HD6413008F RENESAS [Renesas Technology Corp], HD6413008F Datasheet - Page 476

• Manufacturer Part Number: HD6413008F
• Description: Renesas 16-Bit Single-Chip Microcomputer H8 Family/H8/300H Series
• Manufacturer: RENESAS [Renesas Technology Corp]

14. A/D Converter
7. Allowable Signal-Source Impedance
8. Effect on Absolute Accuracy
Rev.4.00 Aug. 20, 2007 Page 432 of 638
REJ09B0395-0400
The analog inputs of the H8/3008 are designed to assure accurate conversion of input signals
with a signal-source impedance not exceeding 10 kΩ. 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 single 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 14.11). To convert
high-speed analog signals or to use scan mode, insert a low-impedance buffer.
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
Figure 14.10 A/D Converter Accuracy Definitions (2)
Digital
output
characteristic
conversion
Ideal A/D
Offset error
Actual A/D conversion
characteristic
SS
.
Nonlinearity
error
Analog input
voltage
Full-scale
error
FS