HCS08 datasheet(116/198 Pages) NXP

116 / 198 page

Analog-to-Digital Converter (S08ADC10V1)

MC9S08QD4 Series MCU Data Sheet, Rev. 6

116

Freescale Semiconductor

In cases where separate power supplies are used for analog and digital power, the ground connection

between these supplies must be at the VSSAD pin. This must be the only ground connection between these

supplies if possible. The VSSAD pin makes a good single point ground location.

8.6.1.2

Analog Reference Pins

In addition to the analog supplies, the ADC module has connections for two reference voltage inputs. The

high reference is VREFH, which may be shared on the same pin as VDDAD on some devices. The low

reference is VREFL, which may be shared on the same pin as VSSAD on some devices.

When available on a separate pin, VREFH may be connected to the same potential as VDDAD, or may be

driven by an external source that is between the minimum VDDAD spec and the VDDAD potential (VREFH

must never exceed VDDAD). When available on a separate pin, VREFL must be connected to the same

voltage potential as VSSAD. Both VREFH and VREFL must be routed carefully for maximum noise

immunity and bypass capacitors placed as near as possible to the package.

AC current in the form of current spikes required to supply charge to the capacitor array at each successive

approximation step is drawn through the VREFH and VREFL loop. The best external component to meet this

current demand is a 0.1

μF capacitor with good high frequency characteristics. This capacitor is connected

between VREFH and VREFL and must be placed as near as possible to the package pins. Resistance in the

path is not recommended because the current will cause a voltage drop which could result in conversion

errors. Inductance in this path must be minimum (parasitic only).

8.6.1.3

Analog Input Pins

The external analog inputs are typically shared with digital I/O pins on MCU devices. The pin I/O control

is disabled by setting the appropriate control bit in one of the pin control registers. Conversions can be

performed on inputs without the associated pin control register bit set. It is recommended that the pin

control register bit always be set when using a pin as an analog input. This avoids problems with contention

because the output buffer will be in its high impedance state and the pullup is disabled. Also, the input

buffer draws dc current when its input is not at either VDD or VSS. Setting the pin control register bits for

all pins used as analog inputs must be done to achieve lowest operating current.

Empirical data shows that capacitors on the analog inputs improve performance in the presence of noise

or when the source impedance is high. Use of 0.01

μF capacitors with good high-frequency characteristics

is sufficient. These capacitors are not necessary in all cases, but when used they must be placed as near as

possible to the package pins and be referenced to VSSA.

For proper conversion, the input voltage must fall between VREFH and VREFL. If the input is equal to or

exceeds VREFH, the converter circuit converts the signal to $3FF (full scale 10-bit representation) or $FF

(full scale 8-bit representation). If the input is equal to or less than VREFL, the converter circuit converts it

to $000. Input voltages between VREFH and VREFL are straight-line linear conversions. There will be a

brief current associated with VREFL when the sampling capacitor is charging. The input is sampled for

3.5 cycles of the ADCK source when ADLSMP is low, or 23.5 cycles when ADLSMP is high.

For minimal loss of accuracy due to current injection, pins adjacent to the analog input pins must not be

transitioning during conversions.

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