Analog Angle Article

Slew of differential amps provide current feedback, VGA, and driver functions at low power

Santa Clara, CA–Supporting the high-speed, low-power push in wireless infrastructure, test and measurement, and mil/aero applications, new linear products from National Semiconductor implement differential current-feedback, variable gain amplifier (VGA), and driver functions.

The LMH6552 high-bandwidth current-feedback differential amplifier has 1.5 GHz bandwidth and power dissipation of 112 mW, which the vendor claims is the industry's lowest for this class of product. It features gain flatness of 0.1 dB to 450 MHz. Total harmonic distortion (THD) is -90 dBc at 20 MHz and -74 dBc at 70 MHz,, while noise is just 1.1 nV/rt-Hz. Translated into application terms, the amplifier can drive 14-bit ADCs up to 70 MHz. The 8-lead device, in SOIC or LLP package, is $3.95 in 1000-piece orders.

National's LMH6515 digitally controlled VGA is designed for maximizing dynamic range in IF sampling signal chains. Third-order intercept point (OIP3) is 40 dBm at 70 MHz,, and noise figure is 8 dB. The 5V device ($5.25), in a 4×4 mm 16-lead LLP package, dissipates 100 mA. Maximum gain is 26 dB over a 31-dB gain adjustment range, with 600 MHz bandwidth (3 dB down). The user can set the gain in precise 1-db steps, with 0.05 dB accuracy.

Optimized for dc-coupled, 8-bit data acquisition signal paths with input frequencies up to 750 MHz, the LMH6555 differential driver has 53 dB spur free dynamic range (SFDR) at 750 MHz. It does not require any external circuitry for output clamping, common-mode control, or gain. In operation, it transforms single-ended signals into differential ones, with fixed gain of 13.7 dB and 1.2 GHz bandwidth. Gain flatness is 0.5 dB to 330 MHz, and noise figure is 15 dB. Second harmonic distortion is -53 dBc and third harmonic distortion is -54 dBc at 750 MHz. The 16-lead LLP device can drive 100-ohm loads, and is $5.95.

All three ICs are supported by complete reference designs from National. The ADC14DS105KARB is a low-IF receiver board featuring the LMH6552 differential driver and dual ADC, for evaluation of a quadrature direct-conversion or near-zero IF receiver for dc to 40 MHz. The ADC14V155KDRB high-IR receiver board uses the LMH6515 VGA and an ADC for evaluation of high-dynamic-range IF-sampling designs. The ADC083000RB is an 8-bit, 3 Gsps data acquisition design using the LMH6555 differential driver and an ADC, designed to be used in conjunction with National's WaveVision data-analysis software.–Bill Schweber

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