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Microcomputer Technical Q&A
128
Q&A No.: QAH8S-243
Category: A/D Converter
Subject: Idea Behind External C and R
Question
What is the idea behind the connection of a signal source impedance (tap resistance, etc.) and low-
pass filter to the A/D converter input pins?
Answer
The ideas behind these connections are as follows.
• Permissible signal source impedance
The analog inputs of H8S Series chips are designed so that conversion accuracy is guaranteed
for an input signal with a permissible signal source impedance, R
out
, of 5 k or less (when
φ
>
12 MHz).
The figure below shows an equivalent circuit of the analog input circuit. In order for A/D
converter to be performed correctly, internal capacitance C
AD
must be charged within the
sampling period, t
SPL
.
Charge time constant τ is expressed as τ = C
AD
× (R
out
+ R
AD
), so that, for a permissible
conversion error of ±4 LSB, for example, the calculation is as follows:
V
CC
× 1020/1024 > V
CC
× (1-exp (–t
SPL
/ τ ))
∴ t
SPL
> 5.6 × τ = 5.6 × C
AD
× (R
out
+ R
AD
)
If t
SPL
= 2 µs (
φ
= 16 MHz, CKS = 1), C
AD
= 20 pF and R
AD
= 10 k ; thus:
τ < 360 ns
∴ R
out
< 8 k
- Application Note 1
- Using this Application Note 4
- Contents 5
- Bus controller 6
- Power-Down State 6
- Electrical Characteristics 6
- A/D Converter 8
- I/O Ports 8
- Clock Pulse Generator 8
- Q&A No.: QAH8S-001 9
- Q&A No.: QAH8S-002 10
- Category: CPU 10
- Q&A No.: QAH8S-003 12
- Q&A No.: QAH8S-004 13
- Q&A No.: QAH8S-005 14
- Q&A No.: QAH8S-006 15
- Subject: MAC Register 15
- Q&A No.: QAH8S-007 16
- Subject: EXR Register 16
- Q&A No.: QAH8S-008 17
- Category: Instruction 17
- Subject: SUBX Instruction 17
- Q&A No.: QAH8S-009 18
- Subject: BRN Instruction 18
- Q&A No.: QAH8S-010 19
- Q&A No.: QAH8S-011 20
- Q&A No.: QAH8S-012 21
- Instruction 21
- Q&A No.: QAH8S-013 22
- Q&A No.: QAH8S-014 23
- Subject: Stack Precautions 23
- Q&A No.: QAH8S-015 24
- Subject: Stack Pointer 24
- Q&A No.: QAH8S-016 25
- Subject: TAS Instruction 25
- Q&A No.: QAH8S-017 26
- Q&A No.: QAH8S-018 28
- Q&A No.: QAH8S-019 30
- Category: MCU Operating Modes 30
- Subject: Mode Pins 30
- Q&A No.: QAH8S-020 31
- Subject: Use of RAME Bit 31
- Q&A No.: QAH8S-021 32
- Category: Exception Handling 32
- Subject: Reset 32
- Q&A No.: QAH8S-022 34
- Category: Interrupts 34
- Disabled 34
- Q&A No.: QAH8S-023 35
- Q&A No.: QAH8S-024 36
- Q&A No.: QAH8S-025 37
- Q&A No.: QAH8S-026 38
- Q&A No.: QAH8S-027 39
- Q&A No.: QAH8S-028 40
- Q&A No.: QAH8S-029 41
- Q&A No.: QAH8S-030 43
- Q&A No.: QAH8S-031 44
- Category: Bus Controller 44
- Q&A No.: QAH8S-032 45
- Subject: 45
- Q&A No.: QAH8S-033 46
- Subject: WAIT (1) 46
- Q&A No.: QAH8S-034 47
