| ??? 01/19/10 15:10 Modified: 01/19/10 15:12 Read: times |
#172596 - no specs violated Responding to: ???'s previous message |
Erik said:
differential-ADC has around 120-130dB. CMRR
when within specified range!!! which 20V is not. usually the CMRR spec only hold till Vin + Vcm is less than Vce Ofcourse ,but it was for 400mV appearing at the ADC input . Erik said:
What you need, for any reasonable precision is an chopper stabilized op amp with same base ("gnd" [20V]) as the loadcell and then some means of level conversion or isolation to the rest of the circuitry -OR- as below one question: whhy cant you make all the circuitry have "gnd" at the 20V? Erik How can I gaurantee the stability of this ground ? thermal drifts etc and galvanic isolation will still be required ? . Stefan said:
Sorry , i cant follow You - there was 20V in first post, now You mention 8V.
I suppose it is some very strange loadcell - - today "standard" loadcells with glued tensoresistors have output of 0,8...3 mV/V . Does Your loadcell have 3.5 mV/V output? (70mv/20V)? - Is Your loadcell tensoresistor bridge type? If so - why You thinking high voltage excitation? If not - this discussion is almost useless without loadcell's datasheet. On other side - You can power load cell with "independent" 20V . With resistive divider You can build loadcell's "gnd" point - - to be connected to ADC's "gnd" point - to be 2.5V down-offseted below loadcell's output. Maybe this proposal is wrong - i was thinking about such connection but never tried. regards The first post was a general query I was thinking of providing that to the sensor but later fealt it was useless .I am not sure if such high 20V common mode voltage permanently exist in such type of sensor application. spec here,after reducing the voltage further(at lowest possible excitation volts) I could go as low as 7.4V~8V CMV max as recommended by manufacturer. Regards -Ap |
| Topic | Author | Date |
| Isolating xducer | 01/01/70 00:00 | |
| not even a way | 01/01/70 00:00 | |
| noise | 01/01/70 00:00 | |
| I wouldn't do that... | 01/01/70 00:00 | |
| agree | 01/01/70 00:00 | |
| when within specified range!!! | 01/01/70 00:00 | |
| why 8V or 20V | 01/01/70 00:00 | |
| no specs violated | 01/01/70 00:00 | |
| that is TOTALLY irrelevant | 01/01/70 00:00 | |
| Jumping to conclusions | 01/01/70 00:00 | |
| AC modulating the bridge excitation voltage? | 01/01/70 00:00 | |
modulating the excitation | 01/01/70 00:00 | |
| make it simple at start | 01/01/70 00:00 | |
| making it | 01/01/70 00:00 | |
| o, i see... | 01/01/70 00:00 | |
| from my experience with loadcells ... | 01/01/70 00:00 | |
| what is exactly "xducer" ? | 01/01/70 00:00 | |
| Offsetting a signal | 01/01/70 00:00 | |
| Yes but...? | 01/01/70 00:00 | |
| Error analysis and budget | 01/01/70 00:00 | |
| Common Mode suppression | 01/01/70 00:00 |