The gain of an operational amplifier is usually set using two external resistors. If you wish to have adjustable gain, you can use a digitally controlled multiplexer to select several different gain-setting resistors. Such an arrangement using several ICs can now be replaced by the Linear Technology LTC 6910 single amplifier or LTC 6911 dual amplifier. These ICs incorporate all of the gain-setting components and can be programmed to eight different gain settings using three digital control inputs.
The amplifier is always configured in the inverting mode and features rail-to-rail input and output. The input and output can be driven to within a few tens of millivolts of the supply voltages. At a gain of 100, the bandwidth still extends to approximately 100 kHz. With a unipolar supply, the supply voltage for the LTC 6910/6911 can range from+2.7 V to +10.5 V. With a bipolar supply, the IC can be operated at ±1.4 V to ±5.25 V. There are several different versions of the IC, which are identified by the suffix -1, -2 or -3. The gains for the various combinations of the digital control signals are shown in the table. It should be noted that due to the internal arrangement of the resistors, the input resistance of the amplifier can range from 1 kΩ to 10 kΩ, depending on the gain setting. This means that a low-impedance signal source must be used to avoid affecting the configured gain setting. The AGND pin (pin 2) is the non-inverting input of the internal opamp. It is connected to an internal voltage divider consisting of two 5-kΩ resistors between V+ und V–. When a single supply voltage is used, a capacitor with a value of at least 1 μF must be connected to this pin (Figure 1). With a bipolar supply, AGND can be connected directly to signal ground (Figure 2). Note also that with a unipolar supply, a coupling capacitor is required at the input, and possibly also at the output, since the input and output are internally biased to half the supply voltage. These coupling capacitors will determine the lower corner frequency of the amplifier.
