SN65HVD1780: COD (Differential Output Capacitance)

Hi Ray,

So in general this is a design value - not directly measured/characterized  (its not in most RS-485 device datasheets) as there is no test conditions (such as what state the device is in, the frequency the capacitance was measured at, etc...)  - for this device specifically when I look into the data it seems that COD has no characterization and is more of a design value. The reason behind this is that COD in most applications is going to be much smaller than cable capacitance (i.e. why you don't find this specification in many applications).

Testing this value can be difficult as it is pretty hard to isolate just the differential capacitance (as A and B will also have parasitic capacitances to ground) and since its output the driver must enabled for measurement or else you will be measuring input capacitance (they should be similar - but they may be different) . The largest effect that may be able to be measured is the change in differential rise and fall time - but without parasitic capacitance, and output impedance of device its hard to make a meaningful measurement.

In the most simplistic view - the driver circuit can be approximated to look like below:

Where the output impedance and parasitic capacitance will also impact the timing of the device. 

With all that being said it may be possible for customer to approximate this value:

1. To find the parasitic capacitance value you can just attach a capacitance meter between A and Ground or B and ground when the driver is disabled (it will be easier when signals aren't firing out of the device) - use a frequency that would be used in the actual application to get a good judge of value during operation. These values are not spec'd but will impact rise/fall timing.

2. By just putting a "high" value on "D" pin you can measure the DC voltage at A, B , and between A and B - these three voltage values will help determine the unknown output/input impedance of the driver  - this should be a relatively smaller value but its not zero and will impact the timing of the device.

3. Once this done measure the differential rise time of  device.

4. Finally run a simulation on rise times by varying COD from 0 to 50pF (it should be around 23pF as designed  - but adding additional margin to have multiple points) along with the measured values - compare these sims to actual measurements to get an idea of the output capacitance.

However one very large caveat here is that if the customer's cable capacitance is >> than what is spec'd on our device datasheet (with most cables greatly exceeding that value after a few meters) this value is completely negligible - in short bus applications where cable capacitance is not >> than output capacitance and is either closer in value or smaller then this parameter will have more impact - but generally that small of a value will do very little to the overall performance of the device - especially considering how slow this device is in general - it is rated for 115Kbps (or eq. to 57.5KHz) the capacitance impedance should be > 120K at max speed of this device - which can be assumed to be essentially an open circuit for non-terminated nodes and for terminated nodes the 120 Ohm resistor recommended is going to have more impact than the differential capacitance. 

So in conclusion:

1. The value is by design - it is not characterized

2. You can estimate its value by taking a few other measurements and simulations but

3. The work required to estimate that value generally isn't necessary as its a very small value compared to speed of the device + small amounts of cabling usually will cause the cable capacitance to be >> than differential output capacitance. It isn't spec'd on most of our devices for a reason - and that generally is that isn't super critical to most designs/applications but it can add unneeded complexity.

Please let me know if your customer has any further inquires and I will see what I can do!

Best,

Parker Dodson