CIRCUITS AND DIGITAL ANALYSIS

Circuits and Digital Analysis

Circuits and Digital Analysis

Introduction

In some circuits it is essential to measure very small values of resistances. To measure very low values of resistances (resistances of m ohms) we should develop a circuit which can works under small amount of currents. Here i took micro ohm meter to measure very low values of resistances which based on lock-in amplifier circuits.

This is the over view of the circuit.

Here we use oscillator as excitation source in order to generate 0.3v. The output probes of the oscillator (excitation source) are given to instrumentation amplifier. The differential mode gain and CMMR of the In-Amp are calculated. In order to develop the signal (by mixing or comparing) the output of the in-amp is given to PSD. Here it takes input signal and compare them to give super position difference and sum. Here it (PSD) acts as heart of lock-in amp. In order to overcome the problem of oscillation in frequency the output of PSD is connected to VCO (VOLTAGE CONTROLED OSCILATOR) 555 timer plays a vital role in this circuit as it act as excitation source and voltage controlled oscillator.

The oscillator should operate at a frequency somewhere around 1kHz, such a frequency being high enough to rectify with a short time constant so that the instrument is quick to respond, but not so high that stray capacitance causes difficulties. You will find that quite a few AC instruments such as conduct meters operate around 1 kHz, being a frequency that is easy to generate and measure, and well above the mains (50/60Hz) and 1/f noise range. The Kelvin probes are four-wire connections. These can be made from sharp probes with double wires to each. The oscillator and the VCO (voltage controlled oscillator) may both employ the 555 timer unless you prefer a different circuit such as the op-amp integrator / Schmitt trigger The 555 timer is a 'classic' integrated circuit with a plethora of uses.

Many measurements in experimental physics involve the detection of an electrical quantity, either a voltage or a current. Some physical quantities are intrinsically electrical in nature, for example the voltage drop across a diode, or the emission current in a vacuum tube. Other quantities such as temperature, pressure, displacement, or light level can be converted to electrical quantities by means of transducers (sometimes called sensors). Perversely, the electrical quantity (signal) of interest is accompanied by noise, the latter sometimes orders of magnitude greater than the former. Various techniques exist to recover the signal of interest from the composite of signal + noise, and one technique, phase sensitive (lock-in) detection will be explored in this experiment. (Farnsworth C.1989)

The output will be a tone, because when tracing a circuit an audible tone is easier and quicker to observe that, for instance, a meter reading. Therefore 555 timer connected as VCO to generate the variable frequency with respect to voltage variations. However, for fixed readings of wire resistance etc you might wish to incorporate some sort of calibrated ...