The switch R_ON resistance will produce DC gain errors and distortion on the transimpedance amplifier. The percent gain error is dependent on the TIA feedback resistor value; and the gain errors are relatively large at the lower feedback resistor values. For example, with the R_F1 = 200kΩ resistor, the maximum switch R_ON resistance of 125Ω will contribute about 0.063% gain error. In the case of the lower R_F2=2kΩ resistor, the gain error due to the switch R_ON resistance is much more significant, about 6.25% gain error. In addition, the switch on resistance changes with temperature producing TIA gain drift errors.

Furthermore, the R_ON switch resistance of each selectable gain varies over the TIA output voltage. Figure 5-7 shows the change of RON resistance vs the voltage difference across the switch.

Figure 5-7 R_ON Resistance Vs Voltage

This ΔR_ON change in switch resistance introduces signal-dependent distortion and gain error linearity based on the switch impedance.

One way to address this issue is to build a multiplexer using the second integrated switch of the OPA3S328. The multiplexer input senses the TIA output connection for the selected gain right at the R_F feedback resistor terminal. The second amplifier stage is configured in a high impedance non-inverting configuration, buffering the multiplexer and providing an accurate Kelvin sense connection of the TIA output for each gain. Since the input bias current of the op-amp is minimal, < 10pA, the voltage drop across the switch R_ON resistance is negligible. Figure 5-8 shows the OPA3S328 circuit using the 3:1 switch B to build the Kelvin sense connection multiplexer, buffered with a second-stage amplifier. The voltage drop across the R_ON resistance of switch SB2 is negligible.

Figure 5-8 Switch Kelvin Sense Connections and High Impedance Buffer

The Kelvin sense connections eliminate the gain error, gain error drift and gain non-linearity due to the switch R_ON resistances though the high impedance second-stage amplifier. The second-stage could also be configured in a non-inverting gain configuration, adding the flexibility for even higher gains while maintaining the circuit bandwidth.