Application Notes; Permissible Signal Source Impedance; Influences On Absolute Precision - Renesas H8 Series Hardware Manual

8-bit single-chip microcomputer

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Application Notes

This LSI's analog input is designed such that conversion precision is guaranteed for an input

signal for which the signal source impedance is 10 kΩ or less. This specification is provided to

enable the A/D converter's sample-and-hold circuit input capacitance to be charged within the

sampling time; if the sensor output impedance exceeds 10 kΩ, charging may be insufficient and it

may not be possible to guarantee A/D conversion precision. However, a large capacitance

provided externally, the input load will essentially comprise only the internal input resistance of

10 kΩ, and the signal source impedance is ignored. However, as a low-pass filter effect is obtained

in this case, it may not be possible to follow an analog signal with a large differential coefficient

(e.g., 5 mV/µs or greater) (see figure 12.8). When converting a high-speed analog signal, a low-

impedance buffer should be inserted.

Adding capacitance results in coupling with GND, and therefore noise in GND may adversely

affect absolute precision. Be sure to make the connection to an electrically stable GND.

Care is also required to ensure that filter circuits do not interfere with digital signals or act as

antennas on the mounting board.

Sensor output

Sensor input

Figure 12.8 Analog Input Circuit Example

A/D converter

equivalent circuit

Rev. 7.00 Mar 10, 2005 page 413 of 652

Section 12 A/D Converter

REJ09B0042-0700