TC7109CLW datasheet(16/30 Pages) MICROCHIP

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TC7109/A

DS21456C-page 16

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,

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 10

μA. 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 2k

Ω precision

potentiometer in series with a 24k

Ω fixed resistor.

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