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output of fully differential amp THS4502.

nilesh mate
Prodigy 30 points
Other Parts Discussed in Thread: ADS5424, THS4502, THS4501

i am operating the fully differential op-amp THS4502  with single supply(5V)  to interface it with ADS5424. the input sine wave available has an amplitude of 0-10V . this input is given to the non-inverting input through a voltage divider of suitable values. The inverting terminal is being given a variable voltage reference using LM336(2.5V) and a voltage divider in order to shift the output .

the circuit diagram is attached. also we have externally supplied 2.4V to the Vocm pin.

the problem that i am facing is that the inputs of the op-amp starts drawing excess of current because of which the required voltage drop at the inputs increases and i am not being able to get the required waveform. it has happened twice, once with inverting terminal and once at the non-inverting terminal. and when i remove the op-amp and check the voltages at the voltage dividers(both inverting and non-inverting) they come to be exact as calculated.

So is the op-amp damaged? if yes, then what could be the reason for the damage? if no, what is wrong in the circuit that is causing this excess voltage drop.

So kindly help me deal with this problem. Any kind of assistance will be highly appreciated.

Regards,

nilesh.

  • 0
    TI__Genius 17905 points

    Nilesh,

    I have several questions about your circuit:

    The op amp may not have been damaged - what excess input current are you measuring and how are you measuring the 'excess' current being drawn by the op amp inputs? I might expect damage if the input pins are drawing more than 10-30mA continuous current for extended periods of time (seconds).

    Do you mean that the circuit works as expected until some event (input signal change, time, etc)? Do you know what change leads to this issue? When the circuit is in this 'excess current' state, what is the input signal amplitude, pot resistance, voltage at + and - pins, voltage at Vout+ and Vout-? This may help identify the issue.

    Is the 0-10V amplitude signal referenced to ground (i.e. bipolar signal)? Is the intention of the circuit to be able to adjust the differential DC offset of the differential output?

    I attach a TINA-TI schematic that shows my recommended approach to adding a differential DC offset to the differential output signal. I've assumed that the input signal is bipolar and referenced to ground. I used a THS4501, which is essentially the same as the THS4502, except that the THS4501 input common mode range extends to the negative rail. Please take a look. You can adjust the pot resistance Rpot and see how the differential DC offset changes.

    THS4501 3-22-2011.TSC
  • 0 in reply to Kris F
    Prodigy 30 points
    Respected Sir,
                                Thanks for your kind concern to my problem.

    The input signal available  is a unipolar signal i.e it is always referenced to 0V and its maximum output could go upto 10V. Regarding the excess current issue, the output of the potential divider is coming to be more than the calculated value with the op-amp in the circuit, whereas it comes to be equal to the calculated value when it is removed and hence i suspect that the input could be drawing excess current. 

    The circuit gives me output waveform, but the dc shift is not the expected one( due to the wrong potentials at the input i guess). so even if the input conditions are kept constant the output obtained is not the desired one.

    My intention of using the op-amp is to provide the correct input signal to the adc i.e ADS5424. The ADC requires a differential input which is symmetric about 2.4V dc level(Vref) and the signal should be of 2.2Vp-p which means that at both the input pins Ain+ and Ain- the maximum and minimum peaks of sine wave are 2.95V and 1.85V i.e 1.1Vp-p so as to give 2.2V differential signal. As the input available is too large in amplitude i have used a voltage divider so as to scale down from 10V to 2.2V.
    Then i have tried to give a reference voltage(ideally 1.1V) in order to shift the input so that the signal swings from +1.1V to -1.1V differentially. And then as mentioned in the datasheet of THS4502 the output signal will be symmetric around the Vocm voltage.

    Thanks for the schematic. But the input signal available is not the one available at my end. the input sine wave swings from 0V to a maximum of 10V. Also i have given a  single power supply connection of +5V to my circuit.

    Also sir, i have attached my schematic with this mail. This is the exact circuit that i have been testing on hardware with the same inputs as in the schematic. By changing the value of rpot i am getting shift in the waveform. 

    I am trying out this circuit for the first time. So it may so happen that my understanding of device operation could be wrong. So kindly guide me the correct way. I hope i have answered all your questions regarding my circuit correctly. Any further assistance would be highly appreciated.

    Regards,
    Nilesh.

    TH4502.TSC
  • 0 in reply to nilesh mate
    TI__Genius 17905 points

    Hi Nilesh,

    Thanks for the information. I understand your requirements now.

    A fully differential amplifier (FDA) is the same as a regular op amp in that the voltages at the + and - pins are driven to be equal by the negative feedback. Some people like to think of an FDA as two parallel op amps that are each in an inverting configuration. Here is a good application note that discusses FDAs:  Fully Differential Amplifiers - sloa054

    Part 3 of the below article series directly discusses your application, too:

    • Using fully differential op amps as attenuators
      • Part 1 - Differential bipolar input signal - slyt336
      • Part 2 - Single-ended bipolar input signal - slyt341
      • Part 3 - Single-ended unipolar input signal - slyt359

    I've modified the schematic. Based on your schematic, you should be able to make the changes on your board by removing the Zener, shorting out the potentiometer, and changing the resistor values. Please let me know if that helps!

    THS4502 0-10V to 2V2pp 2V4DC - 3-24-2011.TSC