TC7109/A
3.5.6
REFERENCE SOURCES
A major factor in the absolute accuracy of the ADC is
the stability of the reference voltage. The 12-bit resolu-
tion of the TC7109A is one part in 4096, or 244 ppm.
Thus, for the on-board reference temperature coeffi-
cient of 70ppm/°C, a temperature difference of 3°C will
introduce a one-bit absolute error. Where the ambient
temperature is not controlled, or where high accuracy
absolute measurements are being made, it is
recommended that an external high quality reference
be used.
A reference output (Pin 29) is provided, which may be
used with a resistive divider to generate a suitable ref-
erence voltage (20mA may be sunk without significant
variation in output voltage). A pull-up bias device is pro-
vided, which sources about 10μA. The output voltage is
nominally 2.8V below V+. When using the on-board
reference, REF OUT (Pin 29) should be connected to
REF IN- (pin 39), and REF IN+ should be connected to
the wiper of a precision potentiometer between REF
OUT and V+. The test circuit shows the circuit for a
204.8mV reference, generated by a 2kΩ precision
potentiometer in series with a 24kΩ fixed resistor.
DS21456C-page 16
© 2006 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
