Back to Back and Ladder Protection Diodes
The drain-current AC voltage, induced by a current through Hcur-Lcur network branch, is measured by a four-terminal-sensing-enabled instrumentation. Anti-parallel diodes D3/D4 (D5/D6) limit voltage swing between Hcur/Hpot (Lcur/Lpot) which may happen in transient regimes and damage your measurement instrumentation. In a normal operation mode of voltage measurement, voltage drop across Hcur/Hpot (bzw. Lcur/Lpot) is small and well below a &quot;knee&quot; voltage of diodes D3/D4; the current terminal Hcur is well isolated from the voltage terminal Hpot of a 4-wire (Kelvin) clip at the high potential probe side; D5 and D6, also almost non-biased, isolate current (Lcur) and voltage (Lpot) terminals of a 4-wire clip at the low potential probe side.
In a 4-wire clip, Hpot is a voltage measurement terminal. The voltage protection network (clamp) is a shunt; when doing precise voltage measurement, the shunt resistance must be held as high as possible. D1(D2) blocks ZD4(ZD3) in the forward direction so that the impedance of the voltage protection network D1/ZD4/ZD3/D2 is high in the enabled voltage region. &quot;Ladder of Diodes&quot; (D7-D12) acts as a low-voltage (~2V) Zener diode.