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MSP430 Compatible DAC

Martin Holyfield
Prodigy 150 points

Other Parts Discussed in Thread: DAC108S085

May I please get help picking a DAC(s) (digital-to-analog converter) that is best suited for my MSP430-RF2500? I need at least 6 analog outputs in the 1.2-1.9 volt, 20-40mA range. I do know of this website (http://www.ti.com/lsds/ti/data-converters/digital-to-analog-converter-products.page) for TI DACs/parts, but I'm not confident in my choice.

I am currently working with the EZ430-RF2500 evaluation kit and the plan is to read 2 analog signals into each of 3 end devices (ED) and send all 6 signals (2x3) to one access point (AP). Hence the need for a 6-8 channel DAC on the AP.

I'm actually not stuck on the MSP430 MCU, but I need the low-voltage operation and I like the Anaren RF chip antenna. So the alternative might be to pick a different MCU for the AP that has at lease 6 analog outputs. -Thanks

  • 0
    Guru 46710 points

    What's desired DAC sample rate?

  • 0 in reply to Ilmars
    Prodigy 150 points

    High rate is not required. For this project, only several samples per seconds (SPS) will work. Pick 10sps.

  • 0 in reply to Martin Holyfield
    Guru 46710 points

    Martin Holyfield said:
    High rate is not required. For this project, only several samples per seconds (SPS) will work. Pick 10sps.

    Then most of SPI or I2C DAC's will do the job. Forget about driving 20..30 mA directly from DAC output, plan to use OPAmp buffers instead.

  • 0 in reply to Ilmars
    Prodigy 150 points

    Do code samples exist that configure and send digital outputs through SPI interface and/or through the I2C bus to the DAC?

    Regarding the OpAmp buffer and the DAC,  can you plz give me an idea how that might work? Does it buffer the DAC output or does it go between the MCU and the DAC?

    Obviously I’m not a EE. I trying to learn more about how to add 6 analog outputs to the MSP430 so I communmicate my design to contract PCB designers. I won't ask you how to solder :)  -Thanks!

  • 0 in reply to Martin Holyfield
    Guru 46710 points

    Martin Holyfield said:
    Do code samples exist that configure and send digital outputs through SPI interface and/or through the I2C bus to the DAC? 

    Most probably not. Because samples are CPU- specific but what you ask is more or less DAC-specific. Each DAC is more or less unique with different registers and so on. Usually reading samples and documentation of msp430 SPI/i2c peripheral and reading DAC user manual is enough.

    Martin Holyfield said:
    Regarding the OpAmp buffer and the DAC,  can you plz give me an idea how that might work? Does it buffer the DAC output or does it go between the MCU and the DAC?

    OpAmp buffer (unity gain amplifier) shall buffer DAC output. http://en.wikipedia.org/wiki/Buffer_amplifier

    Make sure your opamp input/output voltage range includes ground. This one is very nice, RRIO opamp:  http://www.ti.com/product/opa333 Just an example. For sure you can find cheaper opamps still having common mode that includes ground.

  • 0 in reply to Ilmars
    Prodigy 150 points

    Thanks All.

    Any other thoughts on adding an 8 channel DAC to MSP430-RF2500?

    Is there a better low voltage MSP430-XXX that I could use?

    Appreciated.

  • 0 in reply to Martin Holyfield
    Guru 46710 points

    Martin Holyfield said:
    Is there a better low voltage MSP430-XXX that I could use?

    There's no such thing as better msp430. They are just different, each have it's unique feature set. Please define msp430 qualities that are most important for you your application, then you can seek for msp430  here: http://www.ti.com/lsds/ti/microcontroller/16-bit_msp430/products.page

  • 0 in reply to Martin Holyfield
    Guru 12160 points

    Martin Holyfield said:
    Any other thoughts on adding an 8 channel DAC to MSP430-RF2500?

    Hi Martin,

    One other option for you is PWM DACs.  These come very naturally to MSP430 because most models have several timers.  Many models have a "Timer B" that has seven PWM outputs.

    A PWM DAC consists of a single PWM output which is then filtered according to application requirements and typically buffered.  The PWM signal is a square wave with adjustable "duty cycle" but fixed frequency.  The filter averages the high and low times to produce an analog signal with amplitude directly proportional to the duty cycle.

    No SPI or I2C or separate DAC IC required.  Instead, just the filters and the buffers, none of which require software.

    Jeff

  • 0 in reply to Jeff Tenney
    Prodigy 150 points

    Jeff, are you still available for more questions on this topic?

    -Marty

  • 0 in reply to Martin Holyfield
    Guru 46710 points

    Martin Holyfield said:
    Jeff, are you still available for more questions on this topic?

    If you want to ask Jeff alone- then ask using private message. Or just put your questions here, maybe others besides Jeff can help you.

  • 0 in reply to Martin Holyfield
    Guru 12160 points

    Hi Marty, I am still here.  I don't spend a lot of time on the forum but I can try to help you.  Also as llmars said if you combine my availability with others in the forum, I'll bet you can get all the help you need.

    I certainly did not invent the idea of PWM DAC, so there are lots of us who can help you.

    Jeff

  • 0 in reply to Jeff Tenney
    Prodigy 150 points

    Thanks all for hanging in there. 

    It has been suggested that I use an 8-ch DAC, DAC108S085, on the access point (AP) to output the inputs from the end points (ED). So I'm looking for guidance and feasibility feedback.

    I have also observed through other discussions that an SPI interface would be better than I2C interface. Thoughts?

  • 0 in reply to Martin Holyfield
    Guru 46710 points

    Martin Holyfield said:
    It has been suggested that I use an 8-ch DAC, DAC108S085

    Do you really need 10-bit DAC? 8 bits is not enough? 8 bit DACs are cheaper.

    Martin Holyfield said:
    I have also observed through other discussions that an SPI interface would be better than I2C interface. Thoughts?

    You shall consider I2C when you need bunch of chips to connect to uC. SPI is simpler (to implement) - that's why it could be better choice in your case

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