# 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

 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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