Johan Many thanks - I'll make sure that the source impedance is as low as possible!
regards Grant G8UBN > Hi Grant, > > Grant Hodgson wrote: > > The Janus board in the HPSDR project uses LT1128s, which are > > single-eneded low noise op-amps. > > [...] > > or the even lower noise LT1115s > > Both LT1128 and LT1115 are excellent op-amps. Maybe even the best > available! I > am using them both as low noise preamplifiers with loop antennas for > reception > of "natural" ELF/VLF signals. So far I cannot tell the difference > between LT1028 > and LT1115, they are both *very* quiet compared to common op-amps > like TL071 etc. > > However, beeing bipolar chips, with unusually high bias current in > the input > differential stages, LT1028/LT1115 generate a considerable amount of > noise > current that flows OUT from the INPUT pins, about 1pA/sqrt(Hz) > according to the > data sheet. This noise current creates a corresponding noise voltage > across the > internal impedance of the source and there is a "break-even" around > 1kOhm source > resistance where En equals In - the noise generated by the op-amp > across the > source impedance equals the "internal" op-amp "voltage noise". With > s high > impedance signal source, the LT1028 is *extremely* noisy!! > > 72 > Johan SM6LKM
