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 bac= k. --- On Fri, 2/26/10, Spehro Pefhany wrote: > From: Spehro Pefhany > Subject: Re: [EE] poor mans negative rail generation > To: "Microcontroller discussion list - Public." > Date: Friday, February 26, 2010, 8:27 AM > At 10:47 AM 26/02/2010, you wrote: > >OK, so I have a part that requires a +/-12V on it, and > I do have = > >+24V available.=A0 So, a poor mans method would be > to use a voltage = > >divider and generate a +/- 12V and use the centertap > for the ground = > >reference.=A0 The 'ground' or virtual ground, would > be completely = > >isolated from the rest of the circuit of course.=A0 > The kicker is that = > >there are digital signals (SPI), referenced back to the > board = > >ground, and the part requires a digital 3.3V for this > interface = > >(actually can be 5V).=A0 I could generate a local 5V > from the +12, = > >referenced to the virtual ground again but the only way > I can see to = > >get the SPI is to optically/transformer couple it into > this = > >section.=A0 I believe ADI makes a part that can do > this, and its not = > >the fact that its another part, its also room on the > board for another part. > > > >Short of this idea, what other methods to generate a > split supply = > >from the +24V ?=A0 I have very little room for much > of anything, so it = > >has to be small and pretty sure not enough room for a > negative buck = > >switcher unless very tiny.=A0 Total current isnt > going to exceed 1A > = > What do you need the +/-V for? Could you use a "bridging > amplifier"? > (ie. the load would not be ground referenced, but would see > up to +/- 24 > less whatever the amplifiers (one @ near 0V and one @ near > 24V-- those > values added together) need to avoid saturation). > = > Downside is that you need two amplifiers (one for each side > of the load. > = > Best regards, > = > Spehro Pefhany --"it's the network..."=A0 =A0 =A0 > =A0 =A0 =A0 "The Journey is the reward" > speff@interlog.com=A0 > =A0 =A0 =A0 =A0 =A0=A0=A0Info for > manufacturers: http://www.trexon.com > Embedded software/hardware/analog=A0 Info for > designers:=A0 http://www.speff.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