Precision voltage references (TI)

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Precision voltage references (TI)

By Perry Miller, Application Specialist—Data Converters, Texas Instruments, Dallas,
and Doug Moore, Managing Director, Thaler Corp., Tucson, Arizona

Introduction :
    One reason why designing a data conversion system is
such a challenge is the fact that the system accuracy very
much depends on the accuracy of the voltage established
by the internal or external DC voltage reference. The
voltage reference is used to produce a precise value of
output voltage for setting the full-scale input of the data
conversion system. In an analog-to-digital converter
(ADC), the DC voltage reference together with the analog
input signal is used to generate the digitized output signal.
And in a digital-to-analog converter (DAC), the DAC
selects and produces an analog output from the DC reference
voltage according to the digital input signal presented
at the input of the DAC. Any errors in the reference voltage
over the operating temperature range will adversely
affect the linearity and spurious free dynamic range
(SFDR) of the ADC/DAC. Practically all voltage references
vary with time or environmental factors such as
humidity, pressure, and temperature. As a result most
CMOS ADCs/DACs have internal references suitable only
for applications demanding £12-bit resolution even
though the converter may be capable of higher resolution.
Modern CMOS converters operate from 3.3-V or 5-V supplies,
which limits the on-chip voltage reference to a
band-gap reference. By way of the external reference pins
provided on the chip, an external precision reference can
also be connected to a CMOS ADC or DAC. A precision
external voltage reference has a much lower temperature
coefficient, thermal hysteresis, and long-term drift than
an on-chip band-gap voltage reference; therefore, in
applications demanding high accuracy (14-bit or 16-bit
ADCs/DACs), an external precision voltage reference is
often required.
    Precision voltage references are available with varying
degrees of precision and initial accuracy over some
operating temperature range. But often what is not obvious
when reading a manufacturer’s data sheet is how the
initial accuracy of the device is affected by other key device
parameters such as line regulation, load regulation, initial
voltage error, output voltage temperature coefficient (TC),
output voltage noise, turn-on settling time, thermal hysteresis,
quiescent supply current, and long-term stability.

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