Newbie question on RAMDAC ADV476

[Stefan]

Hello Everyone!

I'm new to this forum and I'm pretty much of a layman concerning electronics.
I wanna go into some experiments generating VGA-signals from a breadboard using 74HC-logic, PLDs and such...

Though my question is not directly 6502-related, I still hope that theese nice folks around here can help me with a simple question:

I received an ADV476 Color Palette RAMDAC Chip, and the first thing I wanna do is to proof that it's not dead (since I've got it from china for cheap).
It has an Iref-Input that should be fed with a current of either 4.44 or 8.88 mA. Application-note sais that there should be a lm334 current source connected to it.
I do not have a LM334 and I don't wanna destroy the RAMDAC, so my question is:

Can I just put a 560 Ohm Resistor from +5V VCC to Iref, to provide ~8mA to that pin?
I don't care about how stable the RGB-outputs are, as I only wanna know wether the digital side (palette-RAM and registers) works as it should using an arduino.
I won't connect a monitor to this setup, I'll just measure the RGB-pins with the arduinos analog inputs.

Will it be OK to do so, or is there any chance that it would damage the ADV476?

[BigEd]

Welcome! It sounds to me that it would be (fairly) safe - the RAMDAC won't be able to do the right thing, because that current isn't going to be constant, but as you say, it looks like it won't exceed the spec too much. I think I'd choose 1k though, to be safer. 5/560 is already 8.9e-3, and the resistor and power supply could be off by 10% each, giving you 20% over.

You also need to be sure you've picked the right value of resistor - test it! Those colour codes are easily misread.

If you don't have 1k, a pair of 560 in series will do.

Note that you don't know what that input pin is going to do, voltage-wise, so you won't be getting a constant current.

It's an interesting challenge perhaps, how to cook up a constant current source from things you might have lying around...

[John West]

Looking at the data sheet, the reference current appears to be out of the pin. A resistor to +5V will attempt to put current into it, which you do not want. A resistor to GND will do better, and anything over 568 Ohms should be incapable of drawing more than 8.8mA (unless the chip has a voltage doubler on it, which I very much doubt - there's no need for anything above 5V).

[Stefan]

Thanks for the answers. Staring at theese figures from the datasheet, John seems to be right.
As far as I understand, current flows out of Iref through the limiting part down to ground.
But I do not really understand what C4 and C6 is for.

Anyways, I'll use 1K (measured to be 992R) between Iref and GND.

[BigEd]

oops! I didn't think of the direction of the current.