Hello,


I need some help determining what value of zener diode is in the path
between the LMH1251 RGBout and the input of the LMH6739.

D1, D2, D3 in schematic below...

http://img333.imageshack.us/img333/6...scoder29mx.gif

or

http://www.national.com/ds/LM/LMH1251.pdf page 9 figure 2



Thanks!

Jerry

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


"Jerry Lynds" wrote in message
news:2l6Vh.7705$VF5.3567@edtnps82...
Hello,


I need some help determining what value of zener diode is in the path
between the LMH1251 RGBout and the input of the LMH6739.

D1, D2, D3 in schematic below...

http://img333.imageshack.us/img333/6...scoder29mx.gif

or

http://www.national.com/ds/LM/LMH1251.pdf page 9 figure 2



Thanks!

Jerry

Jerry Lynds wrote:
Hello,


I need some help determining what value of zener diode is in the path
between the LMH1251 RGBout and the input of the LMH6739.

D1, D2, D3 in schematic below...

http://img333.imageshack.us/img333/6...scoder29mx.gif

or

http://www.national.com/ds/LM/LMH1251.pdf page 9 figure 2

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.

John Popelish wrote:

Jerry Lynds wrote:
Hello,


I need some help determining what value of zener diode is in the path
between the LMH1251 RGBout and the input of the LMH6739.

D1, D2, D3 in schematic below...

http://img333.imageshack.us/img333/6...scoder29mx.gif

or

http://www.national.com/ds/LM/LMH1251.pdf page 9 figure 2

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.

I agree 100%.

Graham

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


"John Popelish" wrote in message
. ..
Jerry Lynds wrote:
Hello,


I need some help determining what value of zener diode is in the path
between the LMH1251 RGBout and the input of the LMH6739.

D1, D2, D3 in schematic below...

http://img333.imageshack.us/img333/6...scoder29mx.gif

or

http://www.national.com/ds/LM/LMH1251.pdf page 9 figure 2

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.

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

Cheers,

Jerry



"Ray King" wrote in message
.. .
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


"John Popelish" wrote in message
. ..
Jerry Lynds wrote:
Hello,


I need some help determining what value of zener diode is in the path
between the LMH1251 RGBout and the input of the LMH6739.

D1, D2, D3 in schematic below...

http://img333.imageshack.us/img333/6...scoder29mx.gif

or

http://www.national.com/ds/LM/LMH1251.pdf page 9 figure 2

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.

Ray King wrote:
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.

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.

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


"John Popelish" wrote in message
. ..
Ray King wrote:
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.

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.

Ray King wrote:
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.

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.