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OPA657

Nisha Joshy
Prodigy 160 points
Other Parts Discussed in Thread: OPA657, OPA355, OPA656, OPA380

Hi,

I need to design a transimpedance amplifier with gain greater than 10^7 v/A and banwidth greater than 12 KHz .The photodiode capacitance is 75pF & the current signal is in the range of microamps and there are no negative supplies available. I prefer to use the OPA657 in the single supply mode. In the data sheet,  it is said that a single supply of +8v to +12v can be used with the negative supply grounded.The dark current output of 0V is not of interest. In this case should I apply a voltage to the non inverting input of OPA657 and can I have the same voltage applied to both + input of OPA657 & the anode of the photo diode? A single to differential converter follows the TIA which has an input commom mode range of 200 mV to 3.9 V with a 5V supply.

Thanks in advance,

Nisha

  • 0
    TI__Intellectual 1510 points

    Hello Nisha,

    For your intended design you would have to apply a voltage to the non inverting input of the OP amp due to the use of single supply. As you know this would not be the case with the dual supply since the non inverting input would be at virtual ground; but as you mentioned this is not an option for you as you do not have a negative supply available.

    Also you would be able to apply the same voltage to the + input and the anode of the photodiode as long as it is within the common-mode range of the amplifier.

    For the desired BW and gain, you may be able to use the OPA656, and if you are looking to use a single 5V supply for this you may want to look at the OPA355. 

    Let me know if you have any further concerns; also we can go further into the design if necessary.

    Regards,

    Ranji C. Bhola

    High-Speed Amplifiers

  • 0 in reply to Ranji Bhola
    Prodigy 160 points

    Hi,

    Thanks for the reply.

    I am trying to use an opamp with +5V single supply . Now I am working with OPA380 with 1.5pF feedback capacitor & 10 Mohm resistor.For time being, instead of photodiode signal I am using a current signal (1v ac signal in series with 10Mohm).Both the inverting and noninverting terminals are biased at 2.5V supply (which means when a photodiode is used the cathode  & noninverting terminal will be at the same bias level) and pin 4 is grounded. Will this configuration work ? I am getting very high frequency oscillations at the output... Will decoupling the power supply helps?

    Regards,

    Nisha

  • 0 in reply to Nisha Joshy
    TI__Intellectual 1510 points

    Hello Nisha,

    For the above mentioned configuration, it would be best to add the capacitance of the intended photo-diode either by adding a component or using the photodiode itself; this will help with the stability of the circuit during testing.

    Also, I would recommend decoupling the power supply as this would help narrow that possibility for the oscillation. I would note the frequency of oscillation, that should help determine the cause of the oscillation.

    Unfortunately, I am not as familiar with this device, but I am forwarding this to the team that will be able to assist.

    Regards,

    Ranji C. Bhola

  • 0 in reply to Ranji Bhola
    Guru 21975 points

    Nisha,

    Yes, you should connect a power supply decoupling capacitor from the V+ supply pin to ground (the V- pin in your case) close to the device leads. A 0.1uF ceramic should be adequate.

    Connect the both the 10M resistors (feedback and input resistor to simulate an input current) close to the inverting input terminal. The bias potential connected to the non-inverting input should also be bypassed with a capacitor to ground. Figure 4 in the data sheet shows a good way to do this with a series resistor to avoid coupling noise into this node.

    Regards, Bruce.