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This page describes a low-noise pre-amp for one wire.
Design
Original Design using SST4117
- Preamp Schematic – SCH • PDF
- Preamp Layout – PCB
- <em>Required ECO</em> to fix pre-amp power supply pinout – PDF
Summing amplifier can be built from a standard PCB as follows:
Install connectors, plus only:
C2, C8, C10, C12, C13
U4
R2, R4, R13, R15, R16
Install a wire from J1-1 to U4-2 (solder to pad where R12 goes?)
Revised Design – SST4117 replaced with LMP7721
Miniaturized low power design
- Preamp Schematic – SCH • PDF
- Preamp Layout – x8 PCB
- Preamp BOM – PDF • CSV
Miniaturized summing amp
- Summing Amp Schematic – SCH • PDF
- Summing Amp Layout – PCB
- Summing Amp BOM – PDF • CSV
Parts Info
The biggest problem is the JFET. The 2n4117 (SST4117 SMT version) is a unique low input current part designed for electrometer applications which is going obsolete. Vishay no longer makes the SMT version; the thru-hole parts are in stock here and there, but have disappeared from Vishays website.
Calogic seems to be an alternative supplier, and still lists the SST4117, though it is not in stock anywhere.
We could modify the layout to use the thru-hole part, or see if we have any SST4117 from DRIFT. Possibly we could substitute 4118 or 4119, though those arent much more available.
Another interesting alternative is an op-amp like the LMP7721 or LMC6001.
