I'm guessing your friend was probably successful with his OPA1642 guard ring using an SOIC package?... I would imagine the VSSOP version is likely to be a step too far for a guard ring! :)?
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I found a simple self biased FET circuit very useful when it came to using one as an infinite impedance detector for my RF experiments... As the transistor is used as a switch in that configuration, many of the standard 'desireable' FET features preferred for linear use become somewhat redundant!
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It was Jules Ryckebusch's 'OPA Alice' project that pointed me towards the OPA164* op-amp series for microphone impedance converter use.
It was Jules' own comment that, for most purposes, the noise levels were 'down in the weeds' with those devices, and that's certainly what I've found in practice.
There will of course always be special requirements where absolute minimum noise? is required, and FETs are still hard to beat in those cases, but modern op-amps seem to be quite usable for many hobby applications - and will presuambly continue to improve further?
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On Mon, May 5, 2025 at 07:10 AM, @jp8 wrote:
For hobbyist, guard rings might be difficult to apply correctly and implement on a PC board. But there are plenty of good examples on the internet of how to implement guardrings in combination with SMT ICs. My friend successfully implemented it on an OPA1642 design.
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Indeed, OPAs can be tempting to use, to avoid JFET bias issues and JFET selection. But self-biasing JFET circuits exist, even very simple ones, which work very well with cheap JFETs, such as the 2SK209 series, where the supplier pre-binning (Y, O, GR etc.) is sufficiently accurate.
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Jan
