QWill the switches in the ADA4350 cause gain error and leakage issue?
AThe Kelvin switching technique is used to eliminate voltage and temperature
dependent gain errors induced by switch resistances. The Kelvin
switching technique introduces two switches in each gain selection
loop, one to connect the transimpedance/opamp output to the feedback
network, and the other to connect the feedback network output to the downstream
components. Although this technique introduces twice as many switches,
the TIA output voltage is no longer switch dependent, but only
dependent on the current across the selected resistor. The slide below
shows the design equations, where V2 is the output voltage.
As for switch leakage, it is minimized with the small switch leakage current of
0.5 pA. This means, only a 50 uV offset is produced at the input with a 10MÙ
feedback resistor (assuming all the leakage currents flows towards the input).