AM26C31M: Output impedance of AM26C31MDREP and AM26LV31ESDREP

Bruce,

What's the loading condition in the application? You can refer to Figure 2 of section 6.8 (page 8) on the data sheet of THVD1500 for the driving strength of AM26C31MDREP.

Regards,

Hao

You are saying that the output impedance is dynamic? I am trying to size my series terminations based on the output impedance of the device, not the other way around.

The driver's impedance is not linear. In RS485/422 systems, if the bus is terminated properly at the receiver side, the reflection should be minimized and the driver side won't see much energy bounced back. You can refer to this blog for the discussion about termination.

Regards,

Hao

Do you have any detailed I vs. V curves for the AM26LV31ESDREP?

We are considering a series-terminated application, RS-422 (so one transmitter, one receiver).  This application has no DC current, though there will be a brief but small pulse of AC current to charge the cable capacitance.  We are controlling impedance to 100 ohms, differential.  Can you recommend a value of series termination resistor to use?

Also I think there may be a typo in the app note:

IS: One situation in which you don’t need termination networks is when the two-way loop time of the network is much greater than a single bit time (~<0.1×two-way loop delay). In such scenarios, the reflections will lose energy each time they reach an end of the network.

SHOULD BE:

One situation in which you don’t need termination networks is when the two-way loop time of the network is much less than a single bit time (~<0.1×two-way loop delay). In such scenarios, the reflections will lose energy each time they reach an end of the network.

It might take some time to generate the output voltage vs. current curve of AM26LV31. I'll get the data for you.

About the point on the blog, I think it meant the reflection energy will die out traveling in the long cable. On the other hand, if the distance is short, the impact could be observed at the transmitter output, for example a step at the rising edge. This app note is a good resource for the technical discussion about reflection.

www.ti.com/.../snla027b.pdf

Regards,

Hao

I'll stay tuned for the data and your recommendation.

I see your point on the blog post; I was looking at this from the perspective of critical length which indicates a maximum cable length, given a driver's risetime.  An unterminated connection shorter than the critical length may have reflections, but they will die out before the signal settles out to its new value.  This notion assumes the reflections at the receiver are too small to cross logic thresholds & generate glitches.  

Josh