LFLNA-80

[ZhuraYuk]

Currently have a LFLNA-80 on loan and did some measurements (see attachments).
Performance and handling are excellent — very solid and well-engineered piece of test equipment.

Full schematic attached with help from this thread and following sources:

There are mistakes on schematic definitely, in schematic R210 is 1k but on PCB I clearly see that R210 is 810 - 81Ohms. You need to double check it.

[MiDi]

There are mistakes on schematic definitely, in schematic R210 is 1k but on PCB I clearly see that R210 is 810 - 81Ohms. You need to double check it.

Nice catch, PCB has 81R for each output summing resistor (R202, R206, R210, R214, R218), 1k was copied from p.19 User Manual

[Kleinstein]

The values for the averaging resistors is non critical. The offset between the channels should be relatively small (more an issue with resistor tolerances and a bit larger input signal). The noise is not critical as it is already after most of the gain. The slightly more critical ones are at the input protection: there it is a balance between protection and resistor noise.

[1audio]

In a similar application (audio analyzers) input protection vs noise is a major issue. The usual solution is an incandescent light bulb in series instead of the current limiting resistor. Low resistance when dark and resistance increases when current is demanded. In this case adding some circuitry to detect high voltages and short the output of the coupling cap to ground while the cap charges would be useful.

[Kleinstein]

It could be convenient to have 2 settings for the protection resistor. A relatively low one (could be a light bulb) for low noise and much igher resistor to connecto to a new source. This would not just protect the amplifier, but also the DUT from too much of a current spike. LTZ1000 references without extra protection tend to not like the current spike from charging a larger capacitor. A drop at the output tends to cause a spike in the heater power and thus can take some time to recover.

If really needed one can use a pair of depletion FETs for a relatively low resistance current limit. E.g. an SK2145 (cheap, small dual JFET) would give some 120 ohms and 5 mA current limit (the values depend on the threshold).

[iMo]

..like this..