:-) Mark Skeels Engineer Competition Electronics, Inc. TEL: 815-874-8001 FAX: 815-874-8181 www.competitionelectronics.com On 6/23/2011 3:45 PM, Forrest Christian wrote: > Wow, how did I miss the TI parts? > > Looked everywhere else, and I was sort of stuck between a $1.20 national > part w/shunt, and the Allegro part... The INA21X's have too low of a > CMRR voltage, but the INA270 looks like it might be perfect for the job. > > Thanks! > > -forrest > > On 6/23/2011 12:39 PM, Mark E. Skeels wrote: >> Forest, >> >> It might be worth investigating using a very small series resistor with >> a TI INA21X. current shunt monitor. >> >> http://focus.ti.com/docs/prod/folders/print/ina214.html >> >> The chip output reference can be attached to something other than ground >> and the small value resistor needed allows very low insertion loss and >> low wattage rating. >> >> Different parts in the family have different gains, so you can choose >> one that fits your needs. >> >> Data sheet: http://focus.ti.com/lit/ds/symlink/ina214.pdf >> >> I can't spend a lot of time to see if this really fits your app, but I >> thought I'd mention it here because I bet if you take a look you'll find >> a nice way to use it and I suspect it will save you some $ over the >> Allegro part. >> >> Just make sure you have dedicated sense lead connections to the inputs >> to avoid voltage drop in the main current path which leads to errors. If >> you do this right you can get good results. >> >>> Mark Skeels >>> Engineer >>> Competition Electronics, Inc. >>> TEL: 815-874-8001 >>> FAX: 815-874-8181 >>> www.competitionelectronics.com >>> On 6/23/2011 11:38 AM, Michael Watterson wrote: >>>> what is resolution of ADC compared to accuracy of the sensor? Does it >>>> actually matter if full adc range isn't used? If so save on HW and jus= t >>>> scale in SW. Unless you use precision resistors etc adding an op-amp m= ay >>>> reduce accuracy? >>> I'm going to be using a 10 bit ADC in a PIC. Vdd/Vss references, 3.3V >>> So 3.3/1024 =3D ~3mV/step. >>> >>> For various reasons (primarily increasing the number of places this par= t >>> can be used), i'd really like to use the +-25A version of the Allegro >>> product. In this case, I'm most interested in 0-+5A. >>> >>> 0A corresponds to 1.67V. 5A corresponds to (5*0.055+1.67)=3D 1.945V. >>> So, a range of 275ma over the range of interest... or just under 100 >>> steps. >>> >>> Or using the other math... 55mV per AMP, or about 1/20th of an amp per >>> step. Not sure that is enough resolution for me. >>> >>> I was considering moving the ADC references but there just isn't enough >>> to gain - you need at least 2V from Vref+ to Vref-, which means I'd be >>> lucky to gain another 100 steps. >>> >>> Although I have to admit, the most usable option I've come up with so >>> far is to give up and use the 12.5A version, and move the Vref- to >>> around 1.2V, which should put us at 2.1V/1024 =3D 2mV/step, and the abo= ut >>> 55 steps/amp... or 0.02A per 'step', which seems quite a bit better, an= d >>> then do math in software to get the right values. >>> >>> -forrest >>> --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .