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It is difficult problem

shuai chen
Intellectual 280 points
Other Parts Discussed in Thread: XTR117

Circuit simulation and why not, high input impedance input charge signal output, why not ah, DC bias is wrong

  • 0
    TI__Guru* 93105 points

    Hi Shuai,

    Yes, it appears that your dc bias arrangement is not correct. The 2N7002 and 2SA4127 transistors shouldn't turn on and the 4 mA from the current source must be forced through the series 30 k and 200 k resistors. That would result in a very high voltage being developed across them. The output voltmeter is indicating 0 V, but that may just be a default if the circuit isn't correct. 

    The reason I say the transistors shouldn't be "on" is because of the bias conditions of the 2N7002. The 2N7002 gate threshold voltage is on the order of +1 to +4 V. If the input generator impedance is low, the 200 Ohm resistor connected to the gate provides a low resistance path to the MOSFET's source. The 100 Meg resistor, connected from the 200 Ohm resistor back to the 200 k/30 k resistor divider also acts as a very large voltage divider. The voltage on the gate ends up being microvolts which is completely insufficient to turn on the MOSFET. If the MOSFET is off, there isn't any current flow through the 1.5 k resistor would be needed to bias the PNP base-emitter junction. Therefore, the PNP should remain off too.

    The ac input generator would have to develop a positive dc large enough to overcome the MOSFET threshold voltage for the MOSFET to turn on. When the ac input level falls below the threshold voltage the MOSFET would turn back off.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Intellectual 280 points

    Hi Thomas

    This is my ICP circuit, two wire, a ground wire, a is the output and power line, power 4-20mA, 28-30V power supply, circuit practice I no problem, is my simulation circuit is wrong, do not know where the problem，It is a constant current source is wrong

  • 0 in reply to shuai chen
    TI__Guru 62035 points

    Hi Shuai,

    I would also recommend that you look into some integrated 4-20mA transmitter circuits from Texas Instruments.  You can view through our portfolio here:

    http://www.ti.com/lsds/ti/amplifiers-linear/4-20mA-conditioning-products.page?paramCriteria=no

     

  • 0 in reply to Collin Wells
    Intellectual 280 points

    Hello

    I mean, you may not know, constant current source 4-20mA I, 28V-30V power supply, not the output mode

  • 0 in reply to shuai chen
    TI__Guru* 93105 points

    Hello Shuai,

    I am unable to understand what your circuit applications is. Can you provide a brief explanation?

    Regards, Thomas

    PA - Linear Applicaitons Engineering

  • 0 in reply to Thomas Kuehl
    Intellectual 280 points

    Hello

    Look at my pictures

  • 0 in reply to shuai chen
    TI__Guru 62035 points

    Hi Shuai,

    As we suggested in the post: http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/p/339856/1192345.aspx#1192345

    You will need to use a charge-amplifier to acquire the signal from your charge-based sensor.  Then you will need to derive a DC output voltage based on the sensor output.  This can be accomplished using an absolute value circuit after the charge-amplifier output.  Once you have a circuit that produces a DC output voltage proportional to your sensor output you can connect that voltage to the input of one of our XTR devices, such as the XTR117. 

    I've attached a presentation that discusses charge-amplifiers and includes circuits to convert a capacitive sensor to a DC output.  You will need to design and test the circuit on your own based on the performance and functionality you desire.

    4643.Charge amplifiers_PA.ppt