Measuring Small Signals

reduces noise

environmental noise can be ignored when measuring signals above several hundred millivolts, and cannot be ignored when measuring signals below several tens of millivolts. As a result, to reduce noise picked up by the measurement system, it is necessary to minimize the ground loop and keep the ground wire as short as possible. In extreme cases, power line filters and shielding chambers may be necessary to perform noiseless measurements for signals with very low amplitude

However, before entering extreme conditions, signal averaging should be considered as a simple and economical solution to noise problems. If the signal being measured is repeated and the noise to be eliminated is random, the signal average can effectively improve the SNR of the acquired signal( signal-to-noise ratio). Figure 1 illustrates one example

Figure 1 Figure 1.a and Figure 1.b By averaging the signal (b) Noisy signals can be cleaned (a).

signal averaging is the standard feature of most digital memory oscilloscopes (DSOs) standard. It works by summing the repeated waveforms acquired multiple times, then calculating the average waveform from these acquisitions. Since the long-term average of random noise is zero, the signal averaging process reduces random noise on the repeated signals. The degree of improvement is indicated by SNR . Ideally, the signal averaging function averages power every two times so that SNR improves by 3 dB. Therefore, on average, two waveform acquisitions (21) can make SNR improve by 3 dB, average quadruple waveform acquisition (22) can make SNR improve 6 dB, average eight times (23) can make SNR improve 9 dB, and so on

Improves measurement sensitivity

The measurement sensitivity of the oscilloscope is a function of its input circuit. The input circuit amplifies or attenuates the input signal so that the signal can be displayed on the oscilloscope screen with a calibrated amplitude. You can adjust the vertical sensitivity setting of the oscilloscope to select the required amplification or attenuation for the displayed signal, setting the vertical sensitivity to volts per cell (V/div)

To display and measure small signals, the oscilloscope input must have sufficient gain or sensitivity, and at least increase the signal display height by several bars. For example, displaying 20 mV for two blocks of height peak-peak signal, oscilloscope requirements 10 mV/div vertical sensitivity. For a signal displayed over two divisions of 10 mV, a higher sensitivity setting of 5 mV/div is required. Note that setting a lower voltage per division results in higher sensitivity, and vice versa. Measuring small signals requires not only sufficient oscilloscope sensitivity but also adequate probes. Generally, these are not the ordinary probes provided as standard accessories with most oscilloscopes. Standard probes that come with oscilloscopes are usually 10X probes, which reduce the oscilloscope's sensitivity by a factor of 10. In other words, when using a 10X probe, a 5 mV/div setting on the oscilloscope becomes 50 mV/div. As a result, to maintain maximum signal measurement sensitivity on the oscilloscope, non-attenuated 1X probes must be used. However, note that 1X passive probes have lower bandwidth, lower input impedance, and generally higher probe capacitance. Therefore, extra attention should be paid to the bandwidth limitations of the measured small signals and the possible signal source loading caused by the probe. If either indicates a problem, a better approach is to use 1X active probes, which provide much higher bandwidth and much lower loading. When the small signal amplitude is below the oscilloscope’s sensitivity range, some form of preamplification technique must be used. Because very small signals are easily affected by noise, differential preamplification is typically employed. Differential preamplification provides some degree of noise immunity through common-mode rejection and can also amplify small signals to bring them within the oscilloscope’s sensitivity range. When using differential preamplifiers designed for oscilloscopes, sensitivity levels of 10 µV/div can be achieved. These specially designed preamplifiers have features that allow usable oscilloscope measurements on signals as small as 5 µV, even in high-noise environments. However, remember that taking full advantage of a differential amplifier requires a matched set of high-quality passive probes. Failing to use matched probes will compromise the common-mode noise rejection function of the differential preamplifier. Additionally, when single-ended measurements rather than differential measurements are needed, the negative probe can be connected to the test circuit ground. This is essentially a differential measurement between the signal line and the signal ground. But doing so will lose the common-mode noise rejection capability because the signal line and ground do not share the same noise. Finally, always follow the manufacturer’s recommended procedures for connecting and using all probes and probe amplifiers. Pay special attention to potential overvoltage issues that could damage voltage-sensitive probe components, particularly when using active probes. Reprinted from Tektronix.