Freescale Semiconductor MC68HC908MR32 Manual Do Utilizador

Freescale Semiconductor

51

ADVIN is the input voltage signal from one of the 10 ADC channels to the ADC module.

This section describes the ADC external connections: V

REFH

 and V

REFL

, ANx, and grounding.

Both ac and dc current are drawn through the V

REFH

 and V

REFL

 loop. The AC current is in the form of 

current spikes required to supply charge to the capacitor array at each successive approximation step. 
The current flows through the internal resistor string. The best external component to meet both these 
current demands is a capacitor in the 0.01 

µF to 1 µF range with good high frequency characteristics. This 

capacitor is connected between V

REFH

 and V

REFL

 and must be placed as close as possible to the 

package pins. Resistance in the path is not recommended because the dc current will cause a voltage 
drop which could result in conversion errors.

Empirical data shows that capacitors from the analog inputs to V

REFL

 improve ADC performance. 0.01-

µF 

and 0.1-

µF capacitors with good high-frequency characteristics are sufficient. These capacitors must be 

placed as close as possible to the package pins.

In cases where separate power supplies are used for analog and digital power, the ground connection 
between these supplies should be at the V

SSAD

 pin. This should be the only ground connection between 

these supplies if possible. The V

SSA

 pin makes a good single point ground location. Connect the V

REFL 

pin to the same potential as V

SSAD

 at the single point ground location.

These I/O registers control and monitor operation of the ADC:

•

ADC status and control register, ADSCR

•

ADC data registers, ADRH and ARDL

•

ADC clock register, ADCLK