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DAC714 and SPI control

Skippy VonDrake
Prodigy 45 points
Other Parts Discussed in Thread: DAC714

Hello, 

We purchased the DAC714 with the intent of building a converter as described in Fig 7 of its specification.

It is an old Burr Brown product but the 16 bit control of a +-10v output fit our needs well. However I am having a hard time getting any output to generate via an SPI command. (Vout stays at -0.29 volts). Perhaps my understanding of SPI with respect to this device is lacking.

I've been referencing this Burr Brown application bulletin titled " INTERFACING THE DAC714 TO MICRO-CONTROLLERS VIA SPI". Tried to include the link but this form said "no". It's easily found though.

I'm using an Arduino to generate the serial data in this manner:

Initialize SPI mode to Mode 3  (CPOL = 1, CPHA = 0).

Initialize A0 to High and A1 to High

  1. Write LOW to A0
  2. SPI.transfer the MSB byte to SDI
  3. SPI.transfer the LSB byte to SDI
  4. Write HIGH to A0
  5. Write Low to A1 followed by a HIGH to A1

  • 0
    TI__Guru* 92715 points
    Skippy,


    I don't know this part that well, but it looks like the DAC714 SPI communications is CPOL = 0, CPHA = 1. If you have further problems, you'll also probably need an oscilloscope or a logic analyzer to help debug this.

    I wrote a blog last month on debugging digital communications. Read through it and see if it helps.

    e2e.ti.com/.../help-i-can-t-talk-to-my-data-converter-what-s-wrong


    Joseph Wu
  • 0
    TI__Genius 10615 points

    Hi Skippy,

    Joseph is correct, this device is CPOL=0, CPHA = 1.

    If this still does not solve your problem, it would be best to share a snippet of the schematic as well as an oscilloscope capture of the DIO.

  • 0 in reply to Eugenio Mejia
    Prodigy 45 points
    Thank you both for responding.
    I have tried all 4 possible SPI modes. (i.e. all combinations of CPOL and CPHA)
    I have also tried all 4 modes with 2 different SPI logic sequences.

    1st SPI logic Sequence (as called out by the assembly code referenced in the application bulletin mentioned above):
    Step 1) enable DAC input register (LOW -> A0)
    Step 2) SPI transfer 2 bytes
    Step 3) latch the DAC data (LOW -> A1 followed by HIGH -> A1)
    Step 4) disable DAC input register (HIGH -> A0)

    2nd Sequence:
    Same as 1st sequence except Step 3 is swapped with Step 4.

    Throughout all tests the VOUT of the DAC714 has remained unchanged from the value it has upon power up (negative 0.29 volts).

    At this point this problem can be summarized in two questions.
    Question 1: Are either of the above SPI logic sequences the correct method?
    Question 2: Is there a simple test to prove/disprove the validity of a DAC714 chip?

    The second question I feel is pertinent because of the 4 Application Notes referenced from TI's DAC714 "Technical Documents" tab on the product webpage - there is a document titled "Shelf-Life Evaluation of Lead-Free Component Finishes". Which concludes with an approximation of a shelf life of greater than 8 years. Maybe this doc is linked to all similar components. But this is an old product and I have no idea if the DAC714s we purchased were non-functional upon arrival. Maybe they have been sitting in a hot warehouse for 10 years.

    And if there is no test to prove/disprove the chip's validity - are there alternative chips that can be used with the characteristics of the circuit in question? Which leads me to answer your question regarding schematics.

    We used a circuit described in the DAC714 product specification (written by Burr Brown). The circuit is from figure 7 (page 11) with the caption "Gain and Offset Adjustment in the Bipolar Mode Using D/A Converters (–10V to +10V output range)". We have followed that circuit's description completely.

    Our ultimate goal was to use SPI communication to get a variable reference voltage swing from -10v to +10v. The plan was to use that variable voltage to control a hydraulic proportional valve. We are using Arduinos to provide the SPI communication but we are not "married" to Arduinos. Other micro-controllers can be considered if needed.

    Hopefully there is a way to verify the products we have received as being functional.

    Thanks,

    Skippy
  • 0 in reply to Skippy VonDrake
    TI__Genius 10615 points

    Hi Skippy,

    Skippy VonDrake said:
    Question 1: Are either of the above SPI logic sequences the correct method?

    I can verify that the 2nd sequence will work. I need to see an oscilloscope capture of the digital inputs in order to verify that the interface is indeed correct. A schematic that includes the voltages will also be very useful. If you are hesitant to share this information publicly, is it okay to contact you to your E2E registered email?

    Skippy VonDrake said:
    Question 2: Is there a simple test to prove/disprove the validity of a DAC714 chip?

    There is not. The simplest way is to communicate with the device and update the output. (Although this is the most common problem as well.)

    I suppose that if the devices were truly defective, a good test would be to measure the current going into the supply pins. If excessive current draw is observed, then the devices are probably broken. (Expected values: ~16mA for +VCC and ~26mA for -VCC)

    Skippy VonDrake said:
    The second question I feel is pertinent because of the 4 Application Notes referenced from TI's DAC714 "Technical Documents" tab on the product webpage - there is a document titled "Shelf-Life Evaluation of Lead-Free Component Finishes". Which concludes with an approximation of a shelf life of greater than 8 years. Maybe this doc is linked to all similar components. But this is an old product and I have no idea if the DAC714s we purchased were non-functional upon arrival. Maybe they have been sitting in a hot warehouse for 10 years.

    While this is certainly possible, I would say that this is unlikely. We work with our distributors to maintain fresh samples available and scrap any old material. If you can share the package markings on the device, I can check the date of manufacture to give us peace of mind.

    Skippy VonDrake said:
    And if there is no test to prove/disprove the chip's validity - are there alternative chips that can be used with the characteristics of the circuit in question? Which leads me to answer your question regarding schematics.

    In order to make a good recommendation of another device I would need to get some insight of the application requirements. Since you are using figure 7 and the offset and gain controllers, I assume that you will be using analog calibration for each individual device. Why is this important in your application? What are the acceptable ranges for gain and offset errors? How about linearity?

  • 0 in reply to Eugenio Mejia
    Prodigy 45 points

    Hi Eugenio,

    Eugenio Mejia said:

    A schematic that includes the voltages will also be very useful. If you are hesitant to share this information publicly, is it okay to contact you to your E2E registered email?

    Yes, please *do* respond to my E2E registered email address so that we can continue this discussion in a more appropriate manner. For you have already answered the questions which other forum users might be interested in (e.g. what is the SPI interface of the DAC714 and how can one check the chips validity).

    While I await your email I will collate the answers to your other questions.

    Thanks,

    Skippy