Table of Contents
Advertisement
Figure 15.10 A/D Converter Accuracy Definitions (2)
7. Allowable Signal-Source Impedance: The analog inputs of the H8/3006 and H8/3007 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 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
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.
534
Digital
output
Ideal A/D
conversion
characteristic
Actual A/D conversion
characteristic
Offset error
Full-scale
error
Nonlinearity
error
FS
Analog input
voltage
.
SS
Advertisement
Table of Contents
