematic)

The outputs that feed the Zeners are AC coupled with a zener and a 100K to ground. My guess is that the zeners just clamp the voltage to a safe level for the 6739 inputs. Look at the 6739 data sheet and find out if there are any input voltage constraints for starters.

A bit suprising that Zeners would be used to clamp video lines...since they typically have a high capacitance. This leads me to think that they may be some other type of clamp diode. Oppie

I don't think these are zeners, but Schottkys.

The capacitive coupling is followed by 10k resistors to ground, that would otherwise try to center the signal swing at zero volts. But the diodes clamp the negative swing to about -.3 volts, by charging the capacitors positively, so that, as long as the signal cycle occurs much quicker than the RC time constant, instead of being centered on ground, it has a maximum negative excursion just below ground. So the diodes are part of a level shifting scheme. I suspect almost any small signal Schottky diode would work, as long is it has a voltage rating well above 5 volts.

Jerry, I think you are correct. these are schottkys. these 100uf caps can put quite a negative punch with out limiting the charge current. I would use no less than the IN5817 no 18 nor 19. Ray

You guys are awesome! Thanks a million. You saved me a lot of stress. I'll pick some up tomorrow

Cheers,

Jerry

quite=20

ess=20

The capacitors are not the limiting impedance for these=20 signals. The outputs are not low impedance sources. That=20 is why buffer amps are needed.

Here is a clip from the data sheet:

"The LMH1251 is designed to interface with an ADC or preamplifier through an AC coupling capacitor as shown below in Figure 1. The RGB outputs of the LMH1251 are 700 mVPP video signals with the black level at approximately 2V, which is the chip=92s internal voltage reference level."

"The RGBHV outputs cannot be used drive standard 150- ohm video loads and require high-bandwidth buffers for this kind of application. For example, if the LMH1251 is to be designed into a stand-alone converter box application, the configuration in Figure 2 is recommended. To drive a display monitor over a standard VGA cable, a wideband, low distortion triple video buffer, such as the LMH6739, can be used with a gain of +2 to drive the RGB video signals of the LMH1251, and logic inverters can be used to drive its H and V sync signals."

I don't see that spec on the output impedance, but it must=20 be quite a bit higher than 150 ohms.

So any small signal Schottky should be big enough.

John, If you know the output pull down current of the driver then what ever that current is should be applied to the schottky and this should not exceed 0.55 volts drop in the schottkys forward direction. Is my guess. Ray

The capacitors are not the limiting impedance for these signals. The outputs are not low impedance sources. That is why buffer amps are needed.

Here is a clip from the data sheet:

"The LMH1251 is designed to interface with an ADC or preamplifier through an AC coupling capacitor as shown below in Figure 1. The RGB outputs of the LMH1251 are 700 mVPP video signals with the black level at approximately 2V, which is the chip?s internal voltage reference level."

"The RGBHV outputs cannot be used drive standard 150- ohm video loads and require high-bandwidth buffers for this kind of application. For example, if the LMH1251 is to be designed into a stand-alone converter box application, the configuration in Figure 2 is recommended. To drive a display monitor over a standard VGA cable, a wideband, low distortion triple video buffer, such as the LMH6739, can be used with a gain of +2 to drive the RGB video signals of the LMH1251, and logic inverters can be used to drive its H and V sync signals."

I don't see that spec on the output impedance, but it must be quite a bit higher than 150 ohms.

So any small signal Schottky should be big enough.

My point was, that if the output impedance is more than 150 ohms, and the signal swing is no more than .7 volts, then the peak output current can't be more than a few milliamperes, so 1 amp rated Schottky diodes are not needed. And the capacitance of such a big diode will have a bad effect on the frequency response of the signal.