Motorola MCF5281, MCF5282 user manual Analog Reference Signals, Analog Power Signals

Models: MCF5282 MCF5281

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Signal Connection Considerations

27.9.1 Analog Reference Signals

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 QADC, supplied by signals VRH and VRL, should be low-pass filtered from its source to obtain a noise-free, clean signal. In many cases, simple capacitive bypassing may suffice. In extreme cases, inductors or ferrite beads may be necessary if noise or RF energy is present. 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 limits defined by VDDA and VSSA, as explained in this subsection.

27.9.2 Analog Power Signals

The analog supply signals (VDDA and VSSA) define the limits of the analog reference voltages (VRH and VRL) and of the analog multiplexer inputs. Figure 27-44is a diagram of the analog input circuitry.

VDDAVRH

Sample

AMP

16 Channels Total

C

VSSA

S/H

Comparator

VRL

Figure 27-44. Equivalent Analog Input Circuitry

Because the sample amplifier is powered by VDDA and VSSA, it can accurately transfer input signal levels up to but not exceeding VDDA and down to but not below VSSA. If the input signal is outside of this range, the output from the sample amplifier is clipped.

In addition, VRH and VRL must be within the range defined by VDDA and VSSA. As long as VRH is less than or equal to VDDA, and VRL is greater than or equal to VSSA, and the sample amplifier has accurately transferred the input signal, resolution is ratiometric within the limits defined by VRL and VRH. If VRH is greater than VDDA, the sample amplifier can

MOTOROLA

Chapter 27. Queued Analog-to-Digital Converter (QADC)

27-63

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Motorola MCF5281, MCF5282 user manual Analog Reference Signals, Analog Power Signals