WOBBLopamp-cable.cir

Is this buffer stable driving the cable?

Circuit report
2026-10-15 19:24
FAILS

U1 (LM358) does not work as wired: 2 degrees of phase margin at 47133 Hz: it oscillates or rings hard.

hardest part
CCABLE at 16%
supply power
0.00313 W
hottest part
U1 at 25.2 C
analysis
operating point

Confidence high

  • passed KCL, power balance, tolerance convergence, model provenance

Simulation

Bench setup drawn from this run, with 6 parts on a breadboard, 1 instrument and the measured current on every wire. DC POWER SUPPLY 5.00 V 0.001 A CV a a b b c c d d e e f f g g h h i i j j 1 5 10 15 20 25 104 U_LM358 CCABLE VOLTAGE 2.50 V FAILS 16% of its rating
Bench setup, drawn from this run
Parts and nets of An LM358 buffering a 2.5 V reference down a cable (about 1 uF of capacitance) to an ADC, listed from the netlist. SCHEMATIC An LM358 buffering a 2.5 V reference down a cable (about 1 Drawn as a list, since this circuit is not a simple ladder. PARTS NETS V1 v5 0 5 R1 v5 ref 10k R2 ref 0 10k C1 ref 0 100n XU1 ref out v5 0 out U_LM358 CCABLE out 0 1u RADC out 0 100k ground V1, R2, C1, XU1, CCABL out XU1, CCABLE, RADC ref R1, R2, C1, XU1 v5 V1, R1, XU1
Schematic, drawn from the netlist

Checks

statuscheckresult
passKCLcurrents into every node sum to zero within 7.50e-10 A (largest at ref, against 6.25e-04 A flowing)
passpower balancesupplies give 0.00313 W and the parts dissipate 0.00313 W (0.00016% apart)
passtolerance convergenceevery part's power and current agree to 0% with solver tolerances 100 times tighter
passmodel provenancemodels: 7 from the parts library
infoloop stabilityU1: loop gain 99879 at low frequency crosses 1 at 47133 Hz with 2 degrees of phase margin and 5 dB of gain margin; it will ring hard or oscillate

How to fix it

Isolate U1 from its capacitive load with 100 ohm
Put 100 ohm between U1's output and CCABLE, with the feedback taken before it, so the load's capacitance is outside the loop. Solved again, U1 now has phase margin 72.3 deg, 72 degrees of phase margin at 444507 Hz: stable.

Assumptions

  1. Wobbl guessed: air temperature around the board not given, so Wobbl assumed 25 C.
  2. V1: USB port: 5 V (4.75 to 5.25 V), 0.15 ohm inside, 0.5 A most.
  3. U1: LM358 datasheet: 0.7 MHz, 0.3 V/us, swing to 1.5 V of V+ and 0.02 V of V-, 40 mA short circuit, 3 to 32 V supply.
  4. Ambient 25 C; temperatures are ambient plus average power times each part's thermal resistance.