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OPA 656 with Demoboard DEM-OPA-SOT-1A

Prodigy 30 points

Replies: 5

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Hello together,

i build a transimpedance amplifier for a sensor. Under point five I read in the description of the demoboard that when I use a high impedance sensor I have to place a 100 Ohm resistor on the probe tip. I don't understand where I have to put this resistor excatly?Shall it be parallel or in series?

And I also don't understand the function of the R1 find in the shematic of the description:

http://focus.ti.com/lit/ug/sbou010a/sbou010a.pdf

It would be very kind if somebody can help me.

Kind regards,

Ahmed

  • Ahmed,

    This section of the guide provides recommendations on how to measure the op amp output using a high impedance oscilloscope probe. Due to the capacitive load presented by the oscilloscope probe tip, this 100ohm resistor should be placed in series with the probe to isolate the op amp output from the capacitive load. R1 is used for series termination where the output will be connected to a 50ohm cable and a 50ohm resistor is placed in series with the cable to provide an impedance match between the amplifier and the cable. In your case, if you are measuring the op amp output with an oscilloscope probe, R1 provides a place for that 100ohm isolation resistor.

  • In reply to Kris F:

    Hi Kristoffer,

    thank you for the fast answer. I want to connect the output with a BNC cable to a DAQ Box from National Instruments and measure with LABVIEW.

    At the moment I set the R1 to 0 Ohm because I didn't find a resistor in  Figure 3 of the Datasheet.

    Do you know which measurnebt environment the DAQ Boxes have?

    I have an other question concerning the capitance CD see in the figure 3 in the data sheet of the OPA656.

    What are the advantages of this capacitator? When I use a sensor and not a photodiode, shall a determine the capacitance of it and set a capacitance on R7 of the demoboard?

     

    Kind regards,
    Ahmed

     

     

     

     

  • In reply to Ahmed Mansour:

    Ahmed,

    You should be able to find either an online manual or description of the DAQ module you are using that says the input impedance of the analog inputs. Regardless of whether or not you require matching, you will want to place some resistance, maybe 20-30ohms, at R1 to isolate the amplifier output from the capacitance associated with the cable, otherwise, you may see ringing in the measured waveform.

    The capacitance CD in Figure 3 is not a capacitor that you would place on the board. It is the capacitance of the photodiode. In photodiode transimpedance applications, this diode capacitance is added in parallel with the amplifier's differential and common mode input capacitance to form an equivalent total capacitance at the inverting node. This total capacitance value is used in calculating the required compensation capacitor CF given the gain bandwidth product (GBP) and the desired transimpedance gain (RF) using the first equation on p. 11 of the OPA656 datasheet.

    If not a photodiode, what is your sensor?

     

     

  • In reply to Kris F:

    Hi Kristoffer,

    thanks for the answer.

    For Rf I use a 1 MOhm. With the GBP of 230 MHz and a Sensor capacitance of 185 nF I get for the Cf a capacitance of 16 pF. Because of the capacitance of the resistor I have to set a 14 pF capacitator. Right?

    At the moment I use a 1pF capacitator to have a higher cutoff frequency.

    What are the disadvantage of using a small capacitance for Cf?

    By using LTSpice I simulated the circuit and I get a a cutoff frequency of 150 kOhm by using a 1pF for Cf.

     

    Kind regards,
    Ahmed

  • In reply to Ahmed Mansour:

    Ahmed,

    What sensor are you using and what is the expected current output signal?

    With the large transimpedance gain (1Mohm) and large sensor capacitance of 185nF, the bandwidth will not be very wide, maybe on the order of 10kHz. You are using the OPA656 model in your simulations? How are you modeling the sensor current source and capacitance?

    The value of Cf is important in stabilizing the transimpedance circuit. A balance must be found in the value of Cf, where too small a value can result in a ringing or oscillating output and larger values will reduce the bandwidth. Here is a good application note that discusses the selection of the feedback capacitance in transimpedance amplifier circuits:  Transimpedance Considerations for High-Speed Operational Amplifiers - sboa122

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