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Low-noise ac amplifier has digital control of gain and bandw

2015-01-06 15:15  
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Low-noise ac amplifier has digital control of gain and bandwidth
In low-noise analog circuits, a high-gain amplifier serves at the input to increase the 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's bandwidth as narrow as possible to pass only the useful input-signal spectrum. The optimum combination of an amplifier's gain and bandwidth is the goal of a low-noise design. In a data-acquisition system, digital control of gain and bandwidth provides dynamic adjustment to variations in input-signal level and spectrum. Figure 1 shows a simplified circuit for an ac amplifier with control of both gain and bandwidth. The amplifier's input is a PGA (programmable-gain amplifier) providing gain control (Gain A). Following the input PGA is a first-order highpass filter formed with capacitor C1 and input resistor R1 of an integrator circuit. Inside the integrator's feedback path, the gain of a second PGA (Gain B) multiplies the integrator's –3-dB frequency, thus providing bandwidth control.