PAC7112 datasheet(25/56 Pages) FREESCALE

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ElectricalCharacteristics

MAC7100 Microcontroller Family Hardware Specifications, Rev. 1.2

Preliminary

Freescale Semiconductor

3.10.1 Factors Influencing Accuracy

Three factors—source resistance, source capacitance and current injection—have an influence on the

accuracy of the ATD.

3.10.1.1 Source Resistance

Due to the input pin leakage current as specified in Table 8 in conjunction with the source resistance there

will be a voltage drop from the signal source to the ATD input. The maximum specified source resistance

RS, results in an error of less than 1/2 LSB (2.5 mV) at the maximum leakage current. If the device or

operating conditions are less than the worst case, or leakage-induced errors are acceptable, larger values

of source resistance are allowed.

3.10.1.2 Source Capacitance

When sampling, an additional internal capacitor is switched to the input. This can cause a voltage drop due

to charge sharing with the external capacitance and the pin capacitance. For a maximum sampling error of

the input voltage

≤ 1 LSB, then the external filter capacitor must be calculated as, C

f ≥ 1024 × (CINS – CINN).

3.10.1.3 Current Injection

There are two cases to consider:

1. A current is injected into the channel being converted. The channel being stressed has conversion

values of 0x3FF (0xFF in 8-bit mode) for analog inputs greater than VRH and 0x000 for values less

than VRL unless the current is higher than specified as disruptive condition.

2. Current is injected into pins in the neighborhood of the channel being converted. A portion of this

current is picked up by the channel (coupling ratio K), This additional current impacts the

accuracy of the conversion depending on the source resistance. The additional input voltage error

on the converted channel can be calculated as VERR = K × RS × IINJ, with IINJ being the sum of

the currents injected into the two pins adjacent to the converted channel.

Table 28. ATD Electrical Characteristics

Conditions are shown in Table 7 unless otherwise noted

Num C

Rating

Symbol

Min

Typ

Max

Unit

C Max input Source Resistance

——

C Total Input Capacitance

Non Sampling

Sampling

CINN

CINS

—

C Disruptive Analog Input Current

INA

–2.5

—

2.5

C Coupling Ratio positive current injection

——

TBD

A / A

C Coupling Ratio negative current injection

——

TBD

A / A

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