Datasheet
SBAS323G − JUNE 2004 − REVISED OCTOBER 2007
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29
It will then use the Sinc
2
followed by the Sinc
3
filter to
improve noise performance. This combines the low-noise
advantage of the Sinc
3
filter with the quick response of the
Fast Settling Time filter. The frequency response of each
filter is shown in Figure 16.
VOLTAGE REFERENCE
The MSC1211/12/13/14 can use either an internal or
external voltage reference. The voltage reference
selection is controlled via ADC Control Register 0
(ADCON0, SFR DCh). The default power-up
configuration for the voltage reference is 2.5V internal.
The internal voltage reference can be selected as either
1.25V or 2.5V. The analog power supply (AV
DD
) must be
within the specified range for the selected internal voltage
reference. The valid ranges are: V
REF
= 2.5 internal
(AV
DD
= 3.3V to 5.25V) and V
REF
= 1.25 internal
(AV
DD
= 2.7V to 5.25V). If the internal V
REF
is selected,
then AGND must be connected to REF IN−. The
REFOUT/REF IN+ pin should also have a 0.1µF capacitor
connected to AGND as close as possible to the pin. If the
internal V
REF
is not used, then V
REF
should be disabled in
ADCON0.
If the external voltage reference is selected, it can be used
as either a single-ended input or differential input, for
ratiometric measures. When using an external reference,
it is important to note that the input current will increase for
V
REF
with higher PGA settings and with a higher modulator
frequency. The external voltage reference can be used
over the input range specified in the Electrical
Characteristics section.
For applications requiring higher performance than that
obtainable from the internal reference, use an external
precision reference such as the REF50xx. The internal
reference performance can be observed in the noise (and
ENOB) versus input signal graphs in the Typical
Characteristics section. All of the other ENOB plots are
obtained with the inputs shorted together. By shorting the
inputs, the inherent noise performance of only the ADC
can be determined and displayed. When the inputs are not
shorted, the extra noise comes from the reference. As can
be seen in the ENOB vs Input Signal graph, the external
reference adds about 0.7 bits of noise, whereas the
internal reference adds about 2.3 bits of noise. This ENOB
performance of 19.4 represents 21.16 bits of noise. With
an LSB of 298nV, that translates to 6.3µV or a
peak-to-peak noise of almost 42µV. The internal reference
is initialized each time power is applied. That initialization
can cause a shift in the output that is within the specified
accuracy. An external reference provides the best noise,
drift, and repeatability performance for high-precision
applications.
SINC
3
FILTER RESPONSE
(
−
3dB = 0.262
•
f
DATA
)
f
DATA
0
−
20
−
40
−
60
−
80
−
100
−
120
0 12345
012345
012345
Gain (dB)
SINC
2
FILTER RESPONSE
(
−
3dB = 0.318
•
f
DATA
)
f
DATA
0
−
20
−
40
−
60
−
80
−
100
−
120
Gain (dB)
FAST SETTLING FILTER RESPONSE
(
−
3dB = 0.469
•
f
DATA
)
f
DATA
0
−
20
−
40
−
60
−
80
−
100
−
120
NOTE: f
DATA
= Normalized Data Output Rate = 1/t
DATA
Gain (dB)
Figure 16. Filter Frequency Responses