Tracking type ADC (Analog to Digital Converter)
In counter type DAC, the counter has to come to zero for every conversion which is a disadvantageous with respect to high conversion time. However the input analog signal is continuous in signal amplitude with respect to time. I.e. the difference between the two sampled analog values will be less, So for the first analog sampled value if the counter has reached to its equal value the counter has to be stopped there instead of coming to zero. The second analog value will be slightly higher or lower than the previous sampled value so the counter can increase or decrease the count for producing comparable analog output from DAC.
To perform this operation instead of normal counter Up/Down counter is used in tracking type ADC. After the first sampled value the up/down counter will tracks the input analog value so that it is called tracking type of ADC. The detailed block diagram of Tracking type ADC is shown in below figure.
Operation of tracking type DAC
The N bit up/down counter starts counting according to the clock pulse and provides a digital input to the DAC. The DAC converts the digital input into corresponding analog output which is applied to opamp. The opamp compares the DAC analog output with the input analog sampled value, if the input value greater than the DAC output it provides a clock pulse to increment the counter or otherwise if it is lesser than the DAC output it provides a clock pulse to decrement the counter. But normally for first sampled value the counter will incremented to match to the analog value and for subsequent samples only the counter may get decrement pulse.
When the DAC output is equal to the sampled value, the digital output will be taken from the up/down counter directly. Here shift register is not needed to capture the digital date because the counter will not go to reset state. Then onwards for the next sample value, the counter will increment or decrement according to the difference output of opamp.
This is also called as Derivative counter type ADC because the counter output depends difference to the previous and next sampled value like as differentiator.
Advantages of Tracking type ADC
- Shift register is not needed so the cost is less.
- Speed is high compared to digital ramp type as the counter will not reset
Disadvantages of Tracking type ADC
- Up/Down counter leads to complexity of the circuit.
- The digital output will never be constant because of differentiator effect. I.e. for a constant analog value also the output will oscillating this is known as bit bobble.