To avoid pull-down resistor’s value selection trade-off mentioned in Section 3.1, a switch can be added in series with the resistor, as is depicted in Figure 3-7.

Figure 3-7 Bleeder Resistor With a Switch

The switch SW1 can be controlled with the channel selection signal of demultiplexer:

• When this channel is OFF, SW1 is turned ON to connect the pull-down resistor R1 into the circuit. Then the output voltage can be fixed at ground with R1.
• When this channel is ON, SW1 is turned OFF to block the path so that R1 is not connected to the circuit. Because the input resistance of op amp is very large, the input voltage signal can be conveyed to op amp with little loss.

Figure 3-8 Control Scheme of SW1

Refer to application brief How to Prevent Errors in Voltage Readings When Using Multiplexers with High Input Impedance Op-Amps for more information.