I need a digital variable gain over at least 20 dB with a resolution of 1 dB
or better for a noise generator that should operate from DC to at least 50 MHz.
There are quite a few VGAs suitable for dc applications that people seem to be
able to drive with one of the many DACs out there. The AD603 is a single
channel device, works between dc and 50MHz. The AD8337 is an excellent choice
as it is a much newer (and smaller) part, works from dc to >300MHz, and has a
preamp. The gain and gain range of the AD8337 are 26dB, a tradeoff to achieve
flat response and good pulse response over a wide operating frequency; the
AD8264 is a quad version of the AD8337. The AD8336 is also an excellent part,
works at higher supply voltages (to 18V) has an 80 dB gain range, and uses a
DAC when operating under processor control.
For just a little background, processing DC usually implies dual power
supplies, as one needs 3 supply ports for bi-polarity. DGAs (digital gain
amplifiers) are typically single supply in order to accommodate the logic
interface, and thus require level translation at both ends of the amplifier
chain if one is to operate on dc signals.