# AN546 Converter. Datasheet pdf. Equivalent Recommendation AN546 Datasheet
 Part AN546 Description Using the Analog to Digital Converter Feature AN546; M Authors: www.DataSheet4U.com AN546 Note that the digital output value is 00h for the analog input. Manufacture Microchip Technology Datasheet Download AN546 Datasheet M AN546 Using the Analog-to-Digital (A/D) Converter Authors: Sumit Mitra, Stan D’Souza, and Russ Cooper Microchip Technology Inc. www.DataSheet4U.com INTRODUCTION This application note is intended for PIC16C7X users with some degree of familiarity with analog system design.The various sections discuss the following topics: • Commonly used A/D terminology • How to conﬁgure and use the PIC16C71 A/D • Various ways to generate external reference voltage (VREF) • Conﬁguring the RA3:RA0 pins COMMONLY USED A/D TERMINOLOGY The Ideal Transfer Function In an A/D converter, an analog voltage is mapped into an N-bit digital value. This mapping function is deﬁned as the transfer function. An ideal transfer is one in which there are no errors or non-linearity. It describes the “ideal” or intended behavior of the A/D. Figure 1 shows the ideal transfer function for the PIC16C7X A/D. FIGURE 1: PIC16C7X IDEAL TRANSFER FUNCTION FFh FEh Note that the digital output value is 00h for the analog input voltage range of 0 to 1LSb. In some converters, the ﬁrst transition point is at 0.5LSb and not at 1LSb as shown in Figure 2. Either way, by knowing the transfer function the user can appropriately interpret the data. Transition Point The analog input voltage at which the digital output switches from one code to the next is called the “Tran- sition Point.” The transition point is typically not a single threshold, but rather a small region of uncertainty (Figure 3). The transition point is therefore deﬁned as the statistical average of many conversions. Stated dif- ferently, it is the voltage input at which the uncertainty of the conversion is 50%. Code Width The distance (voltage differential) between two transition points is called the “Code Width.” Ideally the Code Width should be 1LSb (Figure 1). FIGURE 2: ALTERNATE TRANSFER FUNCTION FFh FEh Code Width (CW) 04h 03h 02h 01h 00h Analog input voltage 04h 03h 02h 01h 00h Analog input voltage © 1997 Microchip Technology Inc. DS00546E-page 1 AN546 Center of Code Width The midpoint between two transition points is called the “Center of Code Width” (Figure 3). FIGURE 3: TRANSITION POINTS 7 6 5 www.DataSheet4U.com 4 0% 3 2 1 0 Code under test 100% 50% Center of code width Low side transition Transition points Differential Non-Linearity (DNL) It is the deviation in code-width from 1LSb (Figure 4). The difference is calculated for each and every transition. The largest difference is reported as DNL. It is important to note that the DNL is measured after the transfer function is normalized to match offset error and gain error. Note that the DNL cannot be any less than -1LSb. In the other direction, DNL can be >1LSb. FIGURE 4: DIFFERENTIAL NON-LINEARITY 7 DNL = 1/4 LSb 6 5 4 DNL = +3/4 LSb Ideal transfer function (for reference only) 3 Actual transfer function 2 1 0 DNL = -1/4LSb to +3/4LSb Absolute Error The maximum deviation between any transition point from the corresponding ideal transfer function is deﬁned as the absolute error. This is how it is measured and reported in the PIC16C7X (Figure 5). The notable difference between absolute error and integral non-lin- earity (INL) is that the measured data is not normalized for full scale and offset errors in absolute error. Absolute Error is probably the ﬁrst parameter the user will review to evaluate an A/D. Sometimes absolute error is reported as the sum of offset, full-scale and integral non-linearity errors. Total Unadjusted Error Total Unadjusted Error is the same as absolute error. Again, sometimes it is reported as the sum of offset, full-scale and integral non-linearity errors. No Missing Code No missing code implies that as the analog input volt- age is gradually increased from zero to full scale (or vice versa), all digital codes are produced. Stated otherwise, changing analog input voltage from one quantum of the analog range to the next adjacent range will not produce a change in the digital output by more than one code count. Monotonic Monotonicity guarantees that an increase (or decrease) in the analog input value will result in an equal or greater digital code (or less). Monotonicity does not guarantee that there are no missing codes. However, it is an important criterion for feedback control systems. Non-monotonicity may cause oscillations in such sys- tems. The ﬁrst derivative of a monotonic function always has the same sign. FIGURE 5: ABSOLUTE ERROR 7 6 Error = 3/4LSb 5 Actual transfer function 4 Ideal transfer function 3 Error = 1/4LSb 2 Error = 1/4LSb 1 0 Absolute Error = +3/4LSb DS00546E-page 2 © 1997 Microchip Technology Inc. Ratiometric Conversion Ratiometric Conversion is the A/D conversion process in which the binary result is a ratio of the supply voltage or reference voltage, the latter being equal to full-scale value by default. The PIC16C7X is a ratiometric A/D converter where the result depends on VDD or VREF. In some A/Ds, an absolute reference is provided result- ing in “absolute conversion”. Sample and Hold In sample and hold type A/D converters, the analog input has a switch (typically a FET switch in CMOS) which is opened for a short duration to capture the www.DataSheet4U.comCanoanlvoegrsiionnpuist voltage onto typically started an after on-chip capacitor. the sampling switch is closed. Track and Hold Track and Hold is basically the same as sample and hold, except the sampling switch is typically left on. Therefore the voltage on the on-chip holding capacitor “tracks” the analog input voltage. To begin a conversion, the sampling switch is closed. The PIC16C7X A/D falls in this category. Sampling Time Sampling Time is the time required to charge the on-chip holding capacitor to the same value as is on the analog input pin. The sampling time depends on the magnitude of the holding capacitor and the source impedance of the analog voltage input. Offset Error (or Zero Error) Offset Error is the difference between the ﬁrst actual (measured) transition point and the ﬁrst ideal transition point as shown in Figure 6. It can be corrected (by the user) by subtracting the offset error from each conver- sion result. FIGURE 6: OFFSET ERROR 7 6 5 Actual transfer function 4 Ideal transfer function 3 2 1 Offset error 0 AN546 Full Scale Error (or Gain Error) Full Scale Error is the difference between the ideal full scale and the actual (measured) full scale range (Figure 7). It is also called gain error, because the error changes the slope of the ideal transfer function creating a gain factor. It can be corrected (by the user) by multi- plying each conversion result by the inverse of the gain. FIGURE 7: FULL SCALE ERROR FFh FEh FDh FCh 03h 02h 01h 00h Actual transfer function Ideal transfer function Actual full-scale range Ideal full-scale range Integral Non-Linearity (INL), or Relative Error The deviation of a transition point from its corresponding point on the ideal transfer curve is called “Integral Non-Linearity” (Figure 8). The maximum dif- ference is reported as the INL of the converter. It is important to note that Full Scale Error and the Offset Error are normalized to match end transition points before measuring the INL. FIGURE 8: INTEGRAL NON-LINEARITY 7 6 Deviation = +3/4LSb 5 Actual transfer function 4 Ideal transfer function 3 Deviation 2 = +1/4LSb Deviation 1 = -11/4LSb 0 INL in this example is -1/4LSb to +3/4LSb © 1997 Microchip Technology Inc. DS00546E-page 3

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