Scan IF Operation
24-8 Scan IF
Sample-And-Hold
The sample-and-hold is used to sample the sensor voltage to be measured.
The sample-and-hold circuitry is shown in Figure 243. When SIFSH = 1 and
SIFTEN = 0 the sample-and-hold circuitry is enabled and the excitation
circuitry and mid-voltage generator are disabled. The sample-and-hold is used
for resistive dividers or for other analog signals that should be sampled.
Up to four resistor dividers can be connected to SIFCHx and SIFCOM. AVCC
and SIFCOM are the common positive and negative potentials for all
connected resistor dividers. When SIFEX(tsm) = 1, SIFCOM is connected to
SIFVSS allowing current to flow through the dividers. This charges the
capacitors of each sample-and-hold circuit to the divider voltages. All resistor
divider channels are sampled simultaneously. When SIFEX(tsm) = 0 the
sample-and-hold capacitor is disconnected from the resistor divider and
SIFCOM is disconnected from SIFVSS. After sampling, each channel can be
measured sequentially using the channel select logic, the comparator, and the
DAC.
The selected SIFCHx input can be modeled as an RC low-pass filter during
the sampling time tsample, as shown below in Figure 244. An internal MUX-on
input resistance Ri(SIFCHx) (max. 3 k) in series with capacitor CSCH(SIFCHx)
(max. 7 pF) is seen by the resistor-divider. The capacitor voltage VC must be
charged to within ½ LSB of the resistor divider voltage for an accurate 10-bit
conversion. See the device-specific datasheet for parameters.
Figure 244. Analog Input Equivalent Circuit
Ri(SIFCHx)
VC
MSP430
CSHC(SIFCHx)
VI
VI= Input voltage at pin SIFCHx
VS= External source voltage
RS = External source resistance
Ri(SIFCHx) = Internal MUX-on input resistance
CISHC(SIFCHx) = Input capacitance
VC= Capacitance-charging voltage
RS
VS
The resistance of the source RS and Ri(SIFCHx) affect tsample.The following
equation can be used to calculate the minimum sampling time tsample for a
10-bit conversion:
tsample u(RS)RiSIFCHx) ln(211) CSHC(SIFCHx) (1)
Substituting the values for RI and CI given above, the equation becomes:
tsample u(RS)3k) 7. 625 7pF (2)
For example, if RS is 10 k, tsample must be greater than 684 ns.