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DAC8760: Analog output multiplexing with DAC8760

Alessio Galli
Intellectual 330 points
Part Number: DAC8760
Other Parts Discussed in Thread: XTR111, DAC8775

Dear all,

I'm wondering if there is a practical and safe solution to multiplex the DAC8760 analog output on three different outputs.

Basically I'm designing a 3-axis tilt sensor with analog outputs (0-5V, 0-10V, 0.5V-4.5V & 4-20mA) but I cannot use 3 different DAC8760 because the PCB space is an issue and it must be reduced.

I've noted this application node but I'm wondering if someone of you have already solve the issue with an alternative and simple solution.

Thinking on how to multiplex the standard output voltage ((0-5V, 0-10V, 0.5V-4.5V), a capacitor must filter the multiplexed management, but what for the output current.

An enough big series inductor can do the same job?

Thanks in advance to anyone can help me.

  • 0
    TI__Expert 5820 points

    Hi Alessio,

    The reference design you mentioned is a good solution for using one DAC8760 to produce multiple channels for voltage and current output. It uses a multiplexer and 8 sample and hold circuits to produce the 4 channels of voltage outputs and 4 channels of current output. It doesn't directly use the current output of DAC8760 but rather uses voltage sample and hold circuits followed by 4 XTR111 devices that convert the voltage outputs from the sample and hold to current. This is a viable solution but requires many other devices (OPAs+XTR+discretes) and also will have limitations on update rate due to the settling and hold time of the S+H circuitry.

    I would recommend you look into using DAC8775 which is a 4-ch. device with all of the voltage and current ranges you require. This would likely be a smaller solution size since there are no additional XTRs or OPAs required. The device also offers adaptive power management to reduce power dissipation and generates all of the required rails for operation from a single 12V-36V supply input with an internal buck-boost converter. You could use just 3 of the channels in your application and depopulate the components for the 4th channel to reduce solution size. I have attached an image below of a size-optimized layout with all 4 channels with protection components. If the internal buck-boost is not used then many components could be eliminated.

    Let me know if you have additional questions.

    Thanks,
    Garrett

  • 0 in reply to Garrett Satterfield
    Intellectual 330 points

    Thank you Garrett for the suggestion.
    It looks really promising except the cost.

    It's too really expensive.

    It's gonna to be the right solution for me if I find a safe method to multiplex the DAC8760 on 3 different outputs.

    As I said before the issue is the current output filtering to avoid step changing from one output updating to the successive.

  • 0 in reply to Alessio Galli
    TI__Expert 5820 points
    Hi Alessio,

    Understood regarding your cost concern. This is a single chip solution though so it includes most of the overall cost component wise.

    I am not sure of how you could filter the current output so that it would not change while updating the other voltage/current outputs. The reference design discussed previously simply uses a sample and hold followed by XTR111 V-to-I to produce the current. Since the input voltage will not change due to the sample and hold while updating other channels, the XTR will produce a constant current output. This could also be implemented with a voltage output DAC followed by the S+H, op amp gain stage to produce the correct voltage output, and XTR device to produce the current output.

    A couple of questions to better understand your circuit needs:

    1. What is the accuracy and all of the output ranges you require?
    2. Do you require the voltage and current outputs enabled simultaneously?
    3. What is your cost target?

    Thanks,
    Garrett
  • 0 in reply to Garrett Satterfield
    Intellectual 330 points

    Hi Garrett,

    yes, I understood the solution with the XTR for the corrent output and this is what made me doubt regarding the output current multiplexing,

    About your question:

    1- The accuracy is customizable by the user and depends on the output range. Anyway the 16bits is a must.

    2- I don't require the voltage and current outputs simultaneously. Either voltage or current.

    3- The target cost is as less as possible. The less it costs and the more I'm good :)
    Anyway the DAC8775 is really too expensive.

    What I can do is to try the multiplexed solution which I know that works for the voltage output for sure. I will try adding a series inductor on the output, with the tailored value, to figure out if I can multiplex the current also.

    What do you think?

  • 0 in reply to Alessio Galli
    TI__Expert 5820 points
    Hi Alessio,

    Do you have a total error specification like TUE %? With a 16-bit device there will still be error >> 1LSB due to gain error, offset error, and INL.

    I don't see a way to multiplex the current outputs without either an XTR device or discrete V-to-I converter. The same with the voltage output as well if you don't want it to change while updating other channels it will have to be buffered like in the reference design.

    Thanks,
    Garrett
  • 0 in reply to Alessio Galli
    TI__Expert 5820 points
    Hi Alessio,

    I am not sure what happened but your previous reply is blank. Can you re-post your reply?

    Thanks,
    Garrett
  • 0 in reply to Garrett Satterfield
    Intellectual 330 points

    Sorry Garrett it was an error,

    I've got your point...

    I'll keep the post updated as soon as I have other question or suggestion.

    Maybe solution ;)

    Thanks

    Bye

    Alessio