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Maxim > Design Support > Technical Documents > Application Notes > A/D and D/A Conversion/Sampling Circuits > APP 5282
Keywords: a to d, ADC, high voltage, SAR, sigma delta, a/d, analog to digital, pipeline, amplifier, scaling
input, input range, snr, signal to noise
APPLICATION NOTE 5282
Process High-Voltage Input Levels into a Low-
Voltage ADC Without Losing Much SNR
By: Roberto Sadkowski, Senior Business Manager, Standard ADCs and Filters
Apr 16, 2013
Abstract: It is often difficult to find an analog-to-digital converter (ADC) that aligns with the analog input
range, but also has the appropriate number of inputs, the needed size, and the correct sample speed.
Especially for system designers working with wide voltage swings, there is a concern that scaling down
the input signal to drive an ADC's full-scale range can significantly degrade the signal-to-noise ratio
(SNR). This application note discusses what affects this SNR loss, how it can be quantified, and more
importantly, how it can be minimized.
A similar version of this article appeared on Selezione di Elettronica, September 2012.
A typical misconception when designing with an analog-to-digital converter (ADC) is that scaling down the
input signal to drive an ADC's full-scale range will significantly degrade the signal-to-noise ratio (SNR).
This is especially a concern for system designers who are working with wide voltage swings.
Compounding the issue, there is also a much wider offering of ADCs for the low-voltage supply (5V or
less) than for higher voltage supplies. Higher supplies often lend to higher power consumption and board
complexity, needing for instance, more decoupling capacitors. This application note discusses what
affects the SNR loss introduced by scaling, how it can be quantified, and more importantly, how it can be
minimized.
Many signals coming out of a sensor or system are high voltage and bipolar; for instance ±10V is widely
used. However, there are simple ways to drive this signal through an ADC, and integrated high-voltage
ADC solutions are available that can handle such large full-scale inputs without losing SNR. These
solutions usually need extra high-voltage supplies to accommodate the input range and dissipate a fairly
large amount of power (Figure 1). These high-voltage ADCs also narrow the range of usable signal
conditioning (op amp) solutions. If signals need to be multiplexed with a combination of high-voltage and
low-voltage inputs, the implementation could become quite costly (Figure 2).
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