Renesas F-ZTAT H8 Series Hardware Manual page 568

Section 15 A/D Converter
Figure 15.10 A/D Converter Accuracy Definitions (2)
4. Allowable Signal-Source Impedance
The analog inputs of the H8/3052BF 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 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.
Rev. 3.00 Mar 21, 2006 page 540 of 814
REJ09B0302-0300
Digital
output
Ideal A/D
conversion
characteristic
Actual A/D conversion
characteristic
Offset error
Full-scale
error
Nonlinearity
error
FS
Analog input
voltage