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PGA300: Does the DAC output a negative voltage

Scott Miller1
Intellectual 515 points
Part Number: PGA300
Other Parts Discussed in Thread: PGA305, PGA308, PGA900, DAC7760

In the case of pressure sensors bridges that swing positive and negative with respect to each other, with the application of positive and negative pressure (vacuum), does the PGA300, 308 and 900 DAC output a negative voltage when a vacuum is pulled on a Gauge pressure sensor?  Also, is it flexible enough that when you have an absolute sensor (say 15 PSIA), where the low DAC voltage output of 0V will occur when you pull a vacuum on the sensor, and the high output voltage of the DAC (5V) will be when the bridge is nearly balanced at room pressure?

Lastly, how does is the 3rd order compensation accomplished when operating as described above?  

  • 0
    Intellectual 515 points

    I meant to list the PGA305 above, not the PGA308, by the way.  I can't edit it....

  • 0 in reply to Scott Miller1
    TI__Mastermind 31655 points

    Hello Scott,

    The DAC in the PGA300, 305, and 900 is only capable of outputting a positive voltage up to 10V (with the highest DAC gain setting).It is possible to map any voltage within the DAC's output range to any sensor input, so while you cannot have a negative output from the DAC, your zero point for the DAC output could be in the middle of the range, so anything below that output would correspond to a vacuum input.

    You map the DAC outputs to the pressure inputs on the ADC & DAC Calibration page (on the PGA300 and PGA305). For example, if you are doing a 3 pressure calibration, the 3 DAC codes correspond to P1, P2, and P3. So once the calibration is complete any time the pressure input equals the input you have set in P1, the DAC will output the value you have set in the first cell in the DAC settings, and so on for P2 and P3. Anything input between those points will result in an output that is linear with the calibration points.

    The PGA900 does not have a linearization and temperature compensation algorithm inherently, and it will have to be programmed with your own firmware to accomplish the same thing. I recommend trying the PGA300 or PGA305 first to see if they meet your needs before evaluating the PGA900. The analog front end and DAC blocks are all identical among these devices.

    Regards,

  • 0 in reply to Scott Cummins
    Intellectual 515 points

    Hi, thanks for your response.  As I have gage transducers that need to output negative voltage with vacuum, this is a deal breaker as far as a one chip solution for me.  It would have been perfect.  I will have to add another output stage to add a negative voltage output, and then I can't switch between mA and 5V depending on customer requirement as I was planning. This seems like a big oversight.  With your DAC7760 and 8760 you can attach a negative supply and the output can swing +/-.  I wonder why TI didn't carry that over to the signal conditioning line.

  • 0 in reply to Scott Miller1
    TI__Mastermind 31655 points

    Hi Scott,

    I can't personally speak to the design decisions that were made for these devices, but it's a shame it won't work for your application as a single chip solution. You're right that an additional output stage would be the only option to add negative voltage output capability. Since the 4-20mA loop and voltage output both need their own separate output circuitry, how will you be switching between them? Would it be possible for you to add the additional amplifier needed onto the voltage output stage only, or is the concern more one of size or cost?

    Regards,

  • 0 in reply to Scott Cummins
    Intellectual 515 points

    Hi Scott-

    I want to be able to configure the device between 4-20mA, +/-5, +/-10V on the factory floor, electronically (comms).  I haven't figured out physically how we'd do the switching, but each case would obviously be more wiring.  I suppose another stage would work, but it makes things a lot more complicated.  Again, if you could create the same product with an output DAC stage like you have on the DAC product, that would be a much more viable solution.

    Scott

  • 0 in reply to Scott Miller1
    TI__Mastermind 31655 points

    Hi Scott,

    Thanks for the feedback! At the moment there aren't any plans to update the PGA300 or release a new device in that family which produces a negative DAC output. 

    Assuming there was a way to get a negative output from the PGA300, can you describe what you would like the changeover process to look like to switch between voltage output and current output modes? What inputs/controls will go to the sensor module? If you have a preliminary schematic using the PGA300 could you share it? Feel free to add me as a friend on E2E and message it to me privately if you prefer.


    Regards,