Dac0808

Discussion in 'General Electronics Chat' started by MaxHeadRoom, Oct 22, 2013.

  1. MaxHeadRoom

    Thread Starter Expert

    Jul 18, 2013
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    Anyone have success using the DAC0808?
    I am getting varying results for some mystifying reason?
    What is the normal voltage to be seen on pin 14 when fed from 10v when fed through a 5k resistor as per the spec sheet?
    Max.
     
  2. RichardO

    Well-Known Member

    May 4, 2013
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    The only reason I can think of for variation is that the circuit is oscillating because of poor compensation. Maybe you have a bad cap on pin 16.

    Pins 14 and 15 should be nearly the same voltage. The current source amplifier makes pin 14 a virtual ground (assuming that pin 15 is biased to ground). The 5K resistor on pin 15 is to compensate for the input bias current of the amplifier. This bias current will cause a slight positive voltage on pins 14 and 15.

    The resistance on pins 14 and 15 should be the same value to minimize the effect of the bias current caused by variations between devices and changes in temperature..

    This part is the same as the Motorola MC1408. I seem to remember that there were a number of application notes written for this part.
     
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  3. MaxHeadRoom

    Thread Starter Expert

    Jul 18, 2013
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    Thanks for the info, now I now that the readings I have appear normal, I will have to re-check the mock-up again.
    The only difference between the first and second version is using the LF351 per the app sheet and a TLV2371 on the second, which I don't see should cause a problem?
    Max.
     
  4. RichardO

    Well-Known Member

    May 4, 2013
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    Are you saying that your circuit works with the LF351 but not the TLV2371?

    Note that the TLV2371 has a maximum supply voltage of 16 volts. If you are using the -15 volt Vee and 5 volts for Vcc as in the application example you are applying 20 volts.

    You can run the DAC0808 with a Vee of -5 volts but if I remember correctly the performance suffers in some way. Now if my brain cells would only cooperate, I would tell you what parameter(s) degrade...
     
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  5. MaxHeadRoom

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    Jul 18, 2013
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  6. RichardO

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    May 4, 2013
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    I'm confused. Is the DAC0808 output driving an inverting or non-inverting amplifier? I assume that you are replacing the LM351 with the TLV2371 which is powered from +5 volts and ground. This TLV2371 then drives the circuit that is in your link. Is this correct? Can you post your entire circuit?

    The DAC0808 outputs a current sourced from the negative power supply. The current output pin usually drives into the virtual ground of an op-amp -- it has very small voltage swing into a resistive load. Having said that, I don't see a problem replacing the LM351 with the TLV2371 since it's inputs are "rail to rail".
     
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  7. MaxHeadRoom

    Thread Starter Expert

    Jul 18, 2013
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    There been a slight change of plan, I now need more I/O and rather than size a larger PIC, the one I am using has the MSSP feature so I am now looking at implementing I2c using the MAX517.
    Thanks for the help.
    Max.
     
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