MC68HC908LB8_05 Datasheet, PDF (56/234 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Op Amp/Comparator Module
0 = Comparator mode selected
OACE — Op Amp/Comparator Enable Bit
Setting of the corresponding bit in the register enables the associated op amp/comparator and
connects it to the op amp/comparator pins.
1 = Op amp/comparator is connected to pins and powered on
0 = Op amp/comparator is disconnected from pins and powered off
NOTE
Enabling the op amp/comparator prevents PTB[5:7] from being used as
standard I/O. However, the PTB7 pin can be shared with AD6 and FAULT
if the ADC and PWM modules are also enabled.
4.7 Application Information
We make the following assumptions during the design of the operational amplifier.
1. The signal amplified by the operational amplifier is sampled by the internal ADC.
2. Noise resulting from the operation of other circuitry within the MCU will appear at the output of the
circuit due to the amplification set by user.
We recommend the following.
1. An external 500pF capacitor should be added between the output of the operational amplifier
(PTB7) and VSS. This capacitor will act as a filter to internal bus noise caused by the operation of
other digital circuitry in the MCU.
2. Care should be taken to ensure proper filtering at or around the operation bus speed in the
amplification circuit, to prevent noise from being amplified along with the desired signal.
3. The maximum frequency of the signal to be amplified should be limited to 200kHz. This will ensure
that the filtering element will not affect the gain of the desired signal.
4. Do not set the gain of the amplifier to less than 5 (except in the unity gain buffer).
5. Use the circuit component values suggested for the common amplfier configurations shown in the
following figures (Figure 4-4, Figure 4-5, and Figure 4-6).
VDD
Unity Gain Buffer
Vin
+
–
Vout
CL
500pF
RL
>20kΩ
Figure 4-4. Suggested Application Circuit for Unity Gain Buffer
MC68HC908LB8 Data Sheet, Rev. 1
56
Freescale Semiconductor