Digital Ramp Analog to Digital Converter (ADC)

The Digital Ramp ADC is also known as a counter-type ADC. It uses a binary counter as the register and allows the clock to increment the counter one step at a time until Vo > V+. Circuit shown is a 4 bit Digital Ramp ADC

Circuit Operation

  • At the start, the counter is cleared to 0
  • As the counter counts up with each clock pulse, the Digital to Analog Converter (DAC) outputs a slightly higher voltage. This voltage is compared to the input voltage by the comparator.
  • If the input voltage is greater than the DAC output, the comparator's output will be high and the counter will continue counting normally.
  • Eventually, though, the DAC output will exceed the input voltage, causing the comparator's output to go low. This will be the end of conversion and counter output is the adc output.

Notes:

  • To observe the operation of the Digital Ramp ADC, first set the Vin
  • Press the START button to reset the 4 bit counter and begin the binary counting process
  • U5 samples and holds the Vin voltage to V+ of the the comparator (U1) when the START button is pressed. V+ does not change during the conversion process.
  • Press the CLK button to observe the results of counter counting up.
  • U3 is a 4 bit DAC with step voltage of 0.25V
  • EOC LED goes off when the conversion is complete.
  • To repeat the process with a different input voltage, press the START button again.

Output Table
D3 D2 D1 D0 Digital Value Vin
0 0 0 0 0 0
0 0 0 1 1 0.25
0 0 1 0 2 0.5
0 0 1 1 3 0.75
0 1 0 0 4 1.0
0 1 0 1 5 1.25
0 1 1 0 6 1.5
0 1 1 1 7 1.75
1 0 0 0 8 2.0
1 0 0 1 9 2.25
1 0 1 0 10 2.5
1 0 1 1 11 2.75
1 1 0 0 12 3.0
1 1 0 1 13 3.25
1 1 1 0 14 3.5
1 1 1 1 15 3.75

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