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.