In low-noise analog circuits, high-gain amplifiers are used at the input to improve the signal-to-noise ratio (SNR). The input signal level determines the input stage gain; low-level signals require the highest gain. It is also standard practice in low-noise analog signal processing to make the circuit bandwidth as narrow as possible, allowing only the useful input signal spectrum to pass. The optimal combination of amplifier gain and bandwidth is the goal of low-noise design. In data acquisition systems, digital control of gain and bandwidth has dynamic adjustment capabilities to adapt to changes in input signal level and spectrum. Figure 1 shows a simplified circuit of an AC amplifier that can control both gain and bandwidth. The input of the amplifier is a PGA (programmable gain amplifier) with gain control (Gain A). After the input PGA, there is a first-order high-pass filter composed of capacitor C1 and input resistor R1 of the integrator circuit. In the feedback path of the integrator, the gain of the second PGA (Gain B) doubles the -3dB frequency of the integrator, thereby providing bandwidth control.
Figure 1. This AC amplifier configuration has both gain control and bandwidth control.
Figure 2, This circuit is a specific embodiment of the circuit shown in Figure 1, powered by a dual power supply.
Figure 2 shows a complete circuit implementation using two PGAs digitally controlled by the LTC6910-1 and an LT1884 dual op amp. The input LTC6910-1, IC1, features digital gain control from 1 to 100, using a 3-bit digital input to select gains of 1, 2, 5, 10, 20, 50, and 100. The low-end -3dB frequency of the circuit is fixed and set to 1 Hz. The second LTC6910-1, IC3, is inside an integrator loop based on the LT1884 (IC2). The digital gain control of the integrator becomes a digital bandwidth control, which controls the high-end -3dB frequency from 10Hz to 1kHz. The circuit's low-noise LT1884 op amp and LTC6910-1 combine to provide a high signal-to-noise ratio (9 nV/Ω for each device). For example, a 10-mV peak-to-peak signal with a gain of 100 and a bandwidth of 100Hz has a 76-dB signal-to-noise ratio; a 100-mV peak-to-peak signal with a gain of 10 and a bandwidth of 1kHz has a 64-dB signal-to-noise ratio. Using an LT1884 dual op amp (gain-bandwidth product of 1MHz), the high-end frequency response of this circuit can be increased to 10kHz by reducing the capacitance of C2. (The low-end -3dB frequency can also be increased by reducing the capacitance of C1.) The circuit shown in Figure 2 is powered by dual ±5.5V supplies. You can convert it to a single 2.7-10V supply by grounding pins 4 of IC1, IC2, and IC3, connecting a 1μF capacitor between pin 2 of IC1 and ground, and connecting pin 2 of IC1 to pin 3 of IC2 and pin 2 of IC3. Figure 3 shows the frequency response of the circuit shown in Figure 2 (with a gain-of-1 input and three digital bandwidth control inputs).
Figure 3. The frequency response of the circuit of Figure 2 shows unity gain and three digital bandwidth control inputs.
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