The J111 specs suggest 30ohm with 0V VGS, and leakage is only 1nA when VGS is -10V. I have no experience with JFETs but understand they can be used bidirectionally.
J111..113 and 2N4391..4393 were designed specifically for use as analog switches. They are good enough to be used as switches in 4+ digit double-slope DMMs, as a point of reference.
My question is - how RDS(on) of the JFET would be characterized if the D or S was varied over a range of say +10V to -10V?
It doesn't change at all, since these parts are meant to be turned on by setting the gate voltage to be the source voltage. So the RDS(on) is not absolute-voltage dependent. It varies slightly with the channel current, and around the saturation current the incremental channel resistance goes very high, i.e. you get a current source, not a resistor.
Say the source of the JFET is connected to the output of an op-amp. It is a low impedance-driven node. The other side - drain - is connected to a high impedance load, like say an integrator input.
To turn the switch on, a resistor - say 100kΩ - permanently connects gate to the low-impedance source.
To turn the switch off, a current sink is connected to the gate, using a bipolar transistor as a switch, for example. A current is sunk from the low impedance source-side driver, through the 100kΩ GS resistor, establishing a negative gate-source voltage, i.e. the gate is lower than the source.
Since the gate-channel capacitances are in single pF, the charge injection is minuscule.
As long as the source nodes are driven from a low-impedance source, the JFETs are excellent switches. In that particular application, they outperform low-end integrated MOS switches. Many early DMMs used JFETs as switches, with much success.
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