Texas Instruments MSP430x1xx manual 4. Analog Input Equivalent Circuit

ADC10 Operation

Sample Timing Considerations

When SAMPCON = 0 all Ax inputs are high impedance. When SAMPCON = 1, the selected Ax input can be modeled as an RC low-pass filter during the

sampling time tsample, as shown below in Figure 18−4. An internal MUX-on input resistance RI (max. 2 kΩ) in series with capacitor CI (max. 20 pF) is seen

by the source. The capacitor CI voltage VC must be charged to within ½ LSB of the source voltage VS for an accurate 10-bit conversion.

Figure 18−4. Analog Input Equivalent Circuit

MSP430

RS VI RI

VSVC

CI

VI = Input voltage at pin Ax VS = External source voltage RS = External source resistance

RI = Internal MUX-on input resistance CI = Input capacitance

VC = Capacitance-charging voltage

The resistance of the source RS and RI affect tsample.The following equations can be used to calculate the minimum sampling time tsample for a 10-bit conversion.

When ADC10SR = 0:
tsample u (RS ) RI)	ln(211)	CI ) 800ns
When ADC10SR = 1:
tsample u (RS ) RI)	ln(211)	CI ) 2.5ms

Substituting the values for RI and CI given above, the equation becomes:

tsample u (RS ) 2k)	7.625	20pF ) 800ns
tsample u (RS ) 2k)	7.625	20pF ) 2.5ms

(ADC10SR = 0)

(ADC10SR = 1)

For example, if RS is 10 kΩ, tsample must be greater than 2.63 ∝s when ADC10SR = 0, or 4.33 ∝s when ADC10SR = 1.

18-8 ADC10