Single-Supply DC-Coupled 16-Bit, 125 MSPS Analog Front End for Bipolar Input (CN0252)
The circuit shown in Figure 1 solves a problem often encountered in dc-coupled single-supply systems when interfacing bipolar input signals to differential input, low voltage analog-to-digital converters (ADCs). The technique ensures the proper common-mode voltage level at the input to the differential drive amplifier by controlling the input common-mode level using two level shifting resistors. The output common-mode voltage is established independently by applying the correct voltage to the VOCM pin of the ADA4930-1 differential driver.
This flexible arrangement allows the ADA4930-1 differential driver to operate on a single 3.3 V supply, while the AD9265 16-bit 125 MSPS ADC operates on a 1.8 V supply, thereby minimizing the total circuit power dissipation.
In broadband applications, the frequency range of interest often includes dc. To maximize the dynamic range of a differential input ADC, typical input signals can be relatively large, thereby requiring the differential driver to operate at lower gain settings. Under these conditions, the input common-mode voltage of the differential driver must stay within the specified range.
Independent control of the input and output common-mode voltage of differential amplifiers is often required in direct coupled single-supply applications such as processing demodulator outputs with high input common-mode voltages, x-ray applications with a dc component added to a differential component, and other places where the differential driver must handle low value input common-mode voltages. Low input common-mode voltage applications can include either single-ended or differential inputs, with zero, bipolar, or negative inputs.
Figure 1. High Speed, Single-Ended-to-Differential ADC Driver (Simplified Schematic: All Connections and Decoupling Not Shown)
Схемы и диаграммы
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