EEEN30180 Bioinstrumentation Lab

EEEN30180 Bioinstrumentation Lab

Signal Conditioning Circuits 3: Differential Amplifiers

Introduction In this lab you will design and build the front-end differential amplifier of your electrocardiogram (ECG) instrument, which once tested today can be connected to the input of the band-pass filtering stage you previously designed and built, to enable real ECG recordings in the final lab. The main purpose of a differential amplifier is to provide an output vo proportional to the difference between two input signals v1 and v2. Ideally the amplifier should produce zero output for any component of the input that is common to both v1 and v2. This is known as common-mode rejection. Today’s goal is to calculate a metric of how well the amplifier achieves this, known as the common-mode rejection ratio, given by:

where Gc = vo/vc is the common-mode gain that you get when the two input signals are identical vc=v1=v2; and Gd = vo/(v2-v1) is the differential gain, which measures the amplification of the difference in two separate inputs. This property is hugely important in biomedical applications because most electrophysiological signals generated by the body are tiny with respect to the massive interference and noise signals in a typical recording environment. By placing a pair of electrodes such that they tend to pick up opposite ends of the dipolar electric field generated by the physiological event of interest (e.g., on the left leg with respect to right arm when picking up the activity of the heart, whose apex points slightly to the left), but tend to pick up sources of interference and noise equally (e.g., electromagnetic communications signals), the interference is effectively ‘cancelled-out’ by subtraction.

Procedure Design an instrumentation amplifier like that pictured in Figure 1 by choosing appropriate resistor values R1, R2, R1’, R2’, R3, and R4, such that the overall differential gain is as close as possible to 25. Choose values in the 10s of k range, and divide the overall gain approximately evenly between the two stages (i.e. 5 and 5). (Q1) note the chosen values and give a rationale. Build the circuit on your breadboard, if possible, in a different part than your existing filter circuit so that you can connect them in cascade easily later. As in the last lab, power the op amps with ±Vcc = ±15 V from the DC supplies in the bench. All 3 op amps can be supplied in parallel by the same two DC supplies. Don’t forget to measure and note actual values of resistors. Since we need two input signals in this lab, we will use not just one but BOTH waveform generator outputs of the Analog Discovery. Channel 2 of the waveform generator corresponds to the yellow-white lead on the bottom row (see Lab 1, Fig 2). In the waveform generator software, enable and view BOTH signal generator channels and set the Mode to “Synchronized.” This is a very useful feature that locks the phase of both channels to be completely in sync (i.e., they peak at the same time and trough at the same time). We can then just concern ourselves with differences in amplitude only. Make sure that both waveform generator signals are referenced to the same ground. Set the frequency to 10 Hz (nestled within ECG range) for all measurements. By setting the two waveform generator channel amplitudes to the values indicated on each row, fill out Table 1 below. From these data, compute the gains Gd and Gc. To do this, make two plots: one plotting output amplitude against the difference in input amplitudes in discrete points, and another plotting output amplitude against the absolute input amplitude for the data points with v1=v2. By fitting a line to the data of each plot in MATLAB, and hence estimating the slopes, (Q2) estimate Gd and Gc. (Q3) From these calculations, compute CMRR. (Q4) Is your differential amplifier working well? (Q5) What could you do to improve CMRR?