That's on my todo list, thanks guys. Wagner Lipnharski To: PICLIST@MITVMA.MIT.EDU Subject: Re: Sigma Delta Converter Sent by: pic microcontrolle r discussion list 07/13/99 09:53 PM Please respond to pic microcontrolle r discussion list Pretty one, already tested. DIP 8 small package... without nasty programming. Some small problems with grounding, but works nice. Wagner. Marcos Migliorini wrote: > > If you want a good sigma delta converter take a look at www.linear-tech.com > (LTC2400 24 bits sigma delta a/d). > > Marcos > > ----- Mensaje original ----- > De: Jack Shidemantle > Para: > Enviado: Martes, 13 de Julio de 1999 10:38 a.m. > Asunto: Re: Sigma Delta Converter > > This may not apply directly to your situation. I tried rolling my own V to > I converter to use as a single slope A/D converter with a PIC a year ago > (Similiar to one of their app notes). It worked ok but could not get good > stability and repeatability better than 0.5% to 1%. I needed 0.1% so was > forced to go with a commercial dual slope A/D. I would like to get a sigma > delta unit which will give me a good 14 bits. Understand Tel-Com is working > on one but not available yet. If you come up with something I would be very > interested. > > @ > Sent by: pic microcontroller discussion list > 07/13/99 08:25 AM ZE10 > Please respond to pic microcontroller discussion list > > To: PICLIST@MITVMA.MIT.EDU > cc: > bcc: > Subject: Re: Sigma Delta Converter > > At 01:13 13/07/99 +1200, you wrote: > >I asked about this some weeks ago but a second query may be useful. > > > >I am trialling a Sigma Delta analog to digital converter. This SHOULD > >allow low cost high accuracy A2D with modest conversion speed. > > > >Results to data only get so good (about 1/2 percent accuracy - > >slightly worse than 8 bits)(plus a few worse results here and there) > >and then seem to offer rapidly diminishing returns. Using more > >conversion cycles doesn't seem top help much. I'm using 2048 counts > >at present and would EXPECT much better results. > > > >I have used both a standard microprocessor input pin as the sense > >element (PIC) and an internal comparator (Z8) and an external op-amp > >(LM324) as comparator. I am aware of the poor performance of the > >LM324 as a comparator but am compelled to use it if at all possible > >due to cost considerations. If it CANT be used then something else > >will be used but at around $US0.16 in volume for a quad op-amp it is > >very attractive. I also need to deal with a 0 to 0.2V input signal > >so an amplifier is desirable. > > > >Anybody had better than about 1/2% accuracy results from a Sigma > >Delta built discretely? > >Any thoughts on the process? > >This is a stunningly cheap and simple method for A2D if it would only > >work :-) - once sorted it would/will prove a boon to other PIC users. > > > > > >regards > > > > > >Russell McMahon > > > > > > An LMC660 comes close to the bargin price, but the 324 should do. Some more > information is required, are you using a split supply for the OPAMP, if not > how close to 0V do you get. When you say 0.5% error at what voltage input > level is this? Is the error at the same input level or over the entire > range? > The 324 has a 2mV offset that's up to 1% error in 200mV! Also are you using > dual slope conversion or single slope? > The number of counts not altering the value indicates that this is what the > converter is reading, so I would dare say that the reference is also ok. > But a Z8! Bad boy! > Please tell me more > > Dennis -- -------------------------------------------------------- Wagner Lipnharski - UST Research Inc. - Orlando, Florida Forum and microcontroller web site: http://www.ustr.net Microcontrollers Survey: http://www.ustr.net/tellme.htm