Full-scale error
Digital output
Ideal A/D
conversion
characteristic
Nonlinearity
error
Actual A/D conversion
characteristic
FS
Analog input
voltage
Offset error
Figure 15.9 A/D Conversion Accuracy Definitions (2)
Allowable Signal-Source Impedance
15.6.5
The analog inputs of the H8/3337 Series and H8/3397 Series 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, 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 a large differential coefficient (e.g. 5 mV/µs or more).
To convert high-speed analog signals, insert a low-impedance buffer.
351
RENESAS [ RENESAS TECHNOLOGY CORP ]
