Clarification on Digital to analog conversion

Discussion in 'Embedded Systems and Microcontrollers' started by divakarraj, Mar 30, 2011.

  1. divakarraj

    Thread Starter New Member

    Mar 30, 2011
    4
    0
    Hai, I am trying to produce variable analog voltage between 0V to 5V from a DAC0808 IC controlled by PIC16F76 port pins. The DAC0808 circuit used is attached. Few clarification on the circuitry thou, specifically on the power supply circuit.

    1. Is the circuitry attached is the right one ?

    2. If right, how to provide the power supply to DAC0808 ? I understand that pin 13 and 14 are connected to +5 volts, and pin 1, 2,15 are connected to ground (0V). But, how to connect pin 3 and 16 ? I am clear about 15pf connection. But how to provide -12V connection to pin 3 ? Is that supposed to be a separate power supply with common ground with 5V supply (earlier) ? Even if so, how to achieve -12V ? Clarify please. I also checked it as +12V at few diagrams (net).

    3. How to achieve +12 and -12V power supply for 741 IC. Can I convert it as +24V and 0V ?

    4. Is there anything else I should add or sub to the circuit ? Or can I directly connect PIC port 8 pins to the DAC pins <5 to 12> ?

    5. Will the output at Vout terminal from 741 IC will have appropriate analog voltage, immediately as I change the digital number at PIC port pins ? and can you also comment about the accuracy…

    Thank you
    Divakar Raj. R
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    1. No schematic is visible.
    2. The data sheet covers power connections. Pin 14 is the Vref, and needs a trim resistor. The voltage to pin 3 and the capacitor between 3 & 16 are clearly indicated on the dtata sheet.
    3. 741's have +/- 15 volts specified on their data sheets. Why not use a more modern op amp?
    4. See #1.
    5. That depends on the meaning of "immediately" and "accuracy". 741's have very slow slew rates. More modern op amps swing their outputs much faster. Look at a TL081 for a comparison. The D to A conversion is only 8 bits, or 256 states. That is close to 20 mv per step with a 5 volt reference. What kind of accuracy do you need?
     
  3. divakarraj

    Thread Starter New Member

    Mar 30, 2011
    4
    0
    Thanks for reply. Regrets abt the schematic.. It is attached in this post.

    I summarize the power supply connectivities to the circuit as follows. Looking for corrections..

    Three separate power supplies are used, V1, V2, V3.
    V1 -- from 7805 (+V1=+5V, -V1=0V)
    V2 -- from first 7812 (+V2=+12V, -V2=0V)
    V3 -- from second 7812 (+V3=+12V, -V3=0V).

    The 7805 (V1) and the first 7812 IC (V2) takes the same DC input, whereas the second 7812 (V3) is given a different DC input source, i.e., different ground. The V3 source alone is going to be used for -12V source by having reverse connectivity. The power source connections are shown below:

    DAC IC power connections:

    pin 13 -- +V1
    pin 1,2 -- -V1 & +V3
    pin 14 -- +V1 through 5k resistor
    pin 15 -- -V1 through 5k resistor
    pin 3 -- -V3

    741 IC power connections:

    pin 7 -- +V2
    pin 3 -- -V2 & +V3
    pin 4 -- -V3

    I thought of getting the reverse voltage even without ICL7660.. The capacitor between pin 3 and 16 is 0.1 microfarad, as per figure 1 of the datasheet. But figure 5, 6 of the same datasheet shows it to be 15 pF though.. Otherwise the whole circuit is same. Will these connectivities get me the appropriate output at Vout ? i.e., the Vout to be 0V when digit number is 'b00000000 and the Vout will be 5V if the digital number is 'b11111111, rite..?

    Divakar Raj. R
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Other than tying pin 1 to ground (it should be NC), the circuit may work as expected.

    You can set the DAC and op amp up on a breadboard and check operation. That 741 may have a bit much offset.
     
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