Datasheet

 2012-2013 Microchip Technology Inc. DS20002286B-page 37
MCP3911
5.7 Voltage Reference
5.7.1 INTERNAL VOLTAGE REFERENCE
The MCP3911 contains an internal voltage reference
source specially designed to minimize drift
overtemperature. To enable the internal voltage
reference, the VREFEXT bit in the configuration
register must be set to ‘0’ (default mode). This internal
V
REF
supplies reference voltage to both channels. The
typical value of this voltage reference is 1.2V ±2%. The
internal reference has a very low typical temperature
coefficient of ±7 ppm/°C, allowing the output to have
minimal variation with respect to temperature, since
they are proportional to (1/V
REF
).
The noise of the internal voltage reference is low
enough not to significantly degrade the SNR of the
ADC if compared to a precision external low-noise
voltage reference. The output pin for the internal
voltage reference is REFIN+/OUT.
If the voltage reference is only used as an internal
V
REF
, adding bypass capacitance on REFIN+/OUT is
not necessary for keeping ADC accuracy, but a minimal
0.1 µF ceramic capacitance can be connected to avoid
EMI/EMC susceptibility issues, due to the antenna
created by the REFIN+/OUT pin, if left floating.
The bypass capacitors also help applications where the
voltage reference output is connected to other circuits.
In this case, additional buffering may be needed, as the
output drive capability of this output is low.
Adding too much capacitance on the REFIN+/OUT pin
may slightly degrade the THD performance of the
ADCs.
5.7.2 DIFFERENTIAL EXTERNAL
VOLTAGE INPUTS
When the VREFEXT bit is high, the two reference pins
(REFIN+/OUT, REFIN-) become a differential voltage
reference input. The internal voltage reference circuit is
placed into Shutdown mode and the switch connecting
this circuit to the reference voltage input of the ADC is
opened. The internal voltage reference circuit is placed
into Shutdown mode and the switch connecting this
circuit to the reference voltage input of the ADC is
opened.
The voltage at the REFIN+/OUT is noted
V
REF
+ and the voltage at the REFIN- pin is noted V
REF
-
. The differential voltage input value is given by the
following equation:
EQUATION 5-4:
The specified V
REF
range is from 1.1V to 1.3V. The
REFIN- pin voltage (V
REF
-) should be limited to ±0.1V,
with respect to A
GND
. Typically, for single-ended
reference applications, the REFIN- pin should be
directly connected to A
GND
, with its own separate track
to avoid any spike due to switching noise.
TABLE 5-8: OSR = 32 OUTPUT CODE EXAMPLES
ADC Output code (MSB First) Hexadecimal
Decimal
17-bit Resolution
0111 1111 1111 1111 1
0 0 0 0 0 0 0 0x7FFF80 + 65, 535
0111 1111 1111 1111 00 0 0 0 0 0 0 0x7FFF00 + 65, 534
0000 0000 0000 0000 0
0 0 0 0 0 0 0 0x000000 0
1111 1111 1111 1111 1
0 0 0 0 0 0 0 0xFFFF80 -1
1000 0000 0000 0000 1
0 0 0 0 0 0 0 0x800080 - 65, 535
1000 0000 0000 0000 00 0 0 0 0 0 0 0x800000 - 65, 536
V
REF
= V
REF
+–V
REF
-