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Introduction

OPAL-RT DOUT I/O cards are engineered to deliver ultra-fast digital output transitions, meeting the demanding performance needs of a wide range of applications. Designed to be compatible with all OPAL-RT customer systems—including those with the most stringent requirements for high-speed switching—the cards support output transitions from 0 V to 30 V with rise and fall times in the sub-microsecond range.

The performance and flexibility provided by OPAL-RT’s IO cards also mean that, in certain configurations, the I/O behavior may require fine-tuning to achieve optimal signal integrity. In particular, systems with long cables, high impedance mismatches, or sensitive loads may benefit from the addition of snubber circuits to suppress overshoot.

Common Use-Cases

OPAL-RT I/O cards have an output filter that was optimized in our laboratories to meet the needs of the majority of customer applications. The filter includes a built-in current-limiting mechanism that functions similarly to a small snubber network. It helps ensure compliance with EMC standards while maintaining optimal performance across a wide range of system configurations.

When high capacitive loads are being driven by a DOUT I/O card, the output filter’s inductive characteristic might start to “peak” as it resonates with the high capacitive load.

Below is a transient response simulation of a 250 kHz, 50% duty cycle, 30 V output signal with a 1 nF load capacitance.

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30 V, 250 kHz output waveform with a 1 nF capacitive load. Output overshoot is approximately 6%

We can see that the output waveform is beginning to show some overshoot. The overshoot will continue to increase as the capacitive load is increased until the low-pass filter formed by the cable and the capacitive load begins to attenuate the whole signal waveform (i.e.: too big of a capacitive load for the 250 kHz signal). This overshoot can increase to up to 20% when a cable is added along with the high capacitive load.

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In this simulation of the circuit, a capacitive cable is added, and the load capacitance is increased to 2 nF. The overshoot is now almost 16%

Some situations where a high capacitive load can be present are

• When long cables are used to interface the I/O card to the DUT.

• When overly capacitive cables are used to interface the I/O card to the DUT.

• EMC filtering on the DUT.

• Etc.

NOTE: In the above simulation, a 8-15 foot capacitive cable was used. Cable parasitic elements used for the simulation were:

• L = 600 nH

• C = 1 nF

• R = 18 ohms

Adapting an OPAL-RT DOUT I/O Card for Your Application

In order to limit and reduce the overshoot from the IO card’s output filter, a voltage-limiting, dissipative snubber circuit can be added to the DUT or to the output of the I/O card. This snubber circuit consists of a simple capacitor and resistor network which will absorb the overshoot. The tradeoff with adding this type of circuit will be a limitation to the output speed of the I/O card.

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Representation of a simple snubber circuit

This snubber circuit will need to be adapted to your specific needs. Experimentation might be required to fine-tune the performance of such a circuit with your cable.

Going back to the simulated circuit from above, a snubber circuit consisting of:

• R1 = 100 ohm

• C1 = 5 nF

Allows the transient response of the system to be corrected to this:

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Using a snubber circuit at the output of the I/O card to reduce the peaking from the output filter. Using the snubber circuit, overshoot was reduced from 16% to 0.5%

However, as explained previously, the tradeoff with such a circuit is that the rise time went from 200 ns (with cable and load) to about 500 ns (with same cable and load).

Snubber circuits should be designed and optimized for each specific requirement.

Guidelines on Designing a Snubber Circuit

It is extremely difficult to approximate all parasitic components of your circuit. Therefore, it is recommended to tune your snubber circuit with experimentation in the lab. However, here is a good starting point for a snubber circuit design.

1. Select C to be approximately 3-4 times the size of the characteristic capacitance of the circuit. Without any cable or load, a capacitor of 1 nF is a good approximation. Adjust this value depending on specific circuit needs. If a 1 nF load is present, select C to be between 3 and 4 nF. This value is hard to approximate because if the load capacitance is small, the dominant capacitance will probably be your cable’s capacitance.

2. Select R to be equal to the characteristic impedance of the line. Without any cable or load, a good starting point is between 50 and 100 ohms.

3. Be wary of frequency requirements. If a higher speed is needed, the capacitor might need to be reduced. This will allow for more overshooting and peaking, but the rise/fall times will stay sharp and fast.

NOTES:

• The capacitance of a cable is highly dependent on its construction, but without any other information about the cable, an approximation of maybe 30 pF - 100 pF per foot might be a good starting point.

  • Related to the point above: if a snubber circuit is designed for a specific application and cable, changing the cable means that the snubber will need to be re-validated since it is highly dependent on the cable’s parasitic properties.

• If faster switching speeds are needed, the capacitive load is going to be much smaller than the above example. This is natural, as the capacitive load forms a low-pass filter which will attenuate high frequency signals. For example, if you are looking to achieve 1 Mbps switching speeds, keep the snubber capacitor below 1 nF.

Installing the Snubber Circuit

The snubber circuit can be installed either directly at the I/O card’s output or can be installed at the DUT (device under test). In order to facilitate experimentation and installation, OPAL-RT provides the OP8320-4. The snubber circuit designed can be installed and verified directly with this kit. The DB37 connectors on each end provide an easy way to interface to OPAL-RT simulators.

Please refer to this page for more information regarding the OP8320-4: OP8320-4 - Transmission Line Termination Kit

NOTE: The OP8320-4 also permits testing different kinds of termination circuits, which may or may not be applicable for your design.

References

P. C. Todd, Snubber Circuits: Theory, Design and Application, Power Supply Design Seminar, SEM-1400, Texas Instruments, 1993. [Online]. Available: Snubbing-methods-slup100.pdf

OPAL-RT Technologies, WP CMOS-TTL: Interfacing CMOS and TTL Logic Levels, Rev. 1.2, White Paper, 2023. Available: OP5300 - Related Documents