I would probably set up a galvanically isolated subsystem that 'rode' on the speaker waveform. It would convert the analog value to digital and then transmit it over an isolation barrier (optoisolator) to the rest of the instrument. Bob Ammerman RAm Systems (contract development of high performance, high function, low-level software) ----- Original Message ----- From: "Sanjay Punjab" To: Sent: Sunday, July 29, 2001 7:07 AM Subject: [EE]: Having difficulty interfacing a current sense resistor to create a proportional voltage > I have an application where I need to monitor and > digitize the current waveform output of an audio power > amplifier into a subwoofer (speaker). > Previously, I used a hall-effect based sensor, but for > cost purposes (consumer product), it is impractical. > So instead, I have decided to use a 1 milli-ohm > current sense resistor. The problem now is creating an > electrical interface to convert the instantaneous > current (0 - 50 amps) into a proportional > instantaneous voltage (0-2.5v). Since the audio output > is not referrenced to ground, some type of > differential circuit must be used. But although the > voltage across the resistor will always be small, the > voltage at each of the 2 resistor taps, with respect > to ground can swing as high as much as +/-100 volts. > Unfortunately most instrumentation op-amps that can > handle such a high common-mode input voltage, are also > quite expensive. In addition, a textbook differential > op-amp circuit doesn't provide the needed accuracy, > even with .1% resistors, since any component > tollerance is multiplied by the gain of the circuit > (50). I though it would be easy finding an interface > circuit that would solve my problem, especially since > almost every DMM must use something similar for DC > current measurement. But I have had no luck. I would > appreciate some advice, even better a schematic. > Thanks > > > __________________________________________________ > Do You Yahoo!? > Make international calls for as low as $.04/minute with Yahoo! Messenger > http://phonecard.yahoo.com/ > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body