Have a RS-232 chip that generates a negative voltage on the board? You can sometimes tap that rail for a small amount of current..... -----Original Message----- From: piclist-bounces@MIT.EDU [mailto:piclist-bounces@MIT.EDU] On Behalf Of Xiaofan Chen Sent: Friday, February 26, 2010 7:13 PM To: Microcontroller discussion list - Public. Subject: Re: [EE] poor mans negative rail generation On Sat, Feb 27, 2010 at 12:34 AM, alan smith wrote: > The reason is this chip is used to generate a 0-10V or 4-20mA drive, > in the same part (saving space for doing the same thing with two designs). > Running on a single rail generates 'dead bands' according to what I got back. Yes that is when your accuracy requirement is on the high side. Use a rail to rail input amplifier may help. 4...20mA should be okay. 0...10V is the issue. 0...20mA (if you need that) can be an issue as well. Often we need to provide -10V...10V along with 0...20mA and 4...20mA, in that case, we need to have a +/-15V anyway. But when there is only 0...20mA and 4...20mA requirement, sometimes you can get rid of the negative supply if the requirement is not too high. When the requirement is high, then we have to have a negative -3V (or more negative) supply so you really get 0mA when you mean 0mA. By the way, now ADI (and Maxim) has nice output driver which can product 0...10V and 4...20mA (and other ranges). They are nice product. As for the SPI digital isolation, the digital isolator from ADI and TI are very good. They are more expensive than the high speed opto coupler but consume much less current. -- Xiaofan http://mcuee.blogspot.com -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist