SPMCK16Z4MPV20 datasheet(209/500 Pages) FREESCALE

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M68HC16 Z SERIES

ANALOG-TO-DIGITAL CONVERTER

USER’S MANUAL

8-23

The external circuit described in Table 8-10 is a low-pass filter. A user interested in

measuring an AC component of the external signal must take the characteristics of this

filter into account.

8.8.6.2 Error Resulting from Leakage

A series resistor limits the current to a pin, therefore input leakage acting through a

large source impedance can degrade A/D accuracy. The maximum input leakage cur-

rent is specified in APPENDIX A ELECTRICAL CHARACTERISTICS. Input leakage

is greatest at high operating temperatures and as a general rule decreases by one half

for each 10

°C decrease in temperature.

Assuming VRH –VRL = 5.12 V, 1 count (assuming 10-bit resolution) corresponds to 5

mV of input voltage. A typical input leakage of 50 nA acting through 100 k

Ω of external

series resistance results in an error of less than 1 count (5.0 mV). If the source imped-

ance is 1 M

Ω and a typical leakage of 50 nA is present, an error of 10 counts (50 mV)

is introduced.

In addition to internal junction leakage, external leakage (e.g., if external clamping di-

odes are used) and charge sharing effects with internal capacitors also contribute to

the total leakage current. Table 8-11 illustrates the effect of different levels of total

leakage on accuracy for different values of source impedance. The error is listed in

terms of 10-bit counts.

CAUTION

Leakage from the part of 10 nA is obtainable only within a limited tem-

perature range.

Table 8-10 External Circuit Settling Time (10-Bit Conversions)

Filter Capacitor

(CF)

Source Resistance (RF)

100

Ω

1 k

Ω

10 k

Ω

100 k

Ω

µF

760

µs

7.6 ms

76 ms

760 ms

µF76 µs

760

µs

7.6 ms

76 ms

.01

µF

7.6

µs76 µs

760

µs

7.6 ms

.001

µF

760 ns

7.6

µs76 µs

760

µs

100 pF

76 ns

760 ns

7.6

µs76 µs

Table 8-11 Error Resulting From Input Leakage (IOFF)

Source

Impedance

Leakage Value (10-Bit Conversions)

10 nA

50 nA

100 nA

1000 nA

1 k

Ω

—

0.2 counts

10 k

Ω

—

0.1 counts

0.2 counts

2 counts

100 k

Ω

0.2 counts

1 count

2 counts

20 counts

Freescale Semiconductor, Inc.

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