Datasheet

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SBAS203J − MARCH 2002 − REVISED JANUARY 2008
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26
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 MSC1210 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 the REFOUT pin must be connected to REFIN+, and
AGND must be connected to REFIN−. The REFOUT 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 the rest of the 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. 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