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Making higher accuracy oscilloscope measurements

03 June 2020

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One of the most important functional blocks in a digital oscilloscope is the analogue-to-digital converter, or ADC. This tutorial explains why ADC resolution (number of bits) has become a more important consideration in oscilloscopes, especially for measuring small signal details.

ADC resolution
Many high-precision instruments use high numbers of bits in their ADCs. For example, a precision digital multimeter (DMM) like the Keithley 2002 can make measurements with up to 28 bits. So why has oscilloscope resolution traditionally been lower? Oscilloscopes have traditionally used 8-bit ADCs, because oscilloscope ADC technology has prioritised sample rate. 8-bit ADCs can provide high sample rates and therefore excellent time resolution. Even relatively inexpensive oscilloscopes now sample at GS/s. In contrast, the sampling rates of DMMs are much lower — roughly in the order of tens to hundreds of samples per second.

The 8-bit ADC in an oscilloscope provides a good balance between acquisition speed and amplitude accuracy for signals with frequencies in the hundreds and TTL voltage levels on the order of 0 to 5 V. For this type of signal, 8-bit resolution is usually sufficient.

However, as we move into new technologies, such as IoT devices, mobile devices and autonomous vehicles, that rely on high-speed networks, low interference and low power consumption, engineers are facing new challenges of validating very small amplitude signals with greater accuracy.

This need to address smaller signal details is driving the need to increase the number of ADC bits, while carefully keeping oscilloscope noise low.

Traditional 8-bit ADCs (ignoring oversampling and postprocessing) provide 28 = 256 vertical digitizing levels. This can be too coarse for applications like power supply design, which demands higher vertical resolution over relatively high voltage ranges. In contrast, a 12-bit ADC can deliver up to 212 = 4096 vertical digitizing levels for an extremely large increase in vertical resolution. You can see in the image above zooming in on a switching signal with an 8-bit MDO4000C (left) verses a 12-bit 4 Series MSO (right) oscilloscope.

ASICs at the heart of Tektronix 12-bit oscilloscopes
Tektronix’ new 4 Series, 5 Series and 6 Series MSO oscilloscopes are equipped with 12-bit ADC technology to help you accurately capture tiny signal details. High resolution is delivered by a custom ASIC called the TEK049, and is at the heart of every Tektronix 4, 5 and 6 Series MSO (mixed signal oscilloscope). These scopes support high-definition displays, up to 8 FlexChannel® inputs, 12-bit vertical resolution, and up to 16 bits of resolution with oversampling, all thanks to circuitry integrated into the TEK049. It is a highly-integrated mixed signal system-on-chip (SoC), with 400 million transistors and 2 billion connections, consisting of four ADCs and signal processing. The ADCs can achieve sample rates up to 25 GS/s.  

For a deeper dive into the technology in the new 4, 5, and 6 Series oscilloscopes that combine 12-bit ADCs with low-noise front-ends and innovative DSP, visit our product pages: 4 Series MSO, 5 Series MSO and 6 Series MSO.

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