The transimpedance amplifier (TIA) circuit is located on the lower half of the OPA928EVM. This configuration functions as a front end to convert femtoampere-level input currents into voltage outputs. The input path consists of BNC connector (J5) and 0-Ω series resistor (R4) connected to the inverting input of the OPA928 (U2). The inverting input can be accessed directly through pin sockets P1 and P2.

The noninverting input of the TIA is tied directly to the analog ground of the TIA circuit. The guard copper surrounding the TIA is grounded as well, which provides very good leakage performance because the offset voltage of a guard driver is not present between the input and guard traces. When using grounded guard traces in a PCB layout, make sure to keep power and digital grounds separate from the guard and prevent ground loops from occurring.

The default feedback path for the TIA consists of a 10-GΩ surface mount resistor (RF) in a resistive transimpedance configuration. The feedback loop can be configured through the pin sockets, or by installing and removing jumpers across certain jumper blocks. Section 4.2.1 describes how to configure the feedback path and other functions of the TIA circuit.

The sensitive input traces and feedback components of the TIA are guarded from leakage currents using the techniques described in Section 2.1. The amplifier, input traces, and SMD feedback components are placed 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. For configurability, sensitive input and feedback nodes are also exposed on the top layer at P1, P2, JP2a, and JP3a. The top-side shield encloses these nodes and is sized to accommodate large through-hole components inserted between the TIA pin sockets. If using through-hole components enclosed within the top-side shield, make sure that the component and leads do not come into contact with the shield.

The TIA circuit features two additional functions described in Section 4.2.2. The first function is a low-leakage T-switch that discharges the feedback path to reset measurements in the integrator configuration. The second function uses the OPA928 internal guard buffer in a guarded diode limiter to protect the TIA from overcurrent events. For a detailed description and configuration of these functions, see Section 4.2.2.