Counter (Digital Ramp) ADC

Digital Signal Processing

It is necessary when measured quantities must be in digital form for processing or for display or storage.

Digital signal processing converts signals that naturally occur in analog form, such as sound, video and information from sensors, to digital form and uses digital techniques to enhance and modify analog signal data for various applications.

A digital signal processing system first translates a continuously varying analog signal into a series of discrete levels. This series of levels follows the variations of the analog signal and resembles a staircase, as illustrated for the case of a sine wave in Figure 1. The process of changing the original analog signal to a "stairstep" approximation accomplished by a sample-and-hold circuit. 

Figure 1

Next, the "stairstep" approximation is quantized into binary codes that represent each discrete step on the "stairsteps" by a process called analog-to-digital (AID) conversion. The circuit that performs AID conversion is an analog-to-digital converter (ADC).

Once the analog signal has been converted to a binary coded form, it is applied to a DSP (digital signal processor). The DSP can perform various operations on the incoming data, such as removing unwanted interference, increasing the amplitude of some signal frequencies and reducing others, encoding the data for secure transmissions, and detecting and correcting errors in transmitted codes. DSPs make possible, among many other things, the cleanup of sound recordings, the removal of echos from communications lines, the enhancement of images from CT scans for better medical diagnosis, and the scrambling of cellular phone conversations for privacy. After a DSP processes a signal, the signal can be converted back to a much improved version of the original analog signal. This is accomplished by a digital-to-analog converter (DAC). Figure 2 shows a basic block diagram of a typical digital signal processing system.

Figure 2

Analog – to – Digital Conversion

Analog-to-digital conversion is the process of converting the output of the sample-and-hold circuit to a series of binary codes that represent the amplitude of the analog input at each of the sample times. The sample-and-hold process keeps the amplitude of the analog input signal constant between sample pulses; therefore, the analog-to-digital conversion can be done using a constant value rather than having the analog signal change during a conversion interval, which is the time between sample pulses. Figure 3 illustrates the basic function of an analog-to-digital (ADC) converter. The sample intervals are indicated by dashed lines.

Figure 3

Analog – to – Digital Conversion Methods

As you have seen, analog-to-digital conversion is the process by which an analog quantity is converted to digital form. Some of the more commonly used ADC are listed below.

1.    Flash (simultaneous) ADC
2.  Single/ Dual Slope ADC
3.   Successive Approximation ADC
4. Counter (Digital Ramp or Staircase) ADC

     From the above listed converters this report will discuss about Counter ADC and its operation

Counter (Digital Ramp or Staircase) ADC

This is one of the simplest ADC. This system will be explained with reference to figure 4(a). The clear pulse resets the counter to the zero. The counter then records in the binary form the number of pulses from clock line. The clock is a source of pulses equally spaced in time. Since the number of pulses counted increases as the input linearly with time, the binary word representing this count is used as the input of a DAC (Digital to Analog Converter) whose output is the stair case waveform shown in fig 01(b). As long as the analog input V_A is greater than V_D, the comparator has an output which is high and the AND gate is open for transmission of the clock pulse to the counter. When V_D exceeds V_A the comparator output changes to low value and the AND gate is disabled. This stops the counting at the time when V_A ≈ V_D and the counter can be read out as the digital word representing the analog input voltage.

 Figure 4(a)

Figure 4(b)

If analog voltage varies with time, it is not possible to convert the analog data continuously, but it will be necessary that input signal be sampled at fixed intervals (t_c). If maximum value of the analog voltage is represented by n pulses and if the period of the clock is T seconds, the minimum interval between samples (Conversion Time - t_c) is nT.

Tracker ADC

An improved of the counter ADC, called a tracking or servo converter, is obtained by using an UP - DOWN counter. This modification of FIGURE 01(a) is shown in FIGURE 02. Neither start command nor an AND gate is used. However, an UP - DOWN converter is now required and the comparator output fees the UP - DOWN control of the converter.

Figure 5

To understand the operation of the system, assume initially that the output of the DAC is less than analog input V_A. Then the positive comparator output causes the counter read UP. The DAC output increases with each clock pulse until it exceeds V_A. The UP - DOWN control line changes state so that it now counts DOWN (but only one count, LSB). This causes the control to change to UP and the count to increase by 1 LSB. This process keeps repeating so that the digital output bounces back and forth by ±1 LSB around the correct value. The conversion time is small for small changes in the sampled analog, and hence this system can be used effectively as a tracking ADC. 

Advantages of Counter ADC

1.  Very simple design
2.  Cheap due to its simple design

Disadvantage of Counter ADC

1. Variable conversion time.
2.  Slow operation.

      Reference

1.      J. Millman and A. Grabel, Microelectronics, Edition, McGraw-Hill. (Pages 719 – 721)
2.          L. Floyd, Digital Fundamentals, 9^th edition, Macmillan Publishing Co. (Pages 746 – 750)