Datasheet

© 2011 Microchip Technology Inc. DS25048B-page 27
MCP3903
5.6 Voltage Reference
5.6.1 INTERNAL VOLTAGE REFERENCE
The MCP3903 contains an internal voltage reference
source specially designed to minimize drift over
temperature. In order 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 2.35V ±2%.
The internal reference has a very low typical
temperature coefficient of ±5 ppm/°C, allowing the out-
put codes 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.
When the internal voltage reference is enabled,
REFIN- pin should always be connected to AGND.
For optimal ADC accuracy, appropriate bypass
capacitors should be placed between REFIN+/OUT
and AGND. De-coupling at the sampling frequency,
around 1 MHz, is important for any noise around this
frequency will be aliased back into the conversion data.
0.1 µF ceramic and 10 µF tantalum capacitors are
recommended.
These bypass capacitors are not mandatory for correct
ADC operation, but removing these capacitors may
degrade accuracy of the ADC. The bypass capacitors
also help for 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.
5.6.2 DIFFERENTIAL EXTERNAL
VOLTAGE INPUTS
When the VREFEXT bit is high, the two reference pins
(REFIN+/OUT, REFIN-) become a differential voltage
reference input. The 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 shown in
the following equation:
EQUATION 5-4:
The specified V
REF
range is from 2.2V to 2.6V. The
REFIN- pin voltage (V
REF
-) should be limited to ±0.3V.
Typically, for single-ended reference applications, the
REFIN- pin should be directly connected to AGND.
TABLE 5-5: OSR = 64 OUTPUT CODE EXAMPLES
ADC Output code (MSB First) Hexadecimal
Decimal
20-bit resolution
0111 1111 1111 1111 1111
0 0 0 0 0x7FFFF0 + 524, 287
0111 1111 1111 1111 1110 0 0 0 0 0x7FFFE0 + 524, 286
0000 0000 0000 0000 0000
0 0 0 0 0x000000 0
1111 1111 1111 1111 1111 0 0 0 0 0xFFFFF0 -1
1000 0000 0000 0000 0001 0 0 0 0 0x800010 - 524,287
1000 0000 0000 0000 0000
0 0 0 0 0x800000 - 524, 288
TABLE 5-6: 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 00 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 10 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
-