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Wrong initial output in VoutA and VoutB in DAC7563

Oscar Gomez Fuente
Expert 2066 points
Other Parts Discussed in Thread: DAC7563, DAC8563, OPA140, TPS7A30, DAC8165

Hi,

I am trying to configure the DAC7563. I can write correctly to the DAC-A and the DAC-B. But in the init the DAC7563 has the default values of 0V on the DAC-A and DAC-B.

According to the datasheet the code DAC7563 is the mid-scale, so in the initial state the DACs will have to supply 2.5Volts, won't they?

Connections:

1- VoutA -> oscilloscope

2- VoutB -> oscilloscope

3- GND -> GND

4- LDAC -> GND

5- CLR -> 3.3V

6- SYNC -> CS (uController)

7- SCLK -> SCLK (uController)

8- DIN -> MOSI (uController)

9- Avdd - > 3.3V (with capactior 1uF)

10- Vrefin/Vrefout -> capacitor 470nF

Thank you so much. Best regards.

1º- VoutA -> oscilloscope

  • 0
    TI__Genius 10615 points
    Hi Oscar,

    The DAC8563 has the internal reference off by default. You will need to enabled it via SPI before the outputs will update to midscale. Alternatively you can supply your own external reference voltage at the Vrefin/Vrefout pin.
  • 0 in reply to Eugenio Mejia
    Expert 2066 points
    Hi Eugenio.

    But if I configure the DAC8563 and then power off and power on the power supply of the device, Is this internal reference saved to on?

    My target is init the device with the mid-scale activated without enalbed nothing via SPI because the output of DACs are going to feed the set point of the speed of some thrusters that need to receive midscale to work well as soon as possible.

    Thank you so much. Best regards.
  • 0 in reply to Oscar Gomez Fuente
    TI__Genius 10615 points
    Unfortunately our DACs do not have persistent memory. The memory is reset to the default state at power on. If you need to generate midscale as soon as possible you can supply an external Vref voltage. This way the device will power on to midscale as soon as possible.

    Why is midscale necessary for this application? Maybe I can help you find a solution.
  • 0 in reply to Eugenio Mejia
    Expert 2066 points

    Hi Eugenio,

    As soon as possible our HW platform has to generate six analog signals of 0 Volts. These signals have to control six thrusters. They have an analog input to control the speed from -5V to 5V, really the range is -4.75V to 4.75 because from -5V to -4.75V and from 4.75V to 5V is a dead zone. (-0.25V to 0.25V is a dead zone too).

    fI we don't generate 0 Volts, the thrusters will start to work.

    My Idea is to use one op-amp configuration like this:

    In this way with the midscale [2.5Volts] and this op-amp configuration I can generate 0 Volts when the HW platform start without the need to configured the DAC  through the SPI bus.

    As you told me, due to the fact I'm going to need midscale as soon as possible, I'm going to need an external Vref. Could you recommend me this IC (Voltages reference of 2.5Volts)?

    I need you recommend me the following ICs:

    1. The op-amp (I was thinking to use OPA140. What do you think?).

    2. A voltage regulator (low noise) to power supply the op-amp. [ +Vcc = 5.5 Volts and -Vcc=-5.5 Volts].

    3. A voltages reference of 2.5Volts to feed the dac7563.

    According to 1: As I'm going to need six DACs and I'm going to use three dac7563 with two DACs each. Probably the best option it's to use one IC with two Op-amps, one  for each DAC.

    According to 2: I'm not sure it's a good idea to use only one IC for voltage reference to feeding the three dac7563 or it's better use one for each dac7563 (probably it's better to improve the low noise). What do you think about this?

    According to 2 and 3: I'm not sure it's a good idea to use only one IC to generate +5.5 and -5.5 to feed the two op-amps (2 op-amps for ceah dac7563) or it's better to use one for each op-amps. What do you think about this?

    Thank you so much. Best regards.

  • 0 in reply to Oscar Gomez Fuente
    Expert 2066 points

    Hi Eugenio,

    I am considereing to use the DAC dac8568D(8 Channnels and midscale) instead three dac7563(2 Channnels and midscale)  to try to reduce the space on the PCB.

    The dac8568D works similar to dac7563. I need to use an external Voltage reference with 5Volts to achive to have 0Volts in the output with this op140 configuration:

    Do you know If the power of the IC voltage reference REF5050 it's enough for this configuration using 6 Channels?

    I'm thinking about using this LDO TPS7S49 for Vcc = +6V and TPS7A30 Vcc= -6V. Would It be enough for feed 6 opa140?

    I'm not going to use 2 of 8 channels, what do I have to do with these pins? tied to GND, floated, ..etc.

    Thank you so much, Best regards.

  • 0 in reply to Oscar Gomez Fuente
    TI__Genius 10615 points
    Hi Oscar,

    It seems that you changed your design, but just in case you refer to the original schematic. That circuit will not give you a bipolar output swing. There is a small mistake in your V- equation. It should be "+R1*Vo" not "-R1*Vo".

    The OPA140 is a great choice for precision DACsdue to the good input offset voltage and output swing.

    For a 2.5V reference we usually recommend the REF5025.

    The choice of regulators would firstly depend on your input voltage profile.
  • 0 in reply to Oscar Gomez Fuente
    TI__Genius 10615 points

    Hi Oscar,

    This seems like a better fit for your application. Some customers sometimes prefer to use smaller channel counts for easier routing and it is something you should consider.

    The REF5050 is a great fit and it will supply more than enough power.

    As I mentioned on my previous post, the OPA140 is a good choice. The LDOs are a good choice as well.

    The unused channels should be left floating.

    I would suggest taking a look at DAC8165, it is also a octal channel DAC, but it has an internal reference that is powered on by default. Since the internal reference is +2.5V you will have to use a circuit outlined in TIPD125. The document is pretty in depth about the design, but you can just take a look at the circuit. You can most likely ignore Riso and Ccomp, unless you are driving a large capacitive load.

  • 0 in reply to Eugenio Mejia
    Expert 2066 points
    Yes, you are ok, but the final equation it's ok. It was a typographic error.

    Vo = R2/R1(Vin - Vref)