Figure 8-1 details how to implement a guard in printed circuit board (PCB) layout. This section explores considerations for driving the PCB guard with the OPA928 internal guard buffer, an external guard driver, or by connecting the guard copper directly to the analog ground.

Figure 8-1 Driving the Guard, Internal Guard Buffer

The guard presents a low-impedance path for leakage currents of equal potential to the high-impedance node that is being guarded. For a noninverting configuration, the common-mode changes with the input signal and the guard must be actively driven by a voltage follower that tracks the input signal. Choose a low-offset, low-noise amplifier for the guard driver because any voltage potential between guard and input causes current to leak into the high-impedance trace. The OPA928 features a high-performance internal guard buffer that can be accessed at pin 2 and pin 7 to drive the PCB guard copper; see the Electrical Characteristics. The internal guard buffer tracks the voltage of the OPA928 input signal and is isolated from capacitive loads through a 1-kΩ isolation resistor, R_ISO.

Figure 8-2 shows how the PCB guard is driven with an external guard driver instead of the OPA928 internal guard buffer. To prevent the input bias current of the external guard driver from degrading the input signal, track the low-impedance input of the OPA928. If an external guard driver is used, the OPA928 guard pins can be left unconnected or can be overdriven by the external guard driver. Include an isolation resistor at the output of the guard driver to prevent gain peaking due to capacitive loading and to provide short-circuit protection. Make sure that the guard driver is stable and capable of driving the capacitive load presented by the guard, including long cable lengths, if applicable.

Figure 8-2 Driving the Guard, External Guard Driver

For inverting configurations, the input common-mode voltage is fixed to the analog ground or some dc reference voltage applied to the noninverting input. In this case, tie the PCB guard directly to the ground or low-impedance reference of the signal amplifier. Tying the PCB guard makes sure that the guard potential is always equal to the input common-mode voltage, without the additional offset and noise of an active guard driver. If the PCB guard is connected to the analog ground of the circuit, make sure to ground return paths in the PCB. Keep power and digital grounds separate from the guard and prevent ground loops from occurring. For inverting configurations, the OPA928 internal guard buffer or an external guard driver can be used to drive the PCB guard, and the same considerations apply as discussed for noninverting configurations.