Chapter 10 Analog-to-Digital Converter (S08ADC12V1)
MC9S08DZ128 Series Data Sheet, Rev. 1
Freescale Semiconductor
229
10.6
Application Information
This section contains information for using the ADC module in applications. The ADC has been designed
to be integrated into a microcontroller for use in embedded control applications requiring an A/D
converter.
10.6.1
External Pins and Routing
The following sections discuss the external pins associated with the ADC module and how they should be
used for best results.
10.6.1.1
Analog Supply Pins
The ADC module has analog power and ground supplies (VDDAD and VSSAD) available as separate pins
on some devices. VSSAD is shared on the same pin as the MCU digital VSS on some devices. On other
devices, VSSAD and VDDAD are shared with the MCU digital supply pins. In these cases, there are separate
pads for the analog supplies bonded to the same pin as the corresponding digital supply so that some degree
of isolation between the supplies is maintained.
When available on a separate pin, both VDDAD and VSSAD must be connected to the same voltage potential
as their corresponding MCU digital supply (VDD and VSS) and must be routed carefully for maximum
noise immunity and bypass capacitors placed as near as possible to the package.
If separate power supplies are used for analog and digital power, the ground connection between these
supplies must be at the VSSAD pin. This should be the only ground connection between these supplies if
possible. The VSSAD pin makes a good single point ground location.
10.6.1.2
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 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.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 causes a voltage drop that could result in conversion errors.
Inductance in this path must be minimum (parasitic only).