ADC12 Operation
20-9ADC12
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 205. An internal MUX-on
input resistance RI (max. 2 k) in series with capacitor CI (max. 40 pF) is seen
by the source. The capacitor CI voltage VC must be charged to within 1/2 LSB
of the source voltage VS for an accurate 12-bit conversion.
Figure 205. Analog Input Equivalent Circuit
RSRI
VSVC
MSP430
CI
VI
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 equation
can be used to calculate the minimum sampling time tsample for a 12-bit
conversion:
tsample u(RS)RI) ln(213) CI)800ns
Substituting the values for RI and CI given above, the equation becomes:
tsample u(RS)2kW) 9.011 40pF )800ns
For example, if RS is 10 k, tsample must be greater than 5.13 µs.