ADC Driver Offers Digitally Controlled Gain

June 1, 2003
This low-distortion variable-gain amplifier features digitally selectable gain and a differential output to drive high-speed analog-to-digital converters.

Digitally controlled gain in the new MAX2055 variable-gain amplifier (VGA) from Maxim Integrated Products (Sunnyvale, CA) allows the device to quickly achieve optimum signal levels for analog-to-digital converters (ADCs). Designed for intermediate frequencies (IFs) from 30 to 300 MHz, the MAX2055 is ideal for receivers (Rxs) in a variety of wireless communications standards, including WCDMA, cdma2000, GSM, DCS/PCS, and EDGE.

The MAX2055 (see figure) is a highly integrated (total of 325 transistors) amplifier fabricated with a high-speed silicon BiCMOS process. Designed for use with 50-(omega) single-ended inputs and differential outputs over a frequency range of 30 to 300 MHz, the amplifier is suitable for gain control of signals entering a high-speed ADC. The MAX2055 integrates a digital attenuator with 23-dB selectable attenuation range (from −3 to +20 dB) and a high-linearity single-ended-to-differential amplifier. The amplifier's gain flatness is ±0.5 dB across a 50-MHz bandwidth.

The MAX2055's attenuation/gain range is set with 1-dB resolution (and ±0.2 dB absolute accuracy at +25°C) by means of five logic lines. The attenuator switching speed is a mere 40 ns from 50 percent of a control signal to 90 percent of a new RF level. Negative feedback in the amplifier helps achieve high gain and linearity over the wide operating bandwidth. Even over the wide operating temperature range, the gain drift with temperature is typically only ±0.3 dB.

Good low-noise performance and exceptional linearity make the MAX2055 a good choice for broadband communications systems, cellular systems, test equipment, and any application requiring optimal level control of signals feeding a high-speed ADC. The MAX2055 features typical noise figure of 5.8 dB and output 1-dB compression point of typically +25.7 dBm. The well-designed circuit achieved reverse isolation of typically 29 dB, a second-order output intercept point of typically +75 dBm (+5 dBm/tone), and a third-order output intercept point of typically +40 dBm over all gain conditions (+5 dBm/tone).

The MAX2055 is supplied in a 20-pin TSSOP-EP package. It is designed for supply voltages from +4.75 to +5.25 VDC, and it typically draws 240 mA of current. Maxim Integrated Products, Inc., 120 San Gabriel Dr., Sunnyvale, CA 94086; (408) 737-7600, FAX: (408) 737-7194, Internet:

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