Can someone explain the mysterious non-behavior of the DAC0808 and DAC0832, is there a difference between them on the output side?

Question

So I have been very happy successful with my R-2R network ladder DAC powered by 74LS245 or similar bus transceivers, with 1% resistors, for nearly perfect output voltage going from 0 to +5 rail, with pretty good precision. Enough to produce voltage ramps to drive the X and Y inputs of my oscilloscope to produce a pretty decent raster display.

But I wanted to learn how to use a proper DAC chip and compare if perhaps the precision I can get with those might be better, and the space it takes up on the board. I have here the DAC0808 and the DAC0832. I know the '32 has some magic with dual latches on the digital side where one can stage the new input while the previous value is presented on the analog side, and then instantaneously activate the new stabilized (latched) value, etc. That digital stuff is easy for me to comprehend and control.

I'm pulling my hair out over the apparent complexity of the analog side of these two DAC chips. I cannot make this work for the life of me and I am flabbergasted about why this is so complicated.

First, all datasheets are telling me to use an external OP-amp. I understand why I would need that to drive a load, but I don't care about load right now, I have a high impedance load on my scope, and just reasonable voltage is all I want to see right now. Besides, if I have to add an op-amp I am already using up nearly as much space as in my home-brewed ladder-DAC.

Besides, I have already established that just any op-amp might not have the frequency characteristics that I would want aside from the hassle of procuring one that doesn't have voltage loss, if it can deal with a single GND-Vcc supply without needing negative power rail. So much hassle!

But there is intricate magic going on with these 4 inputs and outputs that I have been searching all over to find someone who explains it to me real slow, and found nothing.

1. +/-Vref
2. Rfb
3. Iout1
4. Iout2

it appears that the principle is somehow not voltage but current based, but frankly, I just don't understand it. And why is it so complicated?

All I want is to have the DAC produce output between the 0 and 5 V digital rail. Isn't there some minimal circuit that does just that? I have seen some, and only in passing on video, and when I build it I get nowhere, because I don't understand why it is like that to begin with?

Here is an example of such an in-passing shot of a simpler circuit, but then the guy shows me how he programs his stupid Arduino, and I don't care about the digital side.

If you don't want to explain it yourself, but you know a good tutorial introduction page I can read, I would much appreciate it.

Answer 1

Strangely you've chosen a current (as in amps) output DAC (DAC0808) and, to get a voltage out you need an op-amp added to the output: -

Same story for the DAC0832: -

Both these devices are as old as the hills so you might want to choose something more modern and accurate. Maybe a voltage output DAC like this might suit: -

Try this link to Farnell - it's set up for 8 bit DACs with an 8 bit parallel port.

Answer 2

Consider using a common-base bipolar circuit. Its very fast, but not extremely accurate.

For extreme speed, use a common-base bipolar device, a single NPN transistor.

Connect either of your 2 DACs to the emitter.

Have 1,000 ohm in the collector to +5v.

Have 51 ohms in the emitter to ground.

Now to bias the base --- use a similar transistor, with base_collector shorted.

• have 510 ohms in the emitter to ground.

• have 20,000 Ohms from collector/base to +5v.

• tie the collector/base to your first transistor's base.

Answer 3

That is a current mode DAC. Switching currents is faster than switching voltages, plus it allows for multiplying an input signal.

The magic is that there are 8 weighted current mirrors inside the chip, and the data bit inputs control whether current is taken from GND and fed to negative supply, or taken from OUTPUT and fed to negative supply.

It means that you must externally set the output pin voltage to GND with op-amp, and the current that is drawn from it is converted with feedback resistor to voltage output on op-amp.

I suggest at least opening the DAC0808 datasheet from TI to have a quick look inside how it works.