SN65HVD234: How to measure Cin

Troyson,

Thank you for bringing this to our attention. When you say "like the red block in the following picture", I'm not sure the picture was posted. 

I have a presentation that will show how to perform the input capacitance test. If you click my username it will take you to a page where my email is listed. Please email me directly and I can send the presentation and answer any other questions you may have.

Regards,

Hi Eric,

   Help us understanding what is the Maximum Input Capacitance of the TI part. How you expect us to measure the Input capacitance. Can we use  a LCR meter to do this.

Regards

Jithu Tharoor

Jithu,

The capacitance can be measured using an LCR meter, what's typically done with our newer parts is measuring the time constant with through a know series resistor and square wave, then calculating the capacitance that way. I sent Troyson a PowerPoint presentation explaining the time constant method because it seems to be more consistent. 

If the LCR meter gives a consistent measurement, then that can be used.

Regards,

Hi Eric

  When we measure with LCR meter we ate getting the Values in the range of 120 - 140pF. Is this expected?? We are doing this to measure the total input capacitance on the CAN line which includes TVS Diode , 33pF Cap and the TI Transceiver. 

   Theoretical values : TVS Diode: 80pF + Capacitor: 33pF + TI Transceiver IC: 40pF = Total 153pF 

    Practical Values using LCR :  TVS Diode: 88pF + Capacitor: 31.3pF + TI Transceiver IC: 133pF = Total 252.3 pF  this is too high than the theoretical value. Our  Board capacitance is in the range of 8-10pf .

 Will you bebale to explain why we are getting this high Transceiver capacitance ?

Regards

Jithu Tharoor

Jithu,

Because the CAN bus pins and the internal circuitry are not merely a capacitor, and there are many protection components for ESD and transient events, the capacitance can appear much greater to an LCR meter.

Other conditions that can affect the capacitance are the state of the device, was it powered on when the capacitance was measured? Was it in normal mode? Was the bus in recessive mode?

In general, the method I sent over in the Powerpoint is more reliable and consistent between devices. Is it possible for you to explain which part of the time constant measurement method wasn't understood and I can walk you through it?

Regards,

Hi Eric,

   While measuring the TI transceiver Input Impedance we remove all external components lie capacitors and the ESD diodes. Still we can see the Input the capacitance is >130pF.

Regards

Jithu Tharoor

Jithu,

Sorry for the confusion, I meant the protection components internal to the IC. Because of the ESD protection structures inside the IC on the CAN bus pins, and the structure of the CAN bus in general, measuring the capacitance isn't always as straightforward as using an LCR meter.

I have some lab time tomorrow and I will try to find some SN65HVD23x devices that I can measure in the lab. I'll report back with the results. I'll be using the method described in the presentation I sent over previously.

Regards,

Jithu,

I wanted to follow up and let you know I wasn't able to get to this in the lab yesterday. I have lab time on Tuesday and will be able to get to it then.

Regards,