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Baldin 22nd January 2010 05:29 PM

LTspice delay
A Class-d amp can be described simplified in the time domain as an amplification or gain, a delay of 1 sample freq and the output filter.

How do you model the delay in LTspice??

Have tried to use a transmission line, but it doesent really work ...
What is needed is perfect (same delay at all feq) delay of e.g. 200nS in the time domain.

I want this to be able to model different feedback network in the frequency domain.

Can anyone help????

Thanks in advance and best regards Baldin

Eva 22nd January 2010 06:30 PM

I don't use LTspice, but in the old version of Pspice that I use analog delay lines need a characteristic impedance to be specified, and they need to be driven and loaded exactly from that impedance to avoid problems, for example 100 ohm. Voltage controlled voltage sources may be used for input and output buffering.

I didn't find a better way to do it, as the other type of delay line available in my PSpice is digital and gives a lot of trouble with A/D conversion.

Baldin 23rd January 2010 02:22 PM

Hi Eva

Playing a bit more with the Transmission line in LTspice, and terminating it correctly in both ends, actually seems to work.

I think it was partly not having the right termination and partly not getting what I expected, causing the problems :) ... I now use 100 ohm in both ends, and 100 ohm line, and added a buffer on the receiving end.

Thanks and best regards Baldin ;)

ChocoHolic 24th January 2010 03:53 PM

It should be sufficient to terminate the output side with the same Z that you have specified for the TL. Effective drive impedance should not change the shape, but result in a voltage divider of drive impedance and load.
So far my LT Spice always handled the TL perfectly fine.

ChocoHolic 24th January 2010 03:58 PM

Add on:
The drive impedance should not change the shape as long as the drive impedance is not complex. With complex drive impedances you will get a frequency depending divider, which will result in changed shapes.

Baldin 24th January 2010 04:02 PM

Hi ChocoHolic

Well, that was also my fault ... I did not load it at the output with the same impedance as specified for the line .....
On the other hand, it overall didn't look as expected ;) ...... so I thought it was only because the modeling was wrong .... some was, some was my thinking :)

As the moment I'm strugleing with my design stopping to oscillate when not loaded (real life test, not simulated) ... funny thing is I have one oscillator board with a MXL1016 that work without problem, but the LT1711 board stops whan the load is removed ... and will not start without load either (and mosfets heat up :() .... ... one difference is of cource the output being 0/5V on the MXL, and +-5V on the LT ..... but going through the rest of the amp this shouldn't matter!!

ChocoHolic 24th January 2010 04:42 PM

Hi Baldin,
is the Pin5 of the 1711 properly connected to -5V?

Baldin 24th January 2010 04:52 PM


Originally Posted by ChocoHolic (
Hi Baldin,
is the Pin5 of the 1711 properly connected to -5V?

Yes :)
(but you can actually keep it floating also ... but it is connected to -5V in the design)

ChocoHolic 24th January 2010 05:00 PM

...too bad. Pin5 would have been such a simple solution...
Anyway, good luck!

You have a nice home page! ...and a seperate room for the home theatre!!! *envy*

sawreyrw 24th January 2010 05:02 PM

Use an LTspice BV source. V=delay(V(in),delay_time). It works great; better that using a delay line.

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