Reference Voltage Attenuator (RVAV1)
MC9S12G Family Reference Manual Rev.1.27
258 NXP Semiconductors
4.7 Functional Description
The RVA is a prescaler for the ADC reference voltage. If the attenuation is turned off the resistive divider
is disconnected from VSSA, VRH_INT is connected to VRH and VRL_INT is connected to VSSA. In this
mode the attenuation is bypassed and the resistive divider does not draw current.
If the attenuation is turned on the resistive divider is connected to VSSA, VRH_INT and VRL_INT are
connected to intermediate voltage levels:
VRH_INT = 0.9 * (VRH - VSSA) + VSSA Eqn. 4-1
VRL_INT = 0.4 * (VRH - VSSA) + VSSA Eqn. 4-2
The attenuated reference voltage difference (VRH_INT - VRL_INT) equals 50% of the input reference
voltage difference (VRH - VSSA). With reference voltage attenuation the resolution of the ADC is
improved by a factor of 2.
NOTE
In attenuation mode the maximum ADC clock is reduced. Please refer to the
conditions in appendix A “ATD Accuracy”, table “ATD Conversion
Performance 5V range, RVA enabled”.
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