The high-impedance amplifier (Hi-Z) circuit features the OPA928 in a noninverting buffer configuration designed as a front end for high-impedance sensors. The Hi-Z circuit is located on the upper half of the EVM. The sensitive input traces are guarded from leakage currents using the techniques described in Section 2.1.

The input path consists of CBJ70 triaxial (or triax) connector (J1) and 0-Ω series resistor (R1) connected to the noninverting input of the OPA928 (U1). A triax connector is similar to a BNC connector, but includes a guarded enclosure between the center (signal) conductor and the outer shield. The J1 triax connector has three index lugs and is incompatible with standard BNC connectors.

The high-impedance input traces are surrounded by a copper guard plane driven by the OPA928 internal guard buffer. The guard buffer tracks the potential of the amplifier input as the common-mode input voltage changes. The guard pin of the OPA928 can be connected to the triax cable guard by installing a shunt jumper at JP1. If the guard of the triax connector is not driven by another source, install the jumper at JP1. The amplifier and input traces are located on the bottom of the PCB and are fully enclosed by a grounded RF shield to prevent noise and EMI from coupling into the signal path.

The HI-Z circuit feedback loop is not sensitive to leakage currents or EMI, and has been extended out of the shielded enclosure on the top layer for configurability. If done carefully, the feedback circuitry (Rg, R3, C4) can be soldered and desoldered without requiring a rigorous cleaning protocol (see Section 1.1). The long feedback traces add some parasitics to the loop; therefore, a small feedback capacitor (C3) is installed inside the shielded enclosure to maintain stability.