HD64F3048VTF8 Datasheet, PDF (588/903 Pages) Renesas Technology Corp – Hardware Manual Renesas 8-Bit Single-Chip

Section 15 A/D Converter
7. Allowable Signal-Source Impedance: The analog inputs of the H8/3048B Group are designed
to assure accurate conversion of input signals with a signal-source impedance not exceeding
10 kΩ (φ ≤ 13 MHz) or not exceeding 5 kΩ (φ > 13 MHz) (table 15.5). The reason for this
rating is that it enables the input capacitor in the sample-and-hold circuit in the A/D converter
to charge within the sampling time. If the sensor output impedance exceeds 10 kΩ, charging
may be inadequate and the accuracy of A/D conversion cannot be guaranteed.
If a large external capacitor is provided in scan mode, then the internal 10-kΩ input resistance
becomes the only significant load on the input. In this case the impedance of the signal source
is not a problem.
A large external capacitor, however, acts as a low-pass filter. This may make it impossible to
track analog signals with high dv/dt (e.g. a variation of 5 mV/µs) (figure 15.11). To convert
high-speed analog signals or to use scan mode, insert a low-impedance buffer.
8. Effect on Absolute Accuracy: Attaching an external capacitor creates a coupling with ground,
so if there is noise on the ground line, it may degrade absolute accuracy. The capacitor must be
connected to an electrically stable ground, such as AV_SS.
If a filter circuit is used, be careful of interference with digital signals on the same board, and
make sure the circuit does not act as an antenna.
Sensor output impedance
Up to 10 kΩ (φ ≤ 13 MHz)
Sensor ^{Up to 5 kΩ (φ > 13 MHz)}
input
Low-pass
filter
Up to 0.1 µF
H8/3048B Group
Cin =
15 pF
Equivalent circuit of
A/D converter
10 kΩ
20 pF
Figure 15.11 Analog Input Circuit (Example)
Rev. 3.00 Sep 27, 2006 page 560 of 872
REJ09B0325-0300