Dual slope a/d converter pdf

Since ramp generator voltage starts at 0V, decreasing down to —Vs and then increasing up to 0V, the amplitude of negative and positive ramp voltages can be equated as follows.

Thus the unknown analog input voltage VA is proportional to the time period t2, because Vref is a known reference voltage and t1 is the predetermined time period. The actual conversion of analog voltage VA into a digital count occurs during time t2. The binary counter gives corresponding digital value for time period t2. The clock is connected to the counter at the beginning of t2 and is disconnected at the end of t2.

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When the conversion begins, initially the reference voltage is less than the input voltage. The output of the comparator is high and because of this, the counter receives clock pulses, and the counter works and counts. So, when the integrated reference voltage becomes equal to the applied input voltage, the comparator changes its state from high to low. The output of the AND gate becomes low, the counter no longer gets any clock pulses and stops the count.

Whatever, the count is then available on the counter will be proportional to the input voltage. The circuit is also accompanied by a switch known as Reset Switch. This is the process through which the single slope analog to digital converter transforms the given analog signal to the digital output signal. The above graph shows the time taken for the integration with respect to the applied input voltage.

The reference voltage and RC are constant that is why the slope i. The input voltage only affects the time. Higher the input voltage, more time will be taken for the integration process while the slope remains the same.

If the input voltage is fixed and R or C changes then the slope changes. Despite a constant input, we will achieve a different time of integration.

This value of RC changes with time due to external factors like temperature etc which is a major reason for its poor accuracy and leads us to its solution which is a dual-slope analog to digital converter. The integrator of a dual-slope analog to digital converter has a switch at its input side which can either connect to a reference voltage or an input voltage.

Initially in the conversion, the switch is connected to the input voltage and the integrator integrates the input voltage until its output equals the applied voltage. After T1 time, the switch gets connected to the reference voltage and the respective voltage is integrated.

The diagram shows that the given reference voltage is negative yet it is usually greater than the input voltage. As negative reference is applied, the integrator integrates in a positive direction and keeps on integrating until the output is equal to the zero voltage.

The time taken is represented by T2. In dual-slope ADC, T1 and reference voltage are constant which gives the result that the time T2 is directly proportional to the input voltage. If the input voltage changes, T2 also changes. This is represented through the graph below.

At the start of the conversion, the input voltage is connected to the integrator. The integrator integrates it in the negative direction. When compared with the ground, the inverting terminal is negative with respect to the non-inverting terminal so the output of the comparator becomes high.

Just like the single slope, the AND signal then provides the clock pulses to the n-bit counter and starts counting from zero onwards until overflow becomes 1.

When the overflow is detected, the switch automatically toggles and joins with the negative reference voltage. As the reference voltage is negative, the integrator begins integrating in the positive direction and the same procedure is repeated. This time the counter will stop counting when the output of the comparator gets low. At that particular time, the binary output of the n-bit counter would be directly proportional to the time T2.

In this way, the dual-slope ADC does the conversion.