- Subject: WAIT (2) 47
- Q&A No.: QAH8S-035 49
- Q&A No.: QAH8S-036 51
- Q&A No.: QAH8S-037 52
- Q&A No.: QAH8S-038 53
- Q&A No.: QAH8S-039 55
- Q&A No.: QAH8S-040 56
- Q&A No.: QAH8S-041 57
- Q&A No.: QAH8S-042 58
- Q&A No.: QAH8S-043 59
- Q&A No.: QAH8S-044 60
- Subject: 2-CAS DRAM Interface 60
- Q&A No.: QAH8S-045 62
- Category: Power-Down State 62
- Subject: Medium-Speed Mode 62
- Q&A No.: QAH8S-046 63
- Q&A No.: QAH8S-047 64
- Q&A No.: QAH8S-048 65
- Q&A No.: QAH8S-049 66
- Q&A No.: QAH8S-050 67
- Subject: Module Stop Mode 67
- Q&A No.: QAH8S-051 68
- Q&A No.: QAH8S-052 69
- Q&A No.: QAH8S-053 70
- Subject: RD Signal Timing 70
- Q&A No.: QAH8S-054 72
- Category: Pins 72
- Q&A No.: QAH8S-055 73
- Q&A No.: QAH8S-056 74
- Q&A No.: QAH8S-057 76
- Q&A No.: QAH8S-058 77
- Subject: WDTOVF Pin 77
- Q&A No.: QAH8S-201 79
- Category: DMAC 79
- Q&A No.: QAH8S-202 80
- Q&A No.: QAH8S-203 81
- Q&A No.: QAH8S-204 82
- Q&A No.: QAH8S-205 84
- Function 84
- Q&A No.: QAH8S-206 86
- Q&A No.: QAH8S-207 87
- Q&A No.: QAH8S-208 88
- Q&A No.: QAH8S-209 90
- Q&A No.: QAH8S-210 91
- Q&A No.: QAH8S-211 92
- Q&A No.: QAH8S-212 94
- Subject: DREQ Signal Input 94
- Q&A No.: QAH8S-213 95
- Category: DTC 95
- Subject: Nature of DTC 95
- Q&A No.: QAH8S-214 96
- Q&A No.: QAH8S-215 97
- Q&A No.: QAH8S-216 98
- Q&A No.: QAH8S-217 99
- Q&A No.: QAH8S-218 100
- Category: TPU 100
- Q&A No.: QAH8S-219 101
- Subject: Cascaded Connection 101
- Q&A No.: QAH8S-220 102
- Q&A No.: QAH8S-221 103
- Q&A No.: QAH8S-222 104
- Q&A No.: QAH8S-223 105
- Subject: Two-Phase PWM Output 105
- Applicable Products 106
- Q&A No.: QAH8S-224 107
- Category: WDT 107
- Q&A No.: QAH8S-225 108
- Category: SCI 108
- Subject: SCI Initialization 108
- Q&A No.: QAH8S-226 109
- Q&A No.: QAH8S-227 110
- Q&A No.: QAH8S-228 111
- Q&A No.: QAH8S-229 112
- Q&A No.: QAH8S-230 113
- Q&A No.: QAH8S-231 114
- Q&A No.: QAH8S-232 117
- Q&A No.: QAH8S-233 118
- Subject: RDRF Flag Set Timing 118
- In Synchronous Mode 120
- Q&A No.: QAH8S-234 121
- Q&A No.: QAH8S-235 122
- External Clock 122
- Q&A No.: QAH8S-236 123
- Q&A No.: QAH8S-237 124
- Q&A No.: QAH8S-238 125
- Q&A No.: QAH8S-239 126
- Q&A No.: QAH8S-240 129
- Subject: Data Transfer to TDR 129
- Q&A No.: QAH8S-241 131
- Subject: TDRE Flag Set Timing 131
- 2. Synchronous mode 134
- Q&A No.: QAH8S-242 135
- Q&A No.: QAH8S-243 136
- Category: A/D Converter 136
- Q&A No.: QAH8S-244 138
- Q&A No.: QAH8S-245 140
- Q&A No.: QAH8S-246 142
- Category: I/O Ports 142
- Q&A No.: QAH8S-247 143
- Subject: Reserved Bits 143
- Q&A No.: QAH8S-248 144
- Subject: Disabling 144
- Q&A No.: QAH8S-249 145
- Q&A No.: QAH8S-250 146
- Q&A No.: QAH8S-251 147
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