Pico Amp Circuit Help

Discussion in 'The Projects Forum' started by greg50, Oct 13, 2011.

  1. greg50

    Thread Starter New Member

    Oct 13, 2011
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    0
    Hi all,

    I have a project that I would like your input on the general design concept and how to achieve the desired outcome. I'll try to explain the concept below.

    We have a photomultiplier (PMT) that is connected to a typical fixed gain pico amplifier circuit using an AD549. The gain of the PMT, and hence its output, is controlled via a DC PMT control voltage. The "final" output will need to be connected to a 4.5 digit digital panel meter (DPM). Let's say 10V equals 100.00 on the meter.

    In practice the PMT will be subjected to near zero light, this is when the zero button is pressed. Then the PMT will have a fixed amount of light, and this is when the full scale button is pressed. Now you can measure the amount of light hitting the PMT between 0 and 100.

    I need a circuit that will do two things:-

    1. A press of a zero button will make the DPM read zero volts (000.00). I imagine applying an offset to the output, or maybe to the input, of the pico amp in some way.

    2. A press of a full scale button will change the DC PMT control voltage until the DPM reads 100.00.

    Apart from the pico amp which is analog, how is the best way to do the above. Keep it all analog or or do we go digital with say a CPU.
    Ideas and more questions would be greatly appreciated.

    TIA

    Greg
     
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi greg50,

    1. The AD549 has a 3V/uS slew rate. How fast is your detected signal?
    2. How many reading per second are required?
    Doing "push button" gain adjust and offset is very difficult with an analog circuit. Using adjustment pots would be easy.

    Digital method...
    If the signal is not too fast, you can read it with an A2D (50,000 count resolution). Then, using a micro (PIC), add an offset to get zero and multiply by a gain factor to get the full scale reading. You can do without the DPM and just use a digit display of some sort. A sample & hold circuit maybe required, and/or a peak detector circuit.

    Regards,
    Ifixit
     
  3. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    I'd say that one button auto-zero and one button auto-gain are in the realm of a microcontroller much easier than analog. The offset and gain can be done in analog, but not with one button convenience...well, not very easily. It would quickly get digital to avoid drift problems, and microcontroller chips excel at that job.

    edit: I don't think thousands of samples per second are part of the job when you consider the time it takes a person to press a button at nearly zero light, wait for the PMT to stabilize at full light level and then press another button.
     
  4. greg50

    Thread Starter New Member

    Oct 13, 2011
    4
    0
    The detected signal is not that fast, it's presently being done with an analog meter. You would also only need 2 - 3 readings per second on a DPM, so not very fast at all.

    Any idea of components one would use, given we go down the digital path?

    Thanks,

    Greg
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,014
    3,234
    If you need 5 digit accuracy, that's over 13 bits. If you can settle for slightly less resolution you could use a 12-bit A/D converter, which would give you 1 out of 4096 or around 4 digit accuracy.

    You would run the A/D converter output into a microprocessor to perform the zero and full scale calculations and calibration.

    If you can use a PC then it would be relatively simple to use an A/D USB measurement module such as this, and use the PC to perform the calculations.
     
  6. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi Greg,

    There are 2 digital approches.

    One...
    PMT ---> Trans Z Amp ---> Inverting buffer ---> DPM & PIC

    The inverting buffer has two digital potentiometers used with it. One adjusts the gain and the other adjusts the offset to zero. The pots are controlled by a PIC MPU from microchip.com via a digital serial link such as SPI or I2C. The PIC also reads the "zero" and "cal" push buttons. The PIC also has a 12 bit A2D built in for reading the buffer output during the calibration procedure.

    Two...
    PMT ---> Trans Z Amp ---> PIC ---> Digital Display
    Same as one except you don't need the DPM, or the digital pots. Since the PIC has to do the A2D conversion, it may as well do the reading during normal operation, zeroing, & calibrating also. You just need a 5 digit display of some sort, or send the data to a PC.

    Which would work best is up to you. Hopefully, someone will recommend which PIC would be best. Have you done any programming before?

    Regards,
    Ifixit
     
    greg50 likes this.
  7. greg50

    Thread Starter New Member

    Oct 13, 2011
    4
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    Thanks Ifixit, that all makes sense. I've done VB/VBS programming but never PIC or a microcontroller. Any suggestions on some reading material(s) or the type of PIC would be good.

    Would you class this as an easy PIC job or would it be an advanced type of job? Just trying to get a feel for it:)

    Greg
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    Hi Greg,

    While anything is possible with either analog or digital, I have a few questions. You already have the system working with an analog meter.

    1. Why do you need to zero the meter? The PMT output should be reasonably linear down to zero. How much error are you getting at zero?

    2. Is your analog meter an ammeter or voltmeter? I presume the AD549 is converting the PMT current into a voltage. You can add a voltage amplifier/buffer to drive a DPM. All you would need is a single pot for GAIN calibration. No push-button is needed.

    If required, you can add a ZERO offset pot to the opamp.
     
  9. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi Greg,

    I have mpu programming experience, but not with the PIC. Looks like it would be fun and fairly easy to do in basic or C. The are forums that specialize in this where you can likely get some help.

    Use google to learn more about the PIC. Here is a link that looks like an interesting place to start regarding programming in basic...
    http://www.best-microcontroller-projects.com/pic-basic.html

    Regards,
    Ifixit
     
  10. greg50

    Thread Starter New Member

    Oct 13, 2011
    4
    0
    The problem is the about of light hitting the PMT when you need to set 100 is never the same. Hence the gain is always different the therefore the offset. On the analog version I'm just introducing an offset into the AD549 to zero its output. Unfortunately pressure from the opposition and marketing dictates we need to go digital with our design.

    Yes the AD549 is converting the PMT current into voltage. The analog meter is an ammeter but could have been a voltmeter if required. As I said above marketing dictates we must have a zero and 100 button.

    Hope this makes sense.

    Greg
     
  11. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    Aha. So if this is a commercial product then the solution is clear. Go digital with an MCU. You will have to decide whether to go with LCD or LED displays. Large numeric displays are available in either LCD or LED while LCD alpha-numeric displays are easier to interface with an MCU.
     
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