clamper

A red LED. I’ve measured some which leaked less than 50 fA at 20 C, from

-5 to +0.5 V.

I used them as switches for a pyroelectric array, with bias current supplied by illuminating them faintly under processor control.

They were cheap CML parts in early transfer- molded board (TMB) SMT packages, and are now discontinued.

Cheers

Phil Hobbs

That's an idea. Maybe use the two LEDs inside a dual optocoupler. They are always in the dark.

Use two diodes in series, with the junction between the diodes actively driven by an op-amp follower to be the same voltage as the low-leakage node. The op-amp can be a low bias current type, like LMC662 etc.

TL431, or active opamp clamp

Maybe I'll just use two isolated switches and let the user select 2w or 4w measurement mode. One ACPL227 should work, and I can use that somewhere else too.

You didn't tell me that *both* ends of your clamp needed to have low leakage.

Something like this? Opamps work well as voltage clamps. But your input voltage must suit for the amp.

Version 4 SHEET 1 880 680 WIRE 16 -64 -80 -64 WIRE 208 -64 16 -64 WIRE 208 48 208 -64 WIRE 272 48 208 48 WIRE 272 64 272 48 WIRE 624 64 272 64 WIRE 368 128 96 128 WIRE 528 128 368 128 WIRE 368 144 368 128 WIRE 96 176 96 128 WIRE -288 192 -288 160 WIRE -192 192 -288 192 WIRE -80 192 -80 -64 WIRE -80 192 -112 192 WIRE 16 192 16 -64 WIRE 64 192 16 192 WIRE 208 208 128 208 WIRE 272 208 272 64 WIRE 320 208 272 208 WIRE -288 224 -288 192 WIRE 64 224 32 224 WIRE 32 288 32 224 WIRE 32 288 0 288 WIRE 320 304 320 288 WIRE 320 304 240 304 WIRE 32 320 32 288 WIRE 320 336 320 304 WIRE -288 352 -288 304 WIRE 240 352 240 304 WIRE 240 352 64 352 WIRE 240 368 240 352 WIRE 0 384 0 288 WIRE 624 416 624 64 WIRE 624 416 448 416 WIRE 672 416 624 416 WIRE 528 448 528 128 WIRE 96 464 96 240 WIRE 240 464 240 448 WIRE 240 464 96 464 WIRE 448 464 448 416 WIRE 496 464 448 464 WIRE 592 480 560 480 WIRE 672 480 672 416 WIRE 672 480 656 480 WIRE 736 480 672 480 WIRE 496 496 448 496 WIRE 0 528 0 464 WIRE 64 528 64 352 WIRE 64 528 0 528 WIRE 96 528 96 464 WIRE 240 528 240 464 WIRE 528 528 528 512 WIRE 528 528 240 528 WIRE 448 544 448 496 WIRE 736 624 736 560 WIRE 736 624 448 624 WIRE 448 656 448 624 FLAG 368 224 0 FLAG -288 352 0 FLAG 320 336 0 FLAG 448 656 0 SYMBOL voltage 368 128 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL Opamps\\LM324-national 96 144 R0 WINDOW 3 -36 131 Left 2 SYMATTR InstName U1 SYMBOL voltage -288 208 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value SINE(0 8 100) SYMBOL voltage 240 352 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 15 SYMBOL diode 272 192 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D2 SYMBOL res -96 176 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL voltage 0 368 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V4 SYMATTR Value 3.3 SYMBOL Opamps\\LM324-national 528 416 R0 WINDOW 3 -36 131 Left 2 SYMATTR InstName U2 SYMBOL diode 592 496 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D1 SYMBOL voltage 448 640 R180 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V5 SYMATTR Value 0 SYMBOL res 720 464 R0 SYMATTR InstName R2 SYMATTR Value 100K TEXT -168 504 Left 2 !.tran 100m

I did explain the function, automatic 2-wire or 4-wire measurements.