Measuring electrical phenomena presents a number of challenges to biomedical signal measurement systems. The tiny, microVolt-level electrical pulses that signal a firing neuron or a muscle response, are often obscured by high-amplitude noise or accompanied by significant DC potentials. Quite often, the signal of interest is a small transient pulse that either occurs intermittently, or only once. In some applications, minute chemical and catalytic changes occur over a matter of several minutes or even hours, making it critical that important experimental events can be captured in a single acquisition.
A useful tool for making these low-level stimulus/response measurements in biomedical research environments is a digital phosphor oscilloscope (DPO) combined with a differential amplifier to improve signal quality. This set-up allows engineers and researchers to pick up extremely small amplitude signals, such as neurons firing during an experiment or a muscle fiber response to external stimulus. It can also be used to capture the fine nuances of electrophysiological signals in the presence of much larger, common-mode noise signals. Results of experiments can be easily captured and stored for generating reports.
Recording low-level lignals in the presence of noise
Noise in the biomedical environment can be divided into two categories: noise inherent in the signal and noise caused by the external environment. Inherently noisy response signals are usually caused by a noisy stimulus signal, or some other source of noise within the test and measurement apparatus itself. External noise can be generated outside the test and measurement equipment by sources such as florescent lights, stray electric or magnetic fields, and poor shielding or grounding. Selective filtering is required to eliminate or significantly reduce noise and enable accurate capture of microVolt signals.
A good starting point is to use averaging to increase the oscilloscope’s effective vertical resolution. Real-time digital filters, when applied to the oscilloscope’s digitizer output prior to writing the acquired signal to memory, reduce high-frequency noise on lower-frequency signals. For medical applications, a boxcar averaging technique that calculates and displays the average of all sequential values in each sample interval is ideal since it can be used for both single-shot or non-repetitive events.
Touching a finger to an oscilloscope probe, for example, will result in a large 50 or 60 Hz signal on the oscilloscope’s display. This is common-mode noise that a body, acting as an antenna, picks up from the environment. Some of these common-mode signals can be eliminated by turning off fluorescent lights and surrounding the lab with a grounded electrical mesh. Even with these precautions, however, some common mode noise will still be present. This remaining common-mode noise is chiefly due to the inability to ground the biological specimen adequately. The solution is a high performance differential amplifier.
A differential amplifier can amplify very small signal differences, while attenuating or decreasing common-mode noise. The ability of a differential amplifier to reject noise is called common-mode rejection ratio (CMRR). Higher-end differential amplifiers offer a CMRR of 100,000:1, allowing capture of small signals in the microvolt range (5-10 µV) when high-amplitude common-mode noise is present.
Differential amplifiers as shown in Figure 1 have two inputs, both of which are designed to be connected to the specimen. A ground electrode is sometimes connected to the specimen to reference it to the measurement system. When the two differential inputs are connected to the specimen and the impedance at the two connections are reasonably well matched to the amplifier’s impedance, the amplifier “sees” only the true difference signal.
The effectiveness of differential amplifiers is illustrated in Figures 2 and 3. Figure 2 shows unfiltered monitor signals. The signal on channel one contains both a simulated cardiac signal, similar to what would be seen on an ordinary heart monitor, as well as a large 60 Hz sinusoidal noise trace. The signal on channel two contains the same 60 Hz sine wave but without the cardiac signal. Because the sine wave noise is so large it’s difficult to view the anomalies in the “at-rest” area following the main beat. After connecting the differential amplifier to the composite signal the large common-mode signal is removed between the two inputs and the resulting differential is displayed in Figure 3.
If the specimen interface creates a different impedance between the electrode pairs the measured signal will not truly represent the signal at the specimen interface. Also, the voltage dividers will be different, causing the CMRR to degrade according to the following formula:
CMRR = |(R3 or R4)/(R1-R2)| = 1 MΩ / (2 kΩ- 0.5 kΩ) = 666:1
When the CMRR is degraded like this, the displayed common-mode noise is much greater. If the CMRR of the differential amplifier is degraded to 666:1, the amplitude of the common-mode noise will occupy the equivalent of 15 vertical divisions on the oscilloscope’s display (which extends beyond the top and bottom of the screen). Even with the high gain for the differential signal, the 15 division display of the common-mode noise will make the two division response signal unreadable.
The solution to the problem of degraded CMRR is to raise the input impedance of the differential amplifier. This is accomplished by disconnecting the internal resistors in the differential amplifier, thus presenting an essentially infinite impedance to the source.
Eliminating noise at the source
Eliminating noise sources such as fluorescent lighting or using grounded mesh around the test setup are good first steps. Here are a few more suggestions.
Leakage of currents to ground through the specimen can be avoided by connecting stimulus pulse generators through stimulus isolators. If a grounded stimulator is used, the grounded electrode should always be placed between the signal electrode and the measurement electrodes.
When making stimulus-response measurements on an excised nerve of a biological specimen, a grounded electrode should be placed across the nerve between the stimulus isolator and the recording electrodes to effectively bypass surface currents to ground.
Very often multiple pieces of line operated equipment are used for biomedical experiments. How this equipment is connected together can greatly affect noise levels. Make sure safety grounds are solid and all equipment is connected to the same ground bus.
Route “single-ended” signal leads away from electrical sources, and twist paired differential leads together to cancel out induced currents.
Place differential amplifier circuitry at the probe end where it is as close as possible to the specimen being tested. Never ground both ends of signal leads as this immediately sets up a ground loop.
By paying careful attention to the grounding of the equipment, isolation of the signal generators, and shielding of the probes and leads, it is possible to obtain very refined biomedical measurements using an oscilloscope and a differential amplifier. Such a test system delivers precise signal conditioning, outstanding acquisition confidence, comprehensive on-board signal processing analysis, and accurate results storage and report generation capabilities.