# What's the concept about external impedance of A-D conversion input?

## Question:

## Answer:

The following shows the relationship between the output impedance of an analog output device and the set value of an external stabilizing capacitor.

**Regarding the analog input pins**

The following describes the output impedance of an analog output device and how to determine the value for the external stabilizing capacitor to meet the requirement. Explanation will also be made for the case where the output impedance of an analog output device is low and the external stabilizing capacitor C1 is unnecessary.

Fig 1. Internal equivalent circuit of the analog input unit

**(a) Example of calculating the external stabilizing capacitor C1 (recommended to be added)**

We will consider the capacitance of C1 by assuming that R1 in Fig 1 is infinitely large and the charge for the internal capacitor C2 is supplied by C1, and that the potential fluctuations, Vp, caused by capacitance division of C1 and C2 is 0.1 LSB or less. For the 10-bit A-D converter where VREF is 5.12 V, 1 LSB determination voltage = 5.12 V / 1024 = 5 mV. In order to suppress potential fluctuations 0.1 LSB or less, the fluctuation should be kept within 0.5 mV.

**(b) Maximum value for the output impedance R1 when C1 is not added**

If the external capacitor C1 in Fig 1 is unused, it is necessary to examine whether C2 can be fully charged. First, the following shows an equation to find i2 for the case where C1 in Fig 1 is nonexistent.

Fig 2. A-D conversion timing diagram

Shown below is an example of calculating the maximum value of R1 in A-D conversion mode where Xin = 10/8 MHz, Cin = 10 pF, C2 = 2.9 pF and R2 = 2K.

Xin | BCLK period | Conversion mode | Speed mode | A-D conversion execution cycle | T (C2 charging time) in ns | Maximum value of R1 (ohms) |
---|---|---|---|---|---|---|

10MHz | 50ns | A-D conversion mode: Single | Normal | 294 | 367 | 28,000 |

10MHz | 50ns | A-D conversion mode: Single | Double | 168 | 210 | 15,829 |

8MHz | 62.5ns | A-D conversion mode: Single | Normal | 294 | 459 | 35,131 |

8MHz | 62.5ns | A-D conversion mode: Single | Double | 168 | 262 | 19,860 |

Note: The conversion cycle above represents the number of cycles not including dummy cycles at the start and end.

In comparator mode, the equation given below applies, because sampling and comparison each are performed only once.

Shown below is an example of calculating the maximum value of R1 in comparator mode where Xin = 10/8 MHz, Cin = 10 pF, C2 = 2.9 pF and R2 = 2K.

Xin | BCLK period | Conversion mode | Speed mode | A-D conversion execution cycle | T (C2 charging time) in ns | Maximum value of R1 (ohms) |
---|---|---|---|---|---|---|

10MHz | 50ns | Comparator mode: Single | Normal | 42 | 525 | 40,248 |

10MHz | 50ns | Comparator mode: Single | Double | 24 | 300 | 22,806 |

8MHz | 62.5ns | Comparator mode: Single | Normal | 42 | 656 | 50,403 |

8MHz | 62.5ns | Comparator mode: Single | Double | 24 | 375 | 28,620 |

Note: The conversion cycle above represents the number of cycles not including dummy cycles at the start and end.

**Suitable Products**

32170, 32174 |

32171 |

32172, 32173 |