QThe main difference between AD 604 and AD 605 is a preamplifier at the input
for the first one. This amplifier has very low noise : 0.7 nV/square(Hz). I
would like to find such a good amplifier. Which reference is it?
AThe low noise preamp in the AD604 has not been released as a stand alone
product. The lowest noise amplifier available from analog devices is the AD797
with wide band voltage noise density of 900pV/sqrt(Hz). This is used
extensively in audio applications. For wider bandwidth applications such as
sonar or ultrasound, the AD8004 has 250MHZ bandwidth and 1.5nV/sqrt(Hz).