Motorola MCF5281, MCF5282 user manual Accommodating Positive/Negative Stress Conditions

Models: MCF5282 MCF5281

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Signal Connection Considerations

Analog ground must be low impedance to all analog ground points in the circuit.

Bypass capacitors should be as close to the power pins as possible.

The analog ground should be isolated from the digital ground. This can be done by cutting a separate ground plane for the analog ground.

Non-minimum traces should be utilized for connecting bypass capacitors and filters to their corresponding ground/power points.

Minimum distance for trace runs when possible.

27.9.5 Accommodating Positive/Negative Stress Conditions

Positive or negative stress refers to conditions which exceed nominally defined operating limits. Examples include applying a voltage exceeding the normal limit on an input (for example, voltages outside of the suggested supply/reference ranges) or causing currents into or out of the pin which exceed normal limits. QADC specific considerations are voltages greater than VDDA or less than VSSA applied to an analog input which cause excessive currents into or out of the input. Refer to MCF5282 Electrical Characteristics for more information on exact magnitudes.

Either stress conditions can potentially disrupt conversion results on neighboring inputs. Parasitic devices, associated with CMOS processes, can cause an immediate disruptive influence on neighboring pins. Common examples of parasitic devices are diodes to substrate and bipolar devices with the base terminal tied to substrate (VSS/VSSA ground). Under stress conditions, current injected on an adjacent signal can cause errors on the selected channel by developing a voltage drop across the selected channel’s impedances.

Figure 27-50shows an active parasitic bipolar NPN transistor when an input signal is subjected to negative stress conditions. Figure 27-51shows positive stress conditions can activate a similar PNP transistor.

 

 

 

 

 

VSTRESS

 

RSTRESS

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Stress

 

 

 

 

 

 

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10 kΩ

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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RSELECTED

IIN

 

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Adjacent

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Figure 27-50. Input Signal Subjected to Negative Stress

MOTOROLA

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

27-69

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Motorola MCF5281, MCF5282 Accommodating Positive/Negative Stress Conditions, Input Signal Subjected to Negative Stress