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  1. UsmanLula

    CMR Help

    Hi Its a portable system and running on a PP3 9V. An AC excitation can produce a more accurate system-true. In terms of the CMRR, i will stick to the values of CMRR stated on the LT1014 of 114dB (in the low frequency range). Also, this will be worst case cmrr as the classic type will improve...
  2. UsmanLula

    CMR Help

    Smokey thanks I have looked into matching resistance - the difference between a carbon film resistor (5%) and a metal film resistor (1%) can be 10dB (in terms of CMRR). However, the problem I am finding is in calculating the CMRR for a 3-opamp. I have used the CMRR equation: CMRR = 20log...
  3. UsmanLula

    CMR Help

    Hey guys Thanks for post. By ratiometric - as smokey said - I used the same reference voltage on the ADC (LT1298) to excite the S250 strain gauge. So, the voltage variation/drift is not a problem (ADC and bridge move together). However, its a full bridge strain gauge (active) and thus even if...
  4. UsmanLula

    CMR Help

    Just another addition. I have attempted to claculate the CMRR of a 3-opamp (home-brew) Instrumentation Amplifier. However, all my resistances were equal (except the gain resistor). This means that my common mode gain was equal to zero!!! And my diiferential mode gain was 427. So how on earth...
  5. UsmanLula

    CMR Help

    Addendum I didnt adjust the offset voltage (of 200uV) on any one of the amplifier inputs using a POT to create a zero output voltage (which I was supposed to do!). What I did instead, was level shift the signal (using a POT) so that it rides between the 0-4.5V supply rail i.e. at around 2.25V...
  6. UsmanLula

    CMR Help

    Guys I have completed the CMR analysis. I found that with my homebrew (classic 3-opamp IA), my CMR was 119dB (with a 5% resistor matching). With a common mode voltage of 2.25V, the IA would attenuate this CM voltage down to 2.5uV. However, if I matched my resistors well (using metal film 1%)...
  7. UsmanLula

    CMR Help

    Well its definitely not correct. Somehow I had this in the portfolio I gave in. I must have been sleeping when I write that statement! Any light on the CMR question???? Usman
  8. UsmanLula

    CMR Help

    A further query - about strain gauge resistance We all know the equation to the change in resistance in a wire - dR = pl/A; dR = change in resistance; p=resitivity; A=area This means that as the length of a wire increases (in tension), the resistance also increases; and vice versa for the...
  9. UsmanLula

    CMR Help

    Hi guys (correct me if im wrong anywhere in my method) I am looking into the minimum CMR that is required in an Instrumentation Amplifier (IA) used to convert a differential signal (from the S250 strain gauge) to a single ended signal (ready to be ADC'd). If the excitation voltage (Vexc)...
  10. UsmanLula

    PICmicro & I/O Pin

    Hi Felix Thanks for the posting. The compiler I used for coding C for the PICmicro was the PCM compiler. I have sent CCS (devloping company of the compiler??) a query regarding serial timing and how the software UART is implemented. Ive tried to download the CCS compiler manual buts its...
  11. UsmanLula

    PICmicro & I/O Pin

    Hey Felix! Yeah, in the future I will keep the Vcc just under 6V (or optimum voltage) ensuring the regulator has a tight tolerance. Also, i'd rather use a Serial driver chip with capacitors inside. less mess really and less noise perhaps. I am working at a fixed baud rate of 19.2 kbps. My clock...
  12. UsmanLula

    PICmicro & I/O Pin

    Hi guys Its like midnight here i was about to crash after making notes on LM7XXX series regulators i used on one of the projects. It was recommended by Harrowitz&Hill (the bible for us in our training). Thanks Smokey. In fact I have used an NOP in my code but i didnt use it in between the...
  13. UsmanLula

    PICmicro & I/O Pin

    Smokey Its a very interesting thought. hmmmmm. Could it be the noise from the MAX232 or could it be the overvoltage from the PC lines??? If your theory is correct smokey, then why did i need a resistor for it to work (without sending garbage)?? Should it not be better without a resistor then...
  14. UsmanLula

    PICmicro & I/O Pin

    Hi Felix Thanks for the comments. I spoke to a collegue today and he mentioned the same - parallel outputs. I think its a good idea. The buffer (74ACT245) you mentioned probably is the best method because of the noise from the MAX chip affecting the data line TTLs. Thats probably what caused...
  15. UsmanLula

    PICmicro & I/O Pin

    Hi Mike Thanks for that. I have already developed the kit. The bit-banging bit was left to the C-compiler. I just stated the RS-232 parameters. thats all I did for the PIC. What is bit-banging in short??? (i have checked the net but i still cant understand it! It goes into too much depth-I...
  16. UsmanLula

    PICmicro & I/O Pin

    Hey smokey Thanks for that. Somehow I couldnt get the MAX232 to work straight off the PIC I/O. MAX may be workin due to the low speed. True. The PIC16C62 does not have a UART. I was using the RC6/RC7 pins (TX/RX) for serial data transmission. But i set the baud rate of 19,200 in the PIC C...
  17. UsmanLula

    PICmicro & I/O Pin

    One more query. :) I powered the MAX232 using a seperate I/O. Assuming I use the data (from the work of Smokey) and say the I/O supplies ~8.8mA and 4.5V. When I did connect this to the MAX232 Vcc, it didnt work. So what I did was use a 100 Ohm resistor in between the I/O and the MAX Vcc...
  18. UsmanLula

    PICmicro & I/O Pin

    Cheers ItsSmoked. I like the regulator idea too. Atleast I will have a larger head room for the current over the regulated voltage level say 4V. The work you did is very interesting. Now i am hoping to spend all day Wednesday doing the calculations for the circuit and its limitations. By the...
  19. UsmanLula

    PICmicro & I/O Pin

    Thankyou so much "Its_smoked" for the work especially over the weekend!!! I am pretty impressed with the results. So my components are powered at about 4.5V being sourced with ~9mA (each of the two ports). Does this mean that if my Vcc has a tolerance of 2% i.e. a range of 4.9 - 5.1V, the...
  20. UsmanLula

    PICmicro & I/O Pin

    Hi Itsmoked I am revising for my medical electronics viva which is on 21st September. It is based on the three project I have done so far. I would appreciate your help in this. Cheers. I didnt actually think about the I/O power that much at the time I was building the circuit - silly me. Now...

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