Low-noise amp fronts for high-speed ADC

DIGITIZATION of wireless base-station intermediate-frequency (IF) signals requires a high-performance analog-to-digital converter (ADC). But digitization also necessitates a low-noise operational amplifier up-front that can match the ADC's wide bandwidth and need to minimize distortion. This ADC is Burr-Brown Corp.'s (Tucson, AZ) ADS852 (see Microwaves & RF: August 1998, p. 158) which is now joined by the company's OPA687 wideband, ultra-low-noise voltage feedback operational amplifier (see figure). The OPA687's internal noise voltage is specified at 0.95 nV/root Hz typical, and with careful calculation of the gain-setting feedback resistors, the amplifier's output noise will not be much greater than that contributed by the input: With a typical gain-bandwidth product of 3800 MHz and 2700 MHz minimum across the 0 to 70 degrees C range (gain of +50), designers can obtain high-signal bandwidths at high gains or very-low-distortion signals at moderate frequencies and lower gains.