11
11-44
A-D Converters
32180 Group User's Manual (Rev.1.0)
Thus, for 10-bit resolution A-D converters where C2 = 2.9 pF, C1 is 0.06 µF or more. Use this value for
reference when setting up C1.
(b) Maximum value of the output impedance R1 when C1 is not added
If the external capacitor C1 in Figure 11.5.1 is not used, examination must be made to see if the analog input
device can fully charge C2 within a predetermined time. First, the equation to find i2 when C1 in Figure 11.5.1
does not exist is shown below.
i2 =
C2(E - V2)
×
exp
{ - t }
------------------------ Eq. B-1
Cin
×
R1+C2(R1+R2) Cin
×
R1+C2(R1+R2)
11.5 Precautions on Using A-D Converters
(a) Example for calculating the external stabilizing capacitor C1 (addition of this capacitor is recommended)
Assuming the R1 in Figure 11.5.1 is infinitely large and that the current necessary to charge the internal
capacitor C2 is supplied from C1, if the potential fluctuation, Vp, caused by capacitance division of C1 and C2
is to be within 0.1 LSB, then what amount of capacitance C1 should have. For 10-bit A-D converters where
VREF is 5.12 V, 1 LSB determination voltage = 5.12 V / 1,024 = 5 mV. The potential fluctuation of 0.1 LSB
means a 0.5 mV fluctuation.
Vp is also obtained by the equation below:
The relationship between the capacitance division of C1 and C2 and the potential fluctuation,
Vp, is obtained by the equation below:
C2
C1 + C2
Vp =
×
(E - V2)
Eq. A-1
1
2
Vp = Vp1
×
<
Eq. A-2
i
VREF
10
×
2
×
x - 1
∑
i = 0
where Vp1 = potential fluctuation in the first A-D conversion performed
and x = 10 for a 10-bit resolution A-D converter
When Eq. A-1 and Eq. A-2 are solved, the following results:
E - V2
Vp1
C1 = C2 {
- 1 }
Eq. A-3
1
2
∴
C1 > C2 {10
×
2
×
- 1 }
Eq. A-4
i
x - 1
∑
i = 0
×
ADINi
Conversion time
for the first bit
Sampling time
Comparison
time
Repeated (10 times) for 10 bits
Second bit
Sampling time
When sample-and-hold
is disabled
* When sample-and-hold is enabled, the analog input is sampled for only the first bit.
Figure 11.5.2 A-D Conversion Timing Diagram
Figure 11.5.2 shows an A-D conversion timing diagram. C2 must be charged up within the sampling time
shown in this diagram. When the sample-and-hold function is disabled, the sampling time for the second and
subsequent bits is about half that of the first bit.
The sampling times at the respective conversion speeds are listed in the table 11.5.1. Note that when the
sample-and-hold function is enabled, the analog input is sampled for only the first bit.
Содержание M32R/ECU Series
Страница 17: ...12 This page is blank for reasons of layout...
Страница 18: ...CHAPTER 1 OVERVIEW 1 1 Outline of the 32180 Group 1 2 Block Diagram 1 3 Pin Functions 1 4 Pin Assignments...
Страница 712: ...CHAPTER 18 OSCILLATOR CIRCUIT 18 1 Oscillator Circuit 18 2 Clock Generator Circuit...
Страница 794: ...CHAPTER 22 TYPICAL CHARACTERISTICS...
Страница 795: ...22 22 2 32180 Group User s Manual Rev 1 0 TYPICAL CHARACTERISTICS To be written at a later time...
Страница 796: ...APPENDIX 1 MECHANICAL SPECIFICAITONS Appendix 1 1 Dimensional Outline Drawing...
Страница 798: ...APPENDIX 2 INSTRUCTION PROCESSING TIME Appendix 2 1 32180 Instruction Processing Time...
Страница 802: ...APPENDIX 3 PROCESSING OF UNUSED PINS Appendix 3 1 Example Processing of Unused Pins...