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East Tester ET4410 ESR Measure

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TimFox:
In the G-R "Digibridges", the analog circuitry was separate from the computation circuitry, which allowed higher currents and lower noise voltage.
The G-R/Quadtech 1659 uses an NE5534 op amp with a high-current buffer to drive the "low" terminal with the high current required from a powerful source through the DUT.
This allows measuring high current at virtual ground (0 V).
An earlier version in my inventory of not-yet-repaired units used a CA3130 CMOS op amp with multiple CMOS inverters to buffer the output, if I remember correctly.

Martin72:
ET4410:
The signal output stage to H-Cur got a push-pull stage, the output resistance is switchable from 100Ohms to 30Ohms, maximum output voltage is 2Vrms.
Pic shows the actual state of my attempt to draw the circuit, according to the keysight handbook, it´s a auto-balancing bridge.

mawyatt:
The "low" terminal should be a transimpedance amplifier with a selectable feedback resistor to "force" the low side to ~ 0 volts. At very low impedances the DUT current may reach ~0.1 amp in lab type LCR meters and the transimpedance amp must sink/source this at 0 volts and use as low as 10 ohm feedback resistor. This is likely a buffered Op-Amp type transimpedance amp. The "high" side is likely driven by another buffered op-amp with a selectable output resistance as low as 30 ohm to drive the DUT.

Both the transimpedance amp output and the DUT voltage are sensed with a synchronous technique which imposes a narrow bandpass filter around the injected signal which dramatically improves noise, interference rejection and sensitivity, a tried and true technique employed for well over half a century.

Best,

Martin72:
Yepp,
Not so bad for a cheap one.
Test a panasonic FM electrolyt cap, 1000µF/25V on 100khz a few minutes ago.
ESR 0.034 Ohm
D 0.128
Xc -0.267  Ohm
Q 7.81
Phase - 82.7°
And cap....5.95µF  ;)

TimFox:
Quickly approaching self-resonance at that frequency...

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