Jump to content
Electronics-Lab.com Community

DAC 0808


ashray
 Share

Recommended Posts

The schematic shows a DAC-08, (Analog Devices) which National Semiconductor calls DAC-0800, and this one is similar to, but not quite identical with the DAC-0808, or MC1408/MC1508 that other manufacturerers may call it.  The DAC-08 is somewhat more flexible, in that it may be used in some different configurations with non-grounded outputs, and with different, though now mostly obsolete, digital logic levels.

It is somewhat unfortunate that the part numbers are so similar...

The differences have to do with the electrical connections of pins 1 and 2, pin 1 being one of the "mystery pins".

Pin 1 is "VLC" on the DAC-08 and simply not connected at all on the DAC-0808. The purpose of this pin, on the DAC-08 is to be able to set different logic thresholds for the digitat inputs, that is, the voltage, at which the circuit determines whether the inputs mean "0" or "1". The data-sheet that I have here indicates that the threshold voltage is 1,4V plus whatever is applied to the VLC pin. For low-voltage logic (3.3V-5V CMOS or 5V TTL) the 1.4V is just about right, so the VLC pin can be tied to 0V for this use. The datasheets shows examples of using 15V high-voltage logic, and then the threshold voltage should be raised to 7.6 V, by feeding 6.2 V generated with a zener diode into VLC.

The DAC-0808 has a fixed theshold, at around 1.4 V, which makes it suitable for being driven by 5V TTL or CMOS, just like the DAC-08 when this pin is at 0V (or ground). As this pin is not connected on the DAC-0808, it doesn't really matter.

Pin 2 is "/IOUT" on the DAC-08 and identified as GND on the DAC-0808. What this means is that the connection from this pin to the non-inverting input on the following op-amp should be at a hard 0V level for the DAC-0808, but does not have to be on the DAC-08. Your circuit diagram shows this connection at a different level; thus in this circuit, the device must be the DAC-08, and a DAC-0808 may not work very well. (it probably will still put out something that looks kinda right, but as it's GND pin isn't really grounded, all bets are off when it comes to accuracy or linearity or such things that are important with D-to-As).

The other pin that needed explanation is pin 16, "COMP", which is there for the same reason on either the DAC-08 or the DAC-0800. This capacitor provides frequency compensation in the reference amplifier inside the converter, where the reference voltages and current between pins 14 and 15 is distributed to the 8 current sinks, one for each bit, that may be turned on or off, to make the resulting combination of current be drawn into  IOUT output. It is likely that the transistors associated with this amplifier stage will be self-oscillating if this capacitor isn't present. Its value depends on the resistances connected to pin 14, the data-sheet indicates minimum values of 15pF for 1 kOhm or 75 pF for 5 kOhm. This may be of interest if the DAC is to be used as a volume control or multiplier for a fast-moving signal.

Many of the examples show this capacitor with a value of 10 nF or 100 nF,  which is useful when Vref is supposed to be a stable DC voltage.

Here are links to some of the manufacturers' datasheets:

http://www.national.com/pf/DA/DAC0808.html
http://www.datasheetcatalog.com/datasheets_pdf/M/C/1/4/MC1408-8.shtml

http://www.analog.com/en/prod/0,2877,DAC08,00.html
http://www.national.com/pf/DA/DAC0800.html

Hope this clarifies things.

Ashtead

Link to comment
Share on other sites

hi... thanks a lot ... it cleared a lot of things....

i found this application note in the dac 0808 datasheet... but i guess this schematic is for unipolar operation...

can i convert it to a bipolar operation circuit by giving a reference voltage to the non-inverting terminal of the op-amp??

post-12053-14279143271434_thumb.jpg

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
  • Create New...