概要

The ADA4930-1/ADA4930-2 are very low noise, low distortion, high speed differential amplifiers. They are an ideal choice for driving 1.8 V high performance ADCs with resolutions up to 14 bits from dc to 70 MHz. The adjustable output common mode allows the ADA4930-1/ADA4930-2 to match the input of the ADC. The internal common-mode feedback loop provides exceptional output balance, suppression of even-order harmonic distortion products, and dc level translation.

With the ADA4930-1/ADA4930-2, differential gain configurations are easily realized with a simple external feedback network of four resistors determining the closed-loop gain of the amplifier.

The ADA4930-1/ADA4930-2 are fabricated using Analog Devices, Inc., proprietary silicon-germanium (SiGe), complementary bipolar process, enabling them to achieve very low levels of distortion with an input voltage noise of only 1.2 nV/√Hz.

The low dc offset and excellent dynamic performance of the ADA4930-1/ADA4930-2 make them well suited for a wide variety of data acquisition and signal processing applications.

The ADA4930-2 is available in a Pb-free, 4 mm × 4 mm 24-lead LFCSP. The pinout has been optimized to facilitate printed circuit board (PCB) layout and minimize distortion. The ADA4930-2 is specified to operate over the −40°C to +105°C temperature range for 3.3 V or 5 V supply voltages.

 

最大消費電力

The maximum safe power dissipation in the ADA4930-1/ ADA4930-2 packages is limited by the associated rise in junction temperature (TJ) on the die. At approximately 150°C, which is the glass transition temperature, the plastic changes its properties. Even temporarily exceeding this temperature limit can change the stresses that the package exerts on the die, permanently shifting the parametric performance of the ADA4930-1/ADA4930-2. Exceeding a junction temperature of 150°C for an extended period can result in changes in the silicon devices, potentially causing failure.

The power dissipated in the package (PD) is the sum of the quiescent power dissipation and the power dissipated in the package due to the load drive. The quiescent power is the voltage between the supply pins (VS) times the quiescent current (IS). The power dissipated due to the load drive depends upon the particular application. The power due to load drive is calculated by multiplying the load current by the associated voltage drop across the device. RMS voltages and currents must be used in these calculations.

Airflow increases heat dissipation, effectively reducing θJA. In addition, more metal directly in contact with the package leads/ exposed pad from metal traces, through holes, ground, and power planes reduces θJA.

 

動作理論

The ADA4930-1/ADA4930-2 differ from conventional op amps in that they have two outputs whose voltages move in opposite directions and an additional input, VOCM. Like an op amp, theyrely on high open-loop gain and negative feedback to force these outputs to the desired voltages. The ADA4930-1/ADA4930-2 behave much like standard voltage feedback op amps and facilitate single-ended-to-differential conversions, common-mode level shifting, and amplifications of differential signals. Like op amps, the ADA4930-1/ADA4930-2 havehigh inputimpedance and low output impedance.Two feedback loops control the differential and common-mode output voltages. The differential feedback, set with external resistors, controls the differential output voltage. The commonmode feedback controls the common-mode output voltage. This architecture makes it easy to set the output common-mode level to any arbitrary value within the specified limits. The output common-mode voltage is forced to be equal to the voltage applied to the VOCM input by the internal common-mode feedback loop.

The internal common-mode feedback loop produces outputs that are highly balanced over a wide frequency range without requiring tightly matched external components. This results in differential outputs that are very close to the ideal of being identical in amplitude and are exactly 180°apart in phase.

 

ANALYZING AN APPLICATION CIRCUIT

The ADA4930-1/ADA4930-2 use high open-loop gain and negative feedback to force their differential and common-mode output voltages to minimize the differential and common-mode error voltages. The differential error voltage is defined as the voltage between the differential inputs labeled +IN and −IN. For most purposes, this voltage can be assumed to be zero. Similarly, the difference between the actual output common-mode voltage and the voltage applied to VOCM can also be assumed to be zero. Starting from these two assumptions, any application circuit can be analyzed.