Hi, Robert Ussery wrote: >> I must ask, is this a one off project ? in that case it might be easier >> to purchase >> an pre-made weight indicator. >Well, I'm building a prototype with a possibility for ~100 production uni=3D >ts. Ok, then I understand why you might want to roll your own. >> Anyway, in principle you are correct, you only need to apply excitation >> and amplify >> the output and feed it to an ADC. Practically however there are some >> things that >> you need to concider: >> >> -Excitation can be AC (square wave) or DC depending on ADC. >AC, meaning it has a negative component? or just a positive sqare wave? Actually it's only loosly 'AC', an more correct term would be 'switched polarity'. I works by alternating the polarity of the excitation for each sample cycle. Thermo encoders are unaffected by excitation and DC based, an switched polarity design can balance out this 'offset'. Also the drift in the thermo encoders will be compensated (as they generally have no affect). >> -Resolution ? >~50-100 lbs with a 0-2000 lb range. Yes that's the capacity, but what resolution do you need ? Or do you mean that you are only aiming for an resolution of 50lbs on the capacity of 2000lbs, that would only be 40 divisions, that can't be right ? in that case an loadcell would be uncalled for as they normally have are designed for=20 +3000 divisions in the full temperature range (-10C=B0 - + 40C=B0) out of one 10000 th part from full range. This makes no sense to me ? >> If you are set to do this yourself, I would suggest to use an >> bridge transducer ADC such as AD7730. This contains PGA, DAC, >> gain registers, offset registers and ADC components, >> bascilly you only need to add buffering opamps, and some transistors >> for the excitation swing(AC). >> Although it's 24bits and quite pricy (~10$) it the best intergrated >> ADC for this purpose I've found. >> Also it's better suited for 5V excitation, but 10V excitation is fully >> possible (used this myself). >How does excitation voltage affect the ADC? Doesn't it just see the scale >output from the op-amp? Well it depends on if you have an regulated excitation (i.e make sure that the excitation stays within x mV from target at all load and temps.) or an sensed excitation (where the ADC uses the excitation to calculated the quota signal/excitation). When you have an sensed excitation (simpler design) the ADC sense inputs are normally limited within some voltage range. For an 5V ADC normally you cannot surpass 5V as this will freeze the ADC, therefore you must make sure that the ADC never 'sees' any voltage above 5V, even though you might have 10V as excitation. >(sorry for all the "stoopid" questions, but I'm something of a newbie) Nope, there's no stupid questions, only answers :) >> If you have any more questions I'll be glad to assist. >Do you have any circuits you'd be willing to share? Are the app notes and >sample circuits for the ADC very useful in this application? Well I have one advanced cicuit (for AD7730) that is owned by my employer, but you will get pretty far on the app notes. If I recall correctly the only thing missing from the AD7730 ones are the input buffers, to protect your ADC and for impendance matching.=20 However if you have smaller requirments (such as less resolution and only one or a few loadcells ) I maybe can find an simple circuit with only some op-amps and an 'normal' ADC. I'll see if I have the time. No promises though, I'm fairly tied up in some projects. >Thanks very much! You're welcome, /Tony -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu