If you are cost-sensitive, why not use an instrumentation opamp, some can sense diff voltages with 200V+ common mode. The supply (+- 15 v or so) can probably be taken from the amp. Must be cheaper, smaller etc than xformer. Cheers, Jan Didden ----- Original Message ----- From: "Sanjay Punjab" To: Sent: Tuesday, July 31, 2001 7:39 AM Subject: Re: [EE]: (Followup) Having difficulty interfacing a current sense resistor to create a proportional voltage > Thanks everyone for all of your advice. > Unfortunately for cost reasons, I must stick with > using a low-ohm resistor as a current sensor. A > hall-effect current sensor, a magnetoresistive device > and > a reasonably accurate current transformer, cost $6 and > above in volume. It looks like these are my only > options: > > 1. Use a 1:1 transformer to isolate the voltage across > the low-ohm resistor (current sensor), from the > interfacing analog circuitry. On the secondary side, > the voltage can be referenced to ground, making it > quite easy to get reasonable accuracy out of an > inverting op-amp circuit with .1% resistors. > This will work, since I am only concerned with AC > current. The problem is finding a 1:1 transformer with > truly accurate winding ratios (within 1%). Or else, I > will have to perform some type of testing in each unit > during manufacturing and come up with a normalization > look-up table in software. > > 2. Use some type of optocoupler. Again the same > problems as solution #1. I am wondering how difficult > it would be to get an monolithic optocoupler > that has consistant voltage/current transfer ratios > between device samples. > > Perhaps some of you can comment on these potential > solutions. Thanks for all of your help. > > > > > > --- Sanjay Punjab wrote: > > 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 > > > > > > > __________________________________________________ > Do You Yahoo!? > Make international calls for as low as $.04/minute with Yahoo! Messenger > http://phonecard.yahoo.com/ > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads