Datasheet

R
S
1 k
V
S+
MSP430
C
S
V
S+
= Positive external source voltage
V
S−
= Negative external source voltage
R
S
= External source resistance
C
S
= Sampling capacitance
R
S
1 k
V
S−
C
S
AV
CC
/ 2
†
Not implemented on all devices − see the device-specific data sheet
†
†
SD24_A Operation www.ti.com
The SD24INCHx bits select one of eight differential input pairs of the analog multiplexer. The gain for each
PGA is selected by the SD24GAINx bits. A total of six gain settings are available.
On some devices SD24AEx bits are available to enable or disable the analog input pin. Setting any
SD24AEx bit disables the multiplexed digital circuitry for the associated pin. See the device-specific data
sheet for pin diagrams.
During conversion any modification to the SD24INCHx and SD24GAINx bits will become effective with the
next decimation step of the digital filter. After these bits are modified, the next three conversions may be
invalid due to the settling time of the digital filter. This can be handled automatically with the
SD24INTDLYx bits. When SD24INTDLY = 00h, conversion interrupt requests will not begin until the fourth
conversion after a start condition.
On devices implementing the high impedance input buffer it can be enabled using the SD24BUFx bits.
The speed settings are selected based on the SD24_A modulator frequency as shown in Table 27-1.
Table 27-1. High Input Impedance Buffer
SD24BUFx Buffer SD24 Modulator Frequency, f
M
00 Buffer disabled
01 Low speed/current f
M
< 200 kHz
10 Medium speed/current 200 kHz < f
M
< 700 kHz
11 High speed/current 700 kHz < f
M
< 1.1 MHz
An external RC anti-aliasing filter is recommended for the SD24_A to prevent aliasing of the input signal.
The cutoff frequency should be less than 10 kHz for a 1-MHz modulator clock and OSR = 256. The cutoff
frequency may set to a lower frequency for applications that have lower bandwidth requirements.
27.2.6 Analog Input Characteristics
The SD24_A uses a switched-capacitor input stage that appears as an impedance to external circuitry as
shown in Figure 27-2.
Figure 27-2. Analog Input Equivalent Circuit
620
SD24_A SLAU144J–December 2004–Revised July 2013
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