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Doubling Channel Counts Using Dual Mode with Analog Discovery 3

Dual mode represents a recent addition to WaveForms, debuting alongside the Analog Discovery 3 (AD3) release (272-8596) . This innovative feature empowers users to interconnect two identical Mixed Signal Oscilloscopes, enabling the synchronization of their acquisitions and effectively doubling the channel count of your acquisition system. It's compatible with analog inputs, analog outputs, and digital inputs and outputs.

It's worth noting that this feature has been integrated into several of our mixed signal oscilloscopes, including the Analog Discovery Pro 3000-Series (222-2682) . In fact, you can pair two ADP3450s (222-2679) to create an impressive 8-channel scope system! The specifics of the connections may vary depending on the hardware, but for 3000 Series devices, a pair of short coax BNC cables is all you need.

In this post, we'll guide you through the process of setting up Dual mode using a pair of AD3's and configuring the system to accommodate clock delay. This feature is fully supported in WaveForms 3.20.1, which was the latest release at the time of writing. Further enhancements are also available in a beta version of WaveForms (3.20.15), introducing degree units as a phase adjustment option, as showcased in this guide. You can access beta versions through the Digilent Forum.

Analog Discovery 3 in Dual Mode

Getting Started

Hardware Setup:

To leverage Dual mode, it's crucial to connect the external trigger pins of both devices. Link T1 to T1 and T2 to T2. T1 is responsible for transmitting a reference clock between the devices to ensure they share the same clock source, while T2 transmits the selected trigger source to synchronize their acquisitions. Don't forget to connect their grounds to minimize crosstalk. For even better crosstalk reduction, consider using a twisted wire pair for the T1 reference clock signal. Keeping the cables short is ideal for optimal performance.

Software Setup:

Choose the first device that will serve as the host device; it doesn't matter which one you select.

Then, click the "Select + Dual" option and pick the second identical device. Follow the provided instructions to connect Trigger 1 to Trigger 1, Trigger 2 to Trigger 2, and ground pins between the two devices.

Next, open the Scope instrument.

Inside the Scope instrument, you'll now see the secondary set of analog inputs from the additional device, suitable for a pair of Analog Discovery 3s, marked as Channel 3 (+1±) and Channel 4 (+2±). Keep an eye on the indicators along the bottom of the screen, displaying device selections, additional device settings, and device status in that order.

Adjusting for Clock Delay:

Even with both devices connected and sharing a reference clock and trigger line, there might still be some phase discrepancies due to the propagation delay of the reference clock from the primary to the secondary device. Without adjustments, the same signal sent to both devices may appear delayed, showing a skew in nanoseconds. You can measure this skew by comparing the time when the two captured signals cross 0 V.

To rectify this, provide a phase adjustment using the "Phase" setting in the Device Options dropdown (the 100 MHz button at the bottom of the window). This allows manual fine-tuning of the clock for the second device, ensuring data is sampled slightly earlier with respect to a clock edge compared to the other device. This compensates for the propagation delay, ensuring that samples are taken at approximately the same time.

Selecting an appropriate phase offset for the clock signals involves using a shared signal source to test the system. In the photo below, you can see the same analog output channel from one device providing a stimulus signal to scope channel 1 of both devices.

A square wave is often a suitable test signal since it's easier to compare when a rising edge crosses a specific threshold. For most of the experiment, you'll want to zoom in closely on the trigger point to check the data transfer through the loopback.

Next, zoom in and begin adjusting the phase offset between the two devices. Determining the exact phase adjustment value can be a trial-and-error process since it depends on your specific devices and wiring setup. To find the right value, compare two channels, one from the primary device and one from the secondary device. Zoom in to a narrow timebase, and enable only one channel from each device on the view plot.

With a trigger condition set for the primary device (such as the trigger for when Channel 1 crosses the 0 V value), you can observe how many nanoseconds before or after the receiving channel on the secondary device crosses the same threshold.

Adjust the Phase dropdown from the Device Options and fine-tune it until both devices' signals cross the set threshold nearly simultaneously. Additionally, the reference clock option allows you to modify the clock frequency passed over the T1 trigger line, affecting the system clock on the other board. Experimenting with different values is recommended, depending on your application.

Don't forget that you can adjust the channel ranges, which can help spread the signal vertically across the plot and make it easier to identify zero crossings. A poor phase offset example is shown earlier in this article. However, through experimentation, you can achieve a significantly improved offset, such as 247.7 ps out of a 10 ns clock period (with a 100 MHz clock), which is ten times better than the default settings.

In Conclusion

With these adjustments, you now have an oscilloscope system capable of simultaneously sampling four signals, significantly enhancing your data acquisition capabilities.

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