Digital Converter. AN546 Datasheet

AN546 Converter. Datasheet pdf. Equivalent

AN546 Datasheet
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





Microchip Technology AN546
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



Microchip Technology AN546
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.



Microchip Technology AN546
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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