Tailored to handle multiple radio channels or to drive high-linearity, high-speed analog-to-digital converters (ADCs) in receiver applications, Linear Technology's LT5514 is a programmable gain amplifier (PGA) with bandwidth extending from low frequency to 850 MHz. It also features an ultra-low distortion to 300 MHz. It offers high linearity with high-output impedance. The amplifier's internal variable attenuator allows the user to digitally control the PGA's gain via four gain control pins with internal pull-down. This gain can be controlled over a range of 22.5 dB in steps of 1.5 dB resolution. Thus, maximum gain offered by the PGA is 33 dB. In the maximum gain state, the PGA offers a noise figure of 7.3 dB. The maximum output power is rated at 21 dBm.

The PGA is unconditionally stable with respect to the load. Its open-loop architecture offers stable operation for any practical load conditions, including peaking-free AC response when driving capacitive loads and excellent reverse isolation. As per the data sheet, the reverse isolation is -100 dB at 50 MHz and -78 d B at 300 MHz. Likewise, its intermodulation distortion of output current is determined by the input signal level and is almost independent of the output load conditions.

In essence, the amplifier is configured with two identical differential transconductance amplifiers hard wired in parallel with individual dedicated enable pins. When both amplifiers are enabled in standard mode, the LT5514 offers an output IP3 (OIP3) of +47 dBm at 100 MHz. As the amplifier's linearity is a strong function of signal frequency, the OIP3 decreases about 13 dB for every octave of frequency increase above 100 MHz. However, when used in low power mode, power dissipation is reduced by enabling a single amplifier. The amplifier in the low power mode offers the same basic characteristics as the standard mode, except that the g_m decreases from 0.3 S to 0.15 S. And the maximum output current is halved. Thus, in low power mode, the LT5514 will produce about 6 dB lower gain and 6 dB lower OIP3.

Besides combining advance bipolar process with proprietary linearization techniques to accomplish ultra-low distortion and high linearity over a wide bandwidth, the amplifier incorporates temperature compensation circuits to maintain high performance over a temperature range of -40°C to +85°C. For lowest noise and highest linearity, the LT5514 must be driven with a differential signal input. Single-ended drive degrades linearity and noise performance. At maximum gain, the amplifier's output noise floor is specified at -134 dBm/Hz.

In short, the PGA consists of an input variable attenuator “gain-control” block with 100 Ohm input impedance; two parallel, differential transconductance amplifiers — each with independent enable inputs; an internal bias block with internal voltage regulator; and a gain control logic block. The internal voltage regulator and filter isolates the amplifier signal path from external noise sources. Available in production, the LT5514 comes in a 20-pin plastic TSSOP package and operates over a single supply range of 4.75 V to 5.25 V.
Linear Technology Corp.
(408) 432-1900
www.linear.com

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