When looking at the oscilloscope feature of a multi-instrument device or when comparing two benchtop oscilloscopes, the first specifications that we look at are the bandwidth and sample rate. While these are incredibly important specs, often most design work is done well within the limits of most products and the more critical specification to consider is resolution. Did you know that while most popular oscilloscopes start at 8-bit, high resolution is becoming an increasingly important (and increasingly available) option?
Since you typically must choose between sample rate and resolution as a feature for ADCs and DACs, this often begs the question, why choose to prioritize high resolution on your oscilloscope?
Signal Integrity and Noise Reduction in Oscilloscopes
Signal integrity and noise reduction are incredibly important in sensitive designs, and resolution plays a huge part in ensuring that the signals you see are coming from the circuit at hand, not an artifact of the instrument you are using. This is especially important when acquiring a large signal and looking for smaller component signals that could negatively affect the end use of the design. For example, with an 8-bit resolution, only differences of about 19.53 mV on a 5 V signal can be detected. A 14-bit oscilloscope like the ADP3450 can detect much smaller variations around 300 µV on the same 5 V signal, making it much easier to detect and debug problems with sensitive designs.
In our work designing test and measurement equipment and FPGA development boards, a high resolution can often play a critical role. Here are some examples:
- When validating the power supplies on our FPGA boards, it is critical to look for and be able to see ripple, load transients, and loop frequency response. This is a situation with small signals in a naturally noisy environment where specifications are very tight, and where high resolution is a must.
- On the test & measurement side, part of validation of products with oscilloscope instruments is evaluating crosstalk between the channels and making sure that it is non-existent or negligible so that measurements of signals on each channel do not affect each other. Since resolution limits the noise floor, without high resolution we would not be able to see any tiny crosstalk components when we look at the spectrum view of the signal. To ensure that our instruments don’t introduce noise and distort signals, we have to find and eliminate potential crosstalk problems.
Resolution also plays a part in more complex tests like ensuring accurate digital triggering, crossover detection, and the time stabilization of signals.
When is Sample Rate More Important than Resolution?
While resolution is critical in many applications, there are of course situations where higher bandwidth and sample rate are more important than resolution. In our own designs, this includes looking at digital signal transients and measuring bode plots of wide bandwidth circuits. That being said, there are plenty of cases where both are important and software-enhanced resolution can help, such as evaluating signal spectrum and measuring parameters like THD, SNR, and SFDR. In an ideal world, we would have devices with high resolution, high bandwidths, and high sample rates – while keeping prices low.
Is Software-Enhanced Resolution Comparable?
Given the importance of resolution and the prevalence of software-based instruments such as our own, software-enhanced resolution has become increasingly common, but can it be relied on like an organic hardware resolution? The answer: it depends.
Software-enhanced resolution involves averaging the signals in specific ways to gain extra bits of resolution. This allows for a temporary tradeoff from sample rate, as it can be enabled or disabled in software as the case requires. For example, Digilent’s ADP3450 has 14-bit resolution that can be software enhanced up to 16-bit . However, even though software-enhanced resolution (through filtering or interpolation schemes) can help smooth the signal, giving the impression of more resolution and lower noise, it cannot discriminate between signal and noise, and can mask small signals of interest. So, in cases where resolution is critical, hardware resolution should be prioritized over software enhancement.
What’s the Best Value Scope with Reasonable Bandwidth AND Sample Rates?
For an example of cost-effective high-resolution instruments, the Analog Discovery 2 and newly introduced Analog Discovery Pro line feature high-resolution mixed-signal oscilloscopes with bandwidths and sample rates that fit most applications. Stay tuned for future additions to the family that will continue to increase in speed.
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