Certain medical and scientific instrumentation applications require amplification and measurement of microvolt-level signals. For example, accurately measuring the output of a thermopile-based micro-calorimeter demands an amplifier that achieves high gain and exhibits excellent thermal stability and low noise.

Figure 1 illustrates how combining two amplifiers yields a programmable-gain amplifier that provides selectable gains of 160 to 10,240. The circuit also offers typical offset voltage of 5 µV, offset drift of 20 nV/°C, and equivalent input-noise voltage of 9 nV at 0.1 Hz. IC₁, a Cirrus Logic CS3301 low-voltage, differential-input, differential-output, chopper-stabilized programmable-gain amplifier, serves as an input-amplifier stage and drives IC₂, a higher voltage INA114 instrumentation-amplifier output stage. The CS3301 provides seven programmable gains of one to 64, and the INA114 provides a fixed gain of 160. The combination achieves gains of 160 to 10,240. A thermopile produces a 1-mV signal, yielding 10.24V output from the INA114. To select other values of gain, change the value of the INA114's gain-setting resistor, R₃.

External DIP switches and pull-up resistors, which connect to the 3.3V supply (not shown), program the CS3301's gain- and multiplexer-control pins. A microcontroller that can drive 3.3V logic can also control these control inputs. Connecting the CS3301's outputs and the INA114's inputs, an RC lowpass filter composed of R₁, R₂, IC₁'s output resistors, and C₁ limits noise above 500 Hz.

Figure 2 illustrates the combined amplifiers' measured input-referred noise performance at a gain of 10,000. With its 1/f noise corner at 0.08 Hz, the amplifier cascade achieves an equivalent input-noise voltage of about 9 nV at 0.1 Hz. The noise-versus-frequency plot represents the results of FFT processing of more than 2 million output samples over an 18-hour period. For simplicity, the schematic doesn't show power supplies and bypass capacitors. Due to the circuit's extreme amplification factor, use construction techniques that maintain thermally balanced component placement and electrically balanced pc-trace lengths.