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SUMMARY OF PRECAUTIONS
Appendix 4
Appendix 4-23
32185/32186 Group Hardware Manual
Rev.1.10 REJ09B0235-0110 May 15, 07
Appendix 4.10 Notes on A/D Converter
Thus, for a 10-bit resolution A/D Converter where C2 = 2.9 pF, C1 is 0.06 F 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 Appendix Figure 4.10.1 is not used, examination must be made to see if the
analog output device can fully charge C2 within a predetermined time. First, the equation to find i2 when C1 in
Appendix Figure 4.10.1 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)
(a) Example for calculating the external stabilizing capacitor C1 (addition of this capacitor is recommended)
Assuming the R1 in Appendix Figure 4.10.1 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 a 10-bit A/D Converter
where VREF0 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
VREF0
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
×
ADiINn
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.
Appendix Figure 4.10.2 A/D Conversion Timing Diagram
Appendix Figure 4.10.2 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 Appendix Table 4.10.1. Note that
when the sample-and-hold function is enabled, the analog input is sampled for only the first bit.