HD6413008VF25 Datasheet, PDF (478/688 Pages) Renesas Technology Corp – Old Company Name in Catalogs and Other Documents

14. A/D Converter
Digital
output
Ideal A/D
conversion
characteristic
Full-scale
error
Nonlinearity
error
Actual A/D conversion
characteristic
Offset error
FS
Analog input
voltage
Figure 14.10 A/D Converter Accuracy Definitions (2)
7. Allowable Signal-Source Impedance
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.
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.
Rev.4.00 Aug. 20, 2007 Page 432 of 638
REJ09B0395-0400