Limitations of the simple RC-Filter/Op-amp Circuit

Although this simple circuit worked (in most cases!), I think you'll agree that the quality of the ECG was pretty poor. Hopefully, you noticed the following problems:

The Differential Amplifier

In order to improve the signal further, we need a new design for our amplifier:

This new design is called a differential amplifier, because it amplifies the difference between the two input voltages.

The gain is given by                    Gain = R2.(V2-V1)
                                                             R1

Common mode rejection

Since the output is proportional to the difference between the two voltages, anything (e.g. noise) which is present on both inputs will be cancelled out. However, a signal (e.g. the EKG) which is different on the two inputs will be amplified, which of course is exactly what we want. The ratio of the gain of the difference gain to the common gain (usually expressed in dB) is called the Common Mode Rejection Ratio (CMRR).

                                                  CMRR = Differential Gain
                                                                  Common Gain

An typical differential amplifie has a CMRR of about 30,000. So, supposing we build a circuit with a differential gain of 1,000, this means that the common gain (acting on the noise) will be:

                                                 Differential Gain / Common Gain= 30,000

So,                                         Common Gain = Differential Gain / CMRR
                                                                      = 1,000/30,000 = 1/30 or 0.03

In other words, instead of getting amplified, the noise will actually be attentuated 30-fold.

CMRR in dB

Just to be awkward, gains and CMRR are usually quoted in dB, so for voltage gains, the equation becomes:

                                                  CMRR (dB) = 20 log (Differential Voltage Gain / Common Voltage Gain)

Thus, a typical differential amp will have a CMRR of 20 log 30,000 = 90 dB

How about going the other way? Well, the inverse of a log to base 10 is 10 raised to its power, so:

                                                Voltage Gain ratio = 10CMRR/20

e.g. 90 dB = 1090/20 , i.e. a voltage gain ratio of 104.5 or 30,000
 

The Instrumentation Amplifier

Unfortunately, the differential amplifier turns out to be rather limited in its performance because of the low input impedance of (R2 + R1). To improve this, two bootstrapped buffer amplifiers (which are just op-amps with unity gain) are commonly added, which results in the simple instrumentation amplifier:

In practice, it is difficult to precisely match resistors that are discrete components.  To overcome this problem the entire circuit is put on a single integrated circuit, since IC manufacturing technology enables precise resistor ratios to be obtained. Such chips as Analog Devices AD620 find widespread use in working with low-level signals with large common-mode components in noisy environments - just the sort of situation we find in biomedical engineering.

AC coupling

The other problems we had were with DC offset, drift and motion artefact. All of these problems are caused by very low-frequencies (DC is zero frequency), so we need to use a high-pass filter with a very low cutoff frequency. We need to keep the cutoff frequency very low to avoid degrading the ECG signal, so the capacitor needs to be large (0.47 or 1 uF) and the resistor similarly large (e.g. 1 MW).

So, our final circuit is:

The AD620 Instrumentation amp

Luckily, we don't have to build all of this because commercial instrumentation amplifiers are available. The AD620 has a CMRR of 100 dB with a differential gain that is adjustable up to 1,000. This is done by changing the value of a resistor, RG on the input of the chip. Here's the datasheet. By buying a commercial amplifier we also don't need to worry about the offset null because the internbal circuits is perfectly balanced in the factory.

 The REF pin is yet another name for ground.