D-to-A advise needed....

Guest · Posted: Fri Dec 22, 2006 9:01 am

Hi,

I need to make a serial-to-analog output device. The output will be 0 to 10V, and will not have to source much (if any) current. The output will be connected to a motor controller that requires a 0 to 10V input to command motor speed. I'm totally set with the PIC end of the project, but I'm not too experienced with the analog end of things. I'm planning on using a 12 bit D/A IC that communicates serially with the PIC. Maxim, Microchip and others all seem to have a good variety of these IC's. With a stable reference, it looks like I can get 0 to 5V out of one of these devices. Then I plan to use an op-amp to scale the voltage output up to 10V. I have +12V available to power my circuit. I do have the following questions:

1. Will the D/A output a "true" 0 to 5V?
2. If I use a rail-to-rail op-amp, will I get a "true" 0 to 10V if the op-amp is powered with 0 and +12V?
3. Can I use the -V generated by a MAX232 to power my D/A and/or my op-amp to get "true" 0V output at the bottom end?

Anyone have some specific device no.'s that they would recommend?

Thanks,

Charlie

newguy · Joined: 24 Jun 2004 Posts: 1909

Can't help with the D/A converter, but my rail-to-rail opamp of choice is the TI TLV237x. True rail-rail input and output, can be powered by a single ended or +/- supply, up to 16V max between Vcc and Vee. And they're cheap.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Guest · Posted: Fri Dec 22, 2006 2:31 pm

Hi,

No, you are right, I fell into the "more is better" trap. I don't need 2.5mV precision, and for that matter, probably not even 1024 step (10 bit) precision either. If I can control the final output in steps of 50mV or less, I'll be fine!

Yes, if this was a new design, I would use PWM techniques to control the motor. Unfortunately, it is a retrofit, and I simply need to replace a very expensive, and now obsolete, D/A card with the PIC and a D/A converter. Everything else (motor, speed controller, etc.) will stay the same. My requirement is driven by the analog input of the speed controller. It simply requires a 0 to 10V analog voltage for control.

Thanks!

Charlie

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

So, an 8-bit DAC will do the job. Many of these devices exist, so in order to narrow down the search could you give some more requirements like:
- Conversion speed?
- Power consumption?
- Cost?
- Size?
- Normal or extended temperature range?
- Startup condition (always off, half-scale, full scale)?
- ROHS compliant?
- Multiple channels (for future extensions)?
- etc.

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

SLo · Guest

If 8 (or less) bit precision is enough and you have the flexibility in the design, then you could make your own DAC with an R-2R resistor ladder on one PIC port. All the pins of the ladder must use the same port to ensure that the changes are all latched at the ssme instant. The drive of a resistor ladder would almost certainly require a following opamp buffer stage.

Guest · Posted: Tue Dec 26, 2006 8:16 pm

Hi Guys,

Thanks for all the input! I've settled on the Maxim MAX517 rail-to-rail D/A and the TI op-amp mentioned in the 2nd reply. Digikey delivered the parts today, and I had something running in a couple of hours! I need to tweak things a bit, but it looks like this solution is going to be perfect for my needs.

Charlie