SPMCK16Z4MPV20 datasheet(200/500 Pages) FREESCALE

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ANALOG-TO-DIGITAL CONVERTER

M68HC16 Z SERIES

8-14

USER’S MANUAL

Refer to APPENDIX D REGISTER SUMMARY for register mapping and configuration.

8.8 Pin Considerations

The ADC requires accurate, noise-free input signals for proper operation. The follow-

ing sections discuss the design of external circuitry to maximize ADC performance.

8.8.1 Analog Reference Pins

No A/D converter can be more accurate than its analog reference. Any noise in the

reference can result in at least that much error in a conversion. The reference for the

ADC, supplied by pins VRH and VRL, should be low-pass filtered from its source to ob-

tain a noise-free, clean signal. In many cases, simple capacitive bypassing may suf-

fice. In extreme cases, inductors or ferrite beads may be necessary if noise or RF

energy is present. Series resistance is not advisable since there is an effective DC cur-

rent requirement from the reference voltage by the internal resistor string in the RC

DAC array. External resistance may introduce error in this architecture under certain

conditions. Any series devices in the filter network should contain a minimum amount

of DC resistance.

For accurate conversion results, the analog reference voltages must be within the lim-

its defined by VDDA and VSSA, as explained in the following subsection.

8.8.2 Analog Power Pins

The analog supply pins (VDDA and VSSA) define the limits of the analog reference volt-

ages (VRH and VRL) and of the analog multiplexer inputs. Figure 8-4 is a diagram of

the analog input circuitry.

Table 8-9 Result Register Formats

Result Data Format

Description

Unsigned

right-justified format

Conversion result is unsigned right-justified data. Bits [9:0] are used for 10-bit resolution,

bits [7:0] are used for 8-bit conversion (bits [9:8] are zero). Bits [15:10] always return zero

when read.

Signed

left-justified format

Conversion result is signed left-justified data. Bits [15:6] are used for 10-bit resolution, bits

[15:8] are used for 8-bit conversion (bits [7:6] are zero). Although the ADC is unipolar, it

is assumed that the zero point is (VRH –VRL) / 2 when this format is used. The value read

from the register is an offset two’s-complement number; for positive input, bit 15 equals

zero, for negative input, bit 15 equals one. Bits [5:0] always return zero when read.

Unsigned

left-justified format

Conversion result is unsigned left-justified data. Bits [15:6] are used for 10-bit resolution,

bits [15:8] are used for 8-bit conversion (bits [7:6] are zero). Bits [5:0] always return zero

when read.

Freescale Semiconductor, Inc.

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