Prahlad..................

Like I said in my posting......You need to look at the data book for the motor drive and find out what the characteristics of the +/- input pairs really are. It will do no good to guess. As I said.....the +/- pair may simply be the connections to an internal optocoupler LED. Alternatively the +/- pair may be connections to an internal optocoupler LED that is in series with an internal resistor. It is also possible that these +/- input pairs may be the inputs to an internal differential signal receiver.

The point of my previous post was not so much to provide a schematic as much as it was to point out these things...

1) That you need to consult with the driver data book.

2) To tell you I have seen motor drivers with various types of inputs....each of which will require a different drive technique.

3) To suggest that I feel that you most likely do not need to have an opto coupler in your circuit.

After all, one of the primary reasons that one uses an off the shelf motor drive is to avoid the hard work of making your own driver. I have used many of these things over the years and have always found that they do a good job of isolating the motor output from the input side. Also take into account that motor drives by nature are normally placed at a location that is distant from the electronics that provides the input control signals. Thus the manufacturer tries to provide a way to permit the connection to the drive inputs via a twisted pair connection that has equal but opposite current flow in each wire of pair. One way this is commonly done is via a current loop that is terminated into an opto coupler LED in the motor drive. Another way that is common is that the pair is driven with a differential voltage drive and terminated into a line terminaton resistor that is biased across the input pins of a 26LS32 type differential receiver inside the motor drive.

All that said however this may or may not describe how your motor drive functions. Only a reading of the data book or the applications notes can tell you for sure. In any case it is most likely that you do not need to have your own opto couplers. And if you take them out then you eliminate the main reason why your own circuit is bandwidth limited.

Michael Karas