C++ DLL Output Issue: Hardcoded 1.0f output appears as ~0.909 (1/1.1)

QSPICE
1

Hi Mike and everyone,

I am experiencing a strange scaling issue with a C++ DLL block in QSPICE.

I have a specific output variable named PWMBUCK. To test the interface, I simplified my code to force this output to exactly 1.0.

Code Snippet:

C++// Forcing output to 1.0 for testing *PWMBUCK = 1.0f;

The Issue: Even though the code explicitly assigns 1.0f, when I probe the PWMBUCK node in the simulation waveform, it settles at approximately 0.90909… (which corresponds exactly to 1/1.1). It never reaches 1.0.

  • I have checked that the variable is defined correctly in the port list.
  • Is there an implicit output impedance or a default scaling factor of 1.1 applied to DLL analog outputs?
  • Could this be related to how the DLL drives the node (Voltage Source vs Current Source behavior)?

Any advice on why this specific attenuation occurs would be helpful.

Thanks,
Yaşar

2

Is it possible that this is due to the loading effect? The DLL block by default has a 1 kohm resistance at each of its output pins. You can add an attribute ROUT=<value> to adjust this impedance. Alternatively, you can buffer your output with an E-source.

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2 Likes
3

@BLTeX ,

Check @KSKelvin post.

If you have a load on the output, the ROUT driving series resistance will make it act as a resistor divider network.

This is why I prefer to set ROUT=1n ohms. This is very close to 0 ohms without causing problems with the analog solver in QSpice.

Len

4

@KSKelvin @lpoma

The issue is resolved. Removing the 10k resistor at the DLL output fixed the problem, and the value now correctly reaches 1.0. Thank you for your support.

Yaşar

1 Like