Analog Discovery 2 vs. Other PC-Based Oscilloscopes. Is AD2 Worth the Hype?

Digilent's Analog Discovery 2, which is developed in conjunction with Analog Devices, is a multi-function test and measurement (T&M) tool. It can be configured to work as traditional instruments such as an oscilloscope, spectrum analyzer, network analyzer, power supply, signal generator, and so forth.

The Analog Discovery 2 (AD2) was originally designed to provide engineering students, hobbyists, and electronics enthusiasts with a relatively low-cost multi-function instrument. However, the device is sometimes used even in more serious projects.

For example, a research group from Stanford University recently utilized the AD2 in a project involving the safety of RF devices during magnetic resonance imaging (MRI) as seen here.

This diagram depicts the test set up researchers used to "characterize the TAUS signal level versus temperature in different media." Image used courtesy of Magnetic Resonance in Medicine

They used the arbitrary waveform generator of the AD2 to generate a linear frequency modulated chirp signal of 2.5 ms duration that spans the 250-750 kHz range. The oscilloscope of the AD2 was also utilized to investigate the output of the system.

This device, which provides the capabilities of several benchtop instruments in a $279 pocket-sized device, raises important questions:

• What features of the traditional instruments are ignored to reduce the price of this PC-based equipment?
• How much of the cost reduction is achieved by applying innovative design techniques?
• How much of the cost is reduced by simply lowering the performance of the device?

Answering these questions is not always easy because there are several different specifications for a T&M tool, and not all of these details are given in the datasheets and reference manuals.

In this article, we’ll focus on the AD2 oscilloscope's performance. We’ll highlight its important features and when possible, compare them with other available products.

Scope Spectral Characteristics

The inputs of an oscilloscope are connected to analog blocks such as a gain control circuitry, buffers, ADC drivers, etc. These analog circuits exhibit a low-pass frequency response. The analog bandwidth of the scope is the frequency at which the amplitude of the transfer function is attenuated by 3 dB.

The following figure shows the frequency response of the AD2 when the high-gain path of the scope is activated (the channel gain is 10x).

Frequency response of the high-gain path. Image used courtesy of Digilent

The figure below shows the frequency response for the low-gain path (gain set at 0.1x). 

Frequency response of the low-gain path. Image courtesy of Digilent

These measurements are made by applying a sinusoid with varying frequency to the scope input. As you can see, the -3 dB bandwidth of the scope is higher than 30 MHz.

In this measurement, a coax cable along with the BNC adapter board is used to connect the sinusoid to the scope input. With cheap wires, the bandwidth will be lower than this (about 9 MHz).

The analog bandwidth is an important specification and almost every scope manufacturer gives this information.

You'll want a scope with a higher bandwidth because it can measure a wider frequency range. However, note that the -3 dB bandwidth cannot fully characterize the frequency response of an oscilloscope. The scope frequency response might have some magnitude fluctuations at frequencies below the -3 dB frequency.

In the two figures above, these fluctuations seem to be acceptable (it is less than -0.2 dB below 5 MHz). However, with low-cost oscilloscopes, particularly those manufactured by small companies, we have to examine the frequency response closely to make sure that the magnitude response is flat and the oscilloscope is not causing undesirable modifications in the magnitude of measured signals. With a bumpy magnitude response, we might have to restrict the input signal to frequencies much lower than the nominal bandwidth of the oscilloscope. 

Sampling Rate

The sampling rate specifies the number of samples that the oscilloscope can take per second. The higher the sampling rate, the higher the maximum input frequency that the oscilloscope can measure. The sampling rate should be about three times the scope bandwidth. The sampling rate of the AD2 oscilloscope is 100 MSPS (million samples per second).

Resolution

The resolution specifies the smallest incremental input voltage that can be recognized by the oscilloscope. As the resolution increases, the sampling rate decreases. That’s why oscilloscopes with high sampling rate and resolution are expensive.

In this table, we'll compare the specifications of the AD2 oscilloscope with three other PC-based oscilloscopes.

*Amazon lists the PCSGU250 (Velleman) as $212 at regular price. As of March 12, 2020, the oscilloscope is marked down at $114. All prices are from the date of this article's publication.

The PCSGU250 offers both lower sampling rate and resolution (of course at a much lower price of $114). The VT DSO-2A10 has the same sampling rate as that of the AD2; however, its vertical resolution is 10 bits. The sampling rate of the DSO3064A is 200 MSPS but its resolution is only 8 bits (priced at $455). 

The PC-based oscilloscopes from Pico Technology are not included in the table above because the specs and/or prices of the oscilloscopes of this company are very different from the devices mentioned in this table. For example, the PicoScope 3000 series are 8-bit oscilloscopes whose prices start at $579. This relatively high price is justified by high sample rate (1 GSPS), memory (512 MS), and waveform update rate (100,000 waveforms per second). 

Memory Depth

A digital oscilloscope acquires samples and stores them in memory for further processing. Assume that the sampling rate of the oscilloscope is fixed and we increase the time/division setting of the oscilloscope.

In this case, the number of samples displayed on the oscilloscope screen increases and we need a large memory to store these samples. If the oscilloscope doesn’t have enough memory to store these samples, it will automatically reduce the sample rate so that the number of samples fit the available memory. Hence, while the ADC might support a high sampling rate, we might lose some details of the input signal simply because of the lack of memory.

The table above compares the memory depth of the AD2 with other PC-based oscilloscopes.

Triggering Systems

Today’s oscilloscopes support advanced triggering options such as width trigger, slew rate trigger, glitch trigger, and so forth. These triggering options allow us to detect rare abnormal transitions of a signal.

Although the AD2 and DSO3064A cannot offer the sophisticated triggering system of an expensive benchtop oscilloscope, they provide more options compared to the PCSGU250 and VT DSO-2A10.       

Analog Discovery 2. Image used courtesy of Digilent

Histogram Function

The AD2 oscilloscope can plot the histogram of a signal. A histogram is a graphical representation of the number of samples with a given value. The histogram of a circuit parameter can give us clues to debug a flaw. Moreover, we can use histograms along with the different triggering modes to find rare abnormal transitions of a signal.

In a future article, we’ll more closely examine the applications of the advanced triggering modes and the histogram view.   

Conclusion

In this article, we looked at some of the important features of the AD2 oscilloscope and compared it with some products that are in the same general price range. Based on the specifications listed in the table, the AD2 oscilloscope provides the highest vertical resolution and offers a high sample rate. Its triggering options and histogram function can also be helpful in debugging circuits. 

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More Insights on Analog Discovery 2

Many Functions, One Tool: The Analog Discovery 2 from Digilent

The Rise of Low-Cost Test and Measurement Equipment

Diversifying Test and Measurement: An Interview with Kaitlyn Franz of Digilent
 

Featured image (modified) used courtesy of Digilent