SLAU144K December 2004 – August 2022 MSP430AFE221 , MSP430AFE222 , MSP430AFE223 , MSP430AFE231 , MSP430AFE232 , MSP430AFE233 , MSP430AFE251 , MSP430AFE252 , MSP430AFE253 , MSP430F2001 , MSP430F2002 , MSP430F2003 , MSP430F2011 , MSP430F2012 , MSP430F2013 , MSP430F2101 , MSP430F2111 , MSP430F2112 , MSP430F2121 , MSP430F2122 , MSP430F2131 , MSP430F2132 , MSP430F2132-EP , MSP430F2232 , MSP430F2234 , MSP430F2252 , MSP430F2252-Q1 , MSP430F2254 , MSP430F2272 , MSP430F2272-Q1 , MSP430F2274 , MSP430F2274-EP , MSP430F233 , MSP430F2330 , MSP430F235 , MSP430F2350 , MSP430F2370 , MSP430F2410 , MSP430F2416 , MSP430F2417 , MSP430F2418 , MSP430F2419 , MSP430F247 , MSP430F2471 , MSP430F248 , MSP430F2481 , MSP430F249 , MSP430F2491 , MSP430F2616 , MSP430F2617 , MSP430F2618 , MSP430F2619 , MSP430G2001 , MSP430G2101 , MSP430G2102 , MSP430G2111 , MSP430G2112 , MSP430G2121 , MSP430G2131 , MSP430G2132 , MSP430G2152 , MSP430G2153 , MSP430G2201 , MSP430G2201-Q1 , MSP430G2202 , MSP430G2203 , MSP430G2210 , MSP430G2211 , MSP430G2212 , MSP430G2213 , MSP430G2221 , MSP430G2230 , MSP430G2231 , MSP430G2231-Q1 , MSP430G2232 , MSP430G2233 , MSP430G2252 , MSP430G2253 , MSP430G2302 , MSP430G2303 , MSP430G2312 , MSP430G2313 , MSP430G2332 , MSP430G2333 , MSP430G2352 , MSP430G2353 , MSP430G2402 , MSP430G2403 , MSP430G2412 , MSP430G2413 , MSP430G2432 , MSP430G2433 , MSP430G2444 , MSP430G2452 , MSP430G2453 , MSP430G2513 , MSP430G2533 , MSP430G2544 , MSP430G2553 , MSP430G2744 , MSP430G2755 , MSP430G2855 , MSP430G2955 , MSP430TCH5E
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 in Figure 22-4. An internal MUX-on input resistance RI (2 kΩ maximum) in series with capacitor CI (27 pF maximum) 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.
The resistance of the source RS and RI affect tsample.The following equations can be used to calculate the minimum sampling time for a 10-bit conversion.
tsample > (RS + RI) × ln(211) × CI
Substituting the values for RI and CI given above, the equation becomes:
tsample > (RS + 2 kΩ) × 7.625 × 27 pF
For example, if RS is 10 kΩ, tsample must be greater than 2.47 µs.
When the reference buffer is used in burst mode, the sampling time must be greater than the sampling time calculated and the settling time of the buffer, tREFBURST:
For example, if VRef is 1.5 V and RS is 10 kΩ, tsample must be greater than 2.47 µs when ADC10SR = 0, or 2.5 µs when ADC10SR = 1. See the device-specific data sheet for parameters.
To calculate the buffer settling time when using an external reference, the formula is:
tREFBURST = SR × VRef − 0.5 µs
Where:
SR = Buffer slew rate (~1 µs/V when ADC10SR = 0 and ~2 µs/V when ADC10SR = 1)
VRef = External reference voltage