900 MHz, Digitally Controlled, Variable Gain Amplifier

The LMH6518 is a digitally controlled variable

gain amplifier with a total gain that varies from –1.16dB to 38.8dB for a 40dB range

in 2dB steps. The −3dB bandwidth is 900MHz at all gains. Gain accuracy at each

setting is typically 0.1dB. When used in conjunction with TI’s gigasamples per

second (GSPS) ADC with adjustable full-scale range, the LMH6518 gain adjustment

accommodates full scale input signals from 6.8mVPP to 920mVPP

to get 700mVPP nominal at the ADC input. The auxiliary output (+OUT AUX

and −OUT AUX) follows the main output and is intended for use in oscilloscope

trigger function circuitry, but can have other uses in other applications.

The LMH6518 gain is programmed through a

SPI-compatible serial bus. A signal path combined gain resolution of 8.5mdB is

achieved when the device gain and the GSPS ADC FS input are both manipulated. Inputs

and outputs are dc-coupled. The outputs are differential with individual common-mode

voltage control (for main and auxiliary outputs), and have selectable bandwidth

limiting circuitry (common to both main and auxiliary) of 20MHz, 100MHz, 200MHz,

350MHz, 650MHz, 750MHz, or full BW.