ADC

contents:
overview of ADC
types of ADC
successive approximation type ADC
ADC specifications

1. ADC Analog to Digital Convertor

2. Overview of ADC

3. Types of ADC Counter type ADC To convert an analog signal to N bit digital data it requires (2 N -1) clock cycles Successive approximation type ADC To convert an analog signal to N bit digital data it requires only ( N ) clock cycles N = 3 2 N - 1 = 2 3 - 1 = 8 – 1 = 7 clock cycles

4. Successive Approximation type ADC

5. Successive approximation type ADC 100 4 010 2 110 6 001 1 011 3 101 5 111 7 100 4 101 5 110 6 111 7 000 001 1 010 2 011 3 Vd <Vi Vd <Vi Vd <Vi Vd <Vi Vd <Vi Vd > Vi Vd <Vi Vd <Vi Vd > Vi Vd > Vi Vd > Vi Vd > Vi Vd > Vi Vd > Vi Vd < Vi == 1 Vd > Vi == 0 _ + Vd 000 100 000 010 100 110

6. Successive approximation type ADC 100 4 110 6 111 7 110 6 Vd <Vi Vd <Vi Vd > Vi Vi == 6.2V Vd == 110 =6V

7. ADC specifications

8. Resolution Offset error Full scale error Differential Nonlinearity Integral Nonlinearity Overview

9. Resolution The number of bits used to represent the analog signal. Increased resolution = increased precision LSB is the smallest step size achievable , 1LSB = full scale / 2 n

10. Continued…

11. The ideal transfer function of 3 bit ADC The output code will be its lowest (000) at less than 1/8 of the full-scale. ADC reaches its full-scale output code (111) at 7/8 of full scale. The transition to the maximum digital output does not occur at full-scale input voltage. The transition occurs at one code width—or least significant bit (LSB)—less than full-scale input voltage (in other words, voltage reference voltage). -1 Output code Quantization error LSB

12. 3-bit ADC transfer function with - 1/2 LSB offset The transfer function can be implemented with an offset of - 1/2 LSB. This shift of the transfer function to the left shifts the quantization error from a range of (- 1 to 0 LSB) to (- 1/2 to +1/2 LSB). These deviations from the perfect transfer function define the DC accuracy and are characterized by the specifications in a data sheet. Gain is 1

13. Offset error, full-scale error The ideal transfer function line will intersect the origin of the plot. The first code boundary will occur at 1 LSB. An error of - 1/2 LSB is intentionally introduced into some ADCs. In this way, the magnitude of quantization error is intended to be < 1/2 LSB You can observe offset error as a shifting of the entire transfer function left or right along the input voltage axis, and the gain >1 if input is lesser and vice versa -1/2 Quantization error LSB + 1/2

14. Full-scale error Full-scale error is the difference between the ideal code transition to the highest output code and the actual transition to the highest output code when the offset error is zero. A similar specification, gain error, also describes the non-ideal slope of the transfer function as well as what the highest code transition would be without the offset error. Full-scale error accounts for both gain and offset

15. Differential Nonlinearity The difference in code widths from one code to the next is differential nonlinearity (DNL) . The code width (or LSB) of an ADC is, Deviation of each code from an LSB is measured as DNL. A selected digital output code width is shown as larger than the previous code's step size. This difference is DNL error. DNL is calculated as, Code0 = 1+e0 Code1 = 1-e1 Code2 = 1+e2 Code3 = 1+e3 DNL(K)= width of code K – 1LSB 1LSB

16. Continued… |DNL| ≤ Ensures there is no missing code No missing code, but assumed to be missed as per the above consideration.

17. Integral Nonlinearity The integral nonlinearity (INL) is the deviation of an ADC's transfer function from a straight line. INL is determined by measuring the voltage at which all code transitions occur and comparing them to the ideal. The difference between the ideal voltage levels at which code transitions occur and the actual voltage is the INL error, expressed in LSBs.

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