The phase-sensitive or lock-in amplifier is capable of extracting excessively low signals in the presence of relatively high noise. In the past several years there has been an increased interest in portable or embedded lock-in amplifiers for instrumentation and sensing purposes. The fundamental approach of a lock-in amplifier is to make the physical quantity to be measured periodic, shifting the DC signal in this way to a known frequency and thus avoiding a high level of low-frequency flicker noise. In this project we will present the possibility of ultra-low power, low voltage (single supply), lock-in amplifier for portable or embedded applications circuitry design based only on the SLG88104 Rail to Rail I/O 375 nA Quad OpAmp and passive components.

Below we described steps needed to understand how to program the amplifier. However, if you just want to get the circuit design, download GreenPAK software available for free and view the already completed project here: GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the device.

1. The Lock-In Amplifier Circuitry

The block schematic of the proposed lock-in amplifier circuitry is presented in Figure 1. The Quadrature Oscillator generates two phase shifted pulsed voltage signals P (in phase) and Q (in quadrature). The in phase signal is also used to power up the Sensor. The signal from the Sensor is amplified by the Differential Preamplifier and then brought to the inputs of Amplifier and Multiplier where it has been amplified (the overall signal processing chain has three relatively low gain amplifiers in order to keep large bandwidth) and multiplied by the in phase and in quadrature signals. After the multiplications, the signals have been added by the Adder in order to eliminate the possible phase shift of the Sensor. After the filtration by the low-pass Filter and amplification by the Amplifier the output signal Vout is proportional to the measured physical quantity.

Figure 1. Block schematics of the proposed lock-in amplifier.

In order to fulfill all these requirements, the corresponding circuitry can be successfully produced based only on the SLG88104 Rail to Rail I/O 375 nA Quad OpAmp and passive components. For the complete circuitry, eight operational amplifiers (two SLG88104 chips) and a relatively low number of passive components (resistors and capacitors) are sufficient.

The corresponding signals of the presented lock-in amplifier block elements are depicted in Figure 2. The first two time diagrams represent the output signals of the Quadrature Oscillator, which generates two phase shifted pulsed voltage signals with a phase shift of 90°. The voltage levels of these two signals are marked with logic levels “0” and “1”, since they serve to power up and down (switch on and off) the corresponding operational amplifiers.

Figure 2. Time diagrams of the corresponding signals of the suggested lock-in amplifier.

The third diagram represents the time dependence of the voltage signals at the outputs of the second stage Amplifiers, which deliver the voltage signals to the Multipliers inputs. These signals have the following value:

where VB = VDD/2 is the common mode voltage (virtual ground) of the circuitry and VDD is the power supply voltage (we need virtual ground as we have single power supply), ΔV is the amplified Sensor signal with respect to the virtual ground, which is given by ΔV = GA1VS(t), where G is the Differential Preamplifier gain, A1 is the second stage Amplifier amplification and VS(t) is the slow varying voltage signal at the Sensor output, τ is the time delay...

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