H8SLASH3028F datasheet(588/923 Pages) RENESAS

588 / 923 page

Rev. 2.00, 09/03, page 558 of 890

Offset error

Nonlinearity

error

Actual A/D conversion

characteristic

Analog input

voltage

Digital

output

Ideal A/D

conversion

characteristic

Full-scale

error

Figure 15.10 A/D Converter Accuracy Definitions (2)

7. Allowable Signal-Source Impedance: The analog inputs of the H8/3028 Group are designed to

assure accurate conversion of input signals with a signal-source impedance not exceeding 10

Ω. 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 k

Ω, 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-k

Ω 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 AVSS.

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.

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