Differential Amp Buffers ADCs To 2 GHz

Jan. 17, 2006
Well suited for RF and IF applications, this high-performance differential amplifier combines low noise with low distortion and impressive transient performance.

Differential circuitry provides a proven means of suppressing noise in analog and digital designs. An increasing number of analog-to-digital converters (ADCs) are being offered with differential inputs, while even RF frontend components, such as mixers, are adopting a differential architecture. Of course, gain is often needed with these function blocks, implying the requirement for a differential gain block. The new AD8352 low-distortion amplifier from Analog Devices ( Norwood, MA) works with both single-ended and differential inputs and offers a wide 3-dB bandwidth of 2 GHz to handle a multiple of RF and intermediatefrequency (IF) applications.

The AD8352 (see figure) is fabricated with the company's XFCB3 silicon-germanium (SiGe) bipolar process. SiGe materials support the fabrication of transistors with higher cutoff frequencies than conventional silicon-bipolar technologies, with inherently low noise. The AD8352 leverages this technology to achieve a high 80-dB spurious-free dynamic range (SFDR) at frequencies to 180 MHz, putting it in range to buffer the inputs of many high-speed 14-and 16-b ADCs (assuming a SFDR of typically about 76 dB for 16-b ADCs). It is also useful in conjunction with surface-acoustic-wave (SAW) filters, as a general gain block for RF and IF circuits, and wherever single-ended-to-differential conversion is needed. The differential amplifier features a gain range from 0 to 24 dB, adjustable with a single resistor; the 3-dB bandwidth of 2 GHz is at a gain setting of 10 dB. Over that gain range, the AD8352 maintains a constant input resistance of 3 k to simply impedance-matching requirements at different gain settings and frequencies. The amplifier's input stage suffers only 2.6 nV/(Hz) 0.5 at a gain setting of 10 dB.

This device is designed for wideband, low-distortion applications, with harmonic performance to support those design claims. The second-and thirdharmonic distortion levels at 19 MHz are ?87 and ?90 dBc, respectively. At 180 MHz, the second-and third-harmonic distortion levels are still only ?81 and ?80 dBc, respectively. At the other end of the dynamic-range scale, the AD8352 offers an output thirdorder intercept point of +41 dBm at 180 MHz with a 2-V peak-to-peak output voltage.

The AD8352 has a nominal differential output resistance of 100 Ω with output capacitance of 3 pF. It has a slew rate of 11 V/ns and settles to a 1-V step in less than 3 ns. The overdrive recovery time, measured by dropping the input from 4 V to 0 V, is less than 2 ns. The amplifier is designed for simple, single-supply operation, running on voltage sources from 3 to 5 V. The current draw is only 37 mA at 5 V. In addition, the device has a power-down mode in which current draw is only 4 mA at 5 V. The AD8352 is supplied in a 3 3 mm, 16-pin LFCSP package and is designed for operating temperatures from ?40 to +85°C. Analog Devices, Inc., One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106; (781) 329-4700, FAX: (781) 461-3113, Internet: www.analog.com.

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