9.3 Typical Application - Switched Gain Amplifier

Switches and multiplexers are commonly used in the feedback path of amplifier circuits to provide configurable

gain control. By using various resistor values on the switch path, the TMUX110x allows the system to have

multiple gain settings. An external resistor ensures the amplifier is not operating in an open loop configuration.

A transimpedance amplifier (TIA) for photodiode inputs is a common circuit that requires gain control using a

switch to convert the output current of the photodiode into a voltage for the MCU or processor. The amount

of light present during a photodiode measurement is dependent on the time of day and available light source.

An external switch such as the TMUX110x can be utilized to increase the gain when a smaller photodiode

current is present. The leakage current, capacitance, and charge injection performance of the TMUX110x are

key specifications to evaluate when selecting a device for gain control. An example switched gain amplifier

circuit is shown in Figure 9-4.

+

-

RF_2

SEL

Processor

VDD

VI/O

1.8V Logic I/O

VDD

0.1µF

Gain / Filter

Network

VDD

Digital Processing

ADC

OP

AMP

VDD

RF_1

Figure 9-4. Configurable Gain Setting of a TIA Circuit

9.3.1 Design Requirements

For this design example, use the parameters listed in Table 9-1.

Table 9-1. Design Parameters

PARAMETERS

VALUES

Supply (VDD)

3.3V

Input / Output signal range

0µA to 10µA

Control logic thresholds

1.8V compatible

TMUX1101, TMUX1102

SCDS410D – MARCH 2019 – REVISED FEBRUARY 2024

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