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Problem With DAC Output

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muashr

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Hi,

I have a problem with the DAC used in a supply. It outputs more or less than what is required and expected (This was found out by comparing the outputs of malfunctioning DAC with that of properly functioning DAC). A glimpse of the expected/required & actual outputs is in the following. These values are obtained by rotating the shaft of the potentiometer.

Expected/Required
  1. 0.6V
  2. 1.2V
  3. 1.8V
  4. 2.4V
  5. 3V

Actual
  1. 0.9V
  2. 1.8V
  3. 2.3V
  4. 0.17V
  5. 1.02V

All the values on other pins of the DACs (functioning and malfunctioning) are almost the same. Note also that the malfunctioning DAC has been replaced once but nothing has changed.
Any observaton or idea will be appreciated!
 
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Looks like you digital input has exceeded the max value.. ie if your max voltage is 3.3v and the DAC is a 10 bit, then assume 3.3v has a maximum 1024 value sent... so for some reason the digital input has risen so sending 1234 will cause an overflow but you will only see the value of 210...

Has the circuit any feedback device?
 
Looks like you digital input has exceeded the max value.. ie if your max voltage is 3.3v and the DAC is a 10 bit, then assume 3.3v has a maximum 1024 value sent... so for some reason the digital input has risen so sending 1234 will cause an overflow but you will only see the value of 210...

Has the circuit any feedback device?

The output of this DAC is the input to an opamp and that opamp has a feedback.
In the following I mention the exact measured values on the pins of both functioning and malfunctiong DACs.

Functioning DAC
  1. VDD = 3.26V, VOUT = 0.59V, SYNC = 3.26V, SCLK = 3.26V, DIN = 0V whereas pins 2,3 & 8 are not connected.
  2. VDD = 3.26V, VOUT = 1.19V, SYNC = 3.26V, SCLK = 3.26V, DIN = 0V whereas pins 2,3 & 8 are not connected.
  3. VDD = 3.26V, VOUT = 1.78V, SYNC = 3.26V, SCLK = 3.26V, DIN = 0V whereas pins 2,3 & 8 are not connected.
  4. VDD = 3.26V, VOUT = 2.38V, SYNC = 3.26V, SCLK = 3.26V, DIN = 0V whereas pins 2,3 & 8 are not connected.
  5. VDD = 3.26V, VOUT = 2.97V, SYNC = 3.26V, SCLK = 3.26V, DIN = 0V whereas pins 2,3 & 8 are not connected.
Malfunctioning DAC
  1. VDD = 3.31V, VOUT = 0.88V, SYNC = 3.31V, SCLK = 3.31V, DIN = 0V or 3.31V (Here the value changes if the shaft of potentiometer is rotated to some extent!) whereas pins 2,3 & 8 are not connected.
  2. VDD = 3.31V, VOUT = 1.75V, SYNC = 3.31V, SCLK = 3.31V, DIN = 0V or 3.31V (Here the value changes if the shaft of potentiometer is rotated to some extent!) whereas pins 2,3 & 8 are not connected.
  3. VDD = 3.31V, VOUT = 2.33V, SYNC = 3.31V, SCLK = 3.31V, DIN = 0V or 3.31V (Here the value changes if the shaft of potentiometer is rotated to some extent!) whereas pins 2,3 & 8 are not connected.
  4. VDD = 3.31V, VOUT = 0.17V, SYNC = 3.31V, SCLK = 3.31V, DIN = 0V or 3.31V (Here the value changes if the shaft of potentiometer is rotated to some extent!) whereas pins 2,3 & 8 are not connected.
  5. VDD = 3.31V, VOUT = 1.02V, SYNC = 3.31V, SCLK = 3.31V, DIN = 0V or 3.31V (Here the value changes if the shaft of potentiometer is rotated to some extent!) whereas pins 2,3 & 8 are not connected.
For pins refer to MSOP on page 6 of datasheet.
 
Can you draw up a bit of a schematic..

I don't understand what a pot has to do with anything

Well unfortunately the diagram is not available and the supply in which the DAC is used is multipurpose and fairly complicated. The circuitry between potentiometer and DAC is not known yet. However, the at the output of the DAC is a non-inverting amplifier and the rotation of the potentiometer's shaft has an effect on the output of DAC. The output of the DAC is the input of an op amp. That is, the more the potentiometer's shaft rotated clockwise the higher the DAC output becomes and since this output is the input of the opamp (non inverting amplifier), therefore higher DC output voltage is achieved.
 
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