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Maxim > Design Support > Technical Documents > Application Notes > Voltage References > APP 3635
Keywords: internal reference, external reference, Gain Adjust, Adjustable gain, ADC, Sigma Delta, Panel
Meter, Digital Panel Meter, DPM, voltmeter, Digital voltmeter, DVM
APPLICATION NOTE 3635
Implementing an External Reference and the
Resulting Effect on Gain for the MAX149x Sigma-
Delta Panel-Meter ADCs
Sep 23, 2005
Abstract: The MAX149x family of sigma-delta panel-meter ADCs can use an external reference over a
500mV to 3.6V range and achieve accurate conversion results. This application note illustrates how to
use an external voltage reference and divide down the output to create a variable reference voltage that
drives the MAX149x family and adjusts the ADC's gain.
How the Reference Voltage Affects the ADC
The MAX1447, MAX1491, MAX1493, MAX1495, MAX1496, MAX1498 sigma-delta panel-meter ICs give
the user a choice of either a 2.048V internal or an external reference source between 500mV and 3.6V.
In the most basic configuration, these devices expect a 2.048V reference at the input across AIN+ and
AIN-, and convert the result to show on the LED/LCD display. The output count is valid for the 4.5-digit
version and is given by the following transfer functions. For the 3.5-digit versions, change the 20,000 in
these equations to 2000.
With AIN+ - AIN- >
0, RANGE = GND
With AIN+ - AIN- < 0, RANGE = GND
With AIN+ - AIN- > 0, RANGE = DVDD
With AIN+ - AIN- < 0, RANGE = DVDD
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