How Oscilloscope Probes Measure Low Amplitude Signals

Measuring low amplitude signals presents a unique set of challenges. The main challenges are noise and adequate measurement sensitivity.

reduce noise

Environmental noise can be ignored when measuring signals above a few hundred millivolts, and can no longer be ignored when measuring signals below a few tens of millivolts. As a result, to reduce the noise picked up by the measurement system, it is necessary to minimize ground loops and keep ground leads as short as possible. In extreme cases, power line filters and shielded rooms may be necessary to make noise-free measurements of very low amplitude signals.

However, before entering extreme conditions, signal averaging should be considered as a simple, cost-effective solution to noise problems. If the signal you are trying to measure is repetitive and the noise you are trying to eliminate is random, signal averaging can effectively improve the SNR (signal-to-noise ratio) of the acquired signal. Figure 6.12 illustrates one such example.

6.12.a and Figure 6.12b. Noisy signals (a) can be cleaned by signal averaging (b)

Signal averaging is a standard feature on most digital storage oscilloscopes (DSOs). It works by summing repeated waveforms from multiple acquisitions and then calculating an average waveform from the multiple acquisitions. Since the long-term average of random noise is zero, the signal averaging process reduces random noise on repeated signals. The degree of improvement is expressed as SNR. In an ideal world, the signal averaging function will improve the SNR by 3dB for every two averaged powers. Therefore, averaging two waveform acquisitions (21) improves SNR by 3dB, averaging four waveform acquisitions (22) improves SNR by 6dB, averaging eight waveform acquisitions (23) improves SNR by 9dB, and so on.

Improve measurement sensitivity

An oscilloscope's measurement sensitivity is a function of its input circuitry. Input circuitry amplifies or attenuates the input signal so that the signal is displayed at a calibrated amplitude on the oscilloscope screen. You can select the amount of amplification or attenuation required to display the signal through the oscilloscope's vertical sensitivity setting, which is adjusted in volts per division (V/div).

To display and measure small signals, the oscilloscope input must have sufficient gain or sensitivity, at least a few divisions higher than the signal display height. For example, to display a 20mV peak-to-peak signal two divisions high, the oscilloscope requires a vertical sensitivity of 10mV/div. The same 10mV signal is displayed for two divisions, and a higher sensitivity setting of 5mV/div is required. Note that a low Volts per Division setting will result in high sensitivity and vice versa.

Measuring small signals requires sufficient probes in addition to sufficient oscilloscope sensitivity. Generally speaking, this is not the ordinary probe that most oscilloscopes offer as a standard accessory. The standard accompanying probe is usually a 10X probe, which reduces the oscilloscope's sensitivity by a factor of 10. In other words, a 5mV/div oscilloscope setting becomes 50mV/div when using a 10X probe. As a result, in order for the oscilloscope to maintain maximum signal measurement sensitivity, it is necessary to use a non-attenuating 1X probe.

However, be aware that 1X passive probes have lower bandwidth, lower input impedance, and generally higher head capacitance. Therefore, you need to pay extra attention to the bandwidth limitations of the small signals being measured and the possible signal source loading caused by the probe. If any of these indicate a problem, a better approach would be to utilize a 1X active probe which offers much higher bandwidth and much lower loading.

In the case of small signal amplitudes below the oscilloscope's sensitivity range, some form of preamplification technology must be used. Since very small signals are easily affected by noise, differential preamplification techniques are often used. Differential preamplification technology achieves noise immunity to a certain extent through common-mode rejection. In addition, it can amplify small signals so that they fall within the sensitivity range of the oscilloscope.

Sensitivity levels in the 10µV/div range can be achieved when using a differential preamplifier designed for oscilloscopes. These specially designed preamplifiers have features that allow usable oscilloscope measurements on signals as small as 5µV, even in noisy environments!

But remember, taking full advantage of a differential amplifier requires a matched set of high-quality passive probes. Failure to use matched probes can compromise the common-mode noise rejection of the differential preamplifier.

Additionally, the negative signal probe can be connected to the test circuit ground in situations where single-ended measurements are required rather than differential measurements. This is essentially a differential measurement between the signal line and signal ground. But in doing so, common-mode noise rejection is lost because the signal lines and ground have no common noise.

Finally, always follow the manufacturer's recommended procedures for connecting and using all probes and probe amplifiers. Especially when using active probes, pay special attention to overvoltage issues that can damage voltage-sensitive probe components.