Questions abut TLC7226 quad DAC?

Discussion in 'The Projects Forum' started by rougie, Sep 24, 2012.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006

    I would like to make a ramping voltage from approximately 0.3VDC up to approximately 1.0VDC.

    To accomplish this I decided to try out the TLC7226C from TI. I was able to get this chip to ramp from 0 volts to any voltage up to 10V max. But now, I don't want to start from 0 volts DC. I would like to start from 0.3VDC and ramp up to 1.0 VDC.

    Please view attachment with circuit. Based on schematic, I have point "A" which when not connected to pin #5 of the TLC7226C part reads 0.294 VDC. But as soon
    as I connect point "A" to pin #5 of the TLC7226C, I read a voltage at pin#5 of .54volts DC. ??? Why?

    Isn't pin#5 an input for the analogue ground... so why doesn't it read 0.294 VDC as intended by point "A"???

    All help very appreciated!
  2. rougie

    Thread Starter Active Member

    Dec 11, 2006
    Another problem I have is interpreting what the spec sheet is really saying... please consider the following!!!!

    Still referring to the TLC7226 part and its spec sheet at:

    If you go to p.4 of the data sheet and look for:

    ri(ref) Reference input resistance 2 kΩ min and 4 kΩ typical.


    1) Does this mean that pin#4 of the TLC7226 part has a typical input resistance from 2kilo ohms to 4 kilo ohms?

    2) Please refer to the attachment below where I tried to calculate the actual input resistance at pin #4. As a test, I have created a voltage divider to provide 2.2V as my reference voltage. I have measured the currents in the circuit. If I measure 2.2V at point "A", according to ohms law why is it that the calculation of the internal resistance comes out to 22K when the spec sheet says from 2 to 4 k???

    • ref.jpg
      File size:
      66.4 KB