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I have problem when using OPA657 as a transimpedance amplifer, please help me.

Guangjun Gao
Prodigy 65 points

Other Parts Discussed in Thread: OPA657, TINA-TI

Dear all,

 

I am trying to use OPA657 as a transimpedance amplifier but recently I met problem.

 

I current signal from APD is about 0.2uA (during my experiment, the current signal has very low frequency, almost DC). I set up the circuit according to the figure on the cover of OPA657 user manual, but I found the output signal on the pin6 is fixed on 0.9 V and without change even when I change the input light intensity of the APD.

the power +vs I used is 5v. Please help me on this problem, thank you.

 

Another phenomenon, the above phenomenon happened when I connect the Ground of Puwer supply to the Ground of amplifier (Pin 3 and Pin4). When I disconnect the Pin3&4 with power supply ground, the signal obversed from osloscope (1M impedance) can change with the intensity of inlut light. However in the later case, it seems that the amplifier does not work.

  • 0
    TI__Expert 3595 points

    What supply rails are you using.  If you are connecting pin 3 to pin 4 that is wrong.  Pin four is -Vs not ground.  From the data sheet it looks like you can not bring the input within at least 1 volt from the negative supply.  Look at the sample circuits on page 10 and beyond.  The ground is 1/2 way between the +5 V and -5 V supplies.

  • 0
    Genius 5430 points

    Guangjun;

    I think your problem is probably that you are using that circuit on a single supply. Try using +/-5V.

  • 0 in reply to Chris M
    Prodigy 65 points

    Dear Chris and Neil,

    Thank you all for your kind help. I change the power supply into +5 V and -5 V, and the phenomenon is still there, the following is my connection chart:

    For your reference , the following is the Pin chart of OPA657,

    Thank you.

  • 0 in reply to Guangjun Gao
    Genius 5430 points

    Guangjun;

    The feedback resistor & capacitor should connect to the output pin, pin 6, not pin7, the +5V supply pin.

    Unless your photodiode has very high capacitance, I think you will find that your circuit will oscillate. Try simulating it with TINA-TI.

  • 0 in reply to Neil Albaugh
    TI__Prodigy 80 points

    Guangjun;

    Just need to add the fact the impedance at pin 2 and 3 are very high and the wires need to be as short as possible as well as the feedback resistor connection to pin 2 to avoid noise or other picked-up signals there.
    Another thing is the importance of having the power supply decoupled with 1uF AND some good 10nF. If not, all power wires are inductive and will cause voltage drop (or raise) at Vcc when the output moves up and down. The energy needs to be stored close to Vcc/Vee to avoid the influence of power wires or copper in PCB's ans so on.
    If there are long wires to Vcc/Vee there are also possible the will be a too big voltage spice there sometimes. To protect the Vcc/Vee input a transient protection is required. Transient-diodes or a fast diode are good here. The decoupling is also helping ro minimize the issue.

    All these components help you to reach a higher performance in your design.

    Good luck,
    B-O

  • 0 in reply to Bernt-Olov Hellstroem
    Prodigy 65 points

    Thank you Neil and Bernt,

    I will try to modify the experimental setup according to your suggestion. The mistaken on the connection chart (Pin7 to feedback resistor) is my fault when drawing the chart. I correct the chart as follow:

    T

    Thank you all for your suggestion.

    Guangjun

  • 0 in reply to Guangjun Gao
    TI__Expert 4950 points

    Guangjun,

    Please try to simulate the circuit.  With 20pF feedback (compensation capacitance), the source capacitance needs to be greater than 240pF to achieve unconditional stability.  The OPA657 is stable for gains greater than 7V/V with flat response for gain > +10V/V.

    Also, to connect to the oscilloscope, the OPA657 can drive 100ohm fairly easily.  I would recommend using a 50ohm termination resistor into the 50ohm load of the scope.

    The pole formed by the feedback component is at ~8kHz, reducing the feedback capacitor will increase the bandiwdth as well.