Hello all,
Someone has asked me to design a system that can take multiple "oscilloscope" type isolated differential inputs ranging form 0 to 500volts AC or DC. In reality they want to be able to hook up to a 3 phase motor or a 0 to 5 volt signal line, or a current clamp that provides 0-1 volt output. Frankly I'm a little stumped. The AD converter, multiplexer and automatic gain selection issues are easy enough to handle, it's the isolation I'm having trouble with. I can't use a transformer because of DC. I want to use an opto-isolator, like the Farichild H11F3, but I have the roughly 1 volt drop on the input LED that gives me a problem with the current clamp or anything in the 0 to about 1.4 volt range. I have considered using the TI (Burr Brown) INA148, but I don't think it really provides the isolation I need if I have 3 of them connected differentially across a 3 phase motor.
Anyone got any ideas?
Thanks,
Steve