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AD8397 : Clarification regarding output swing range

Category: Datasheet/Specs
Product Number: AD8397

Hi Analog, I hope you are having a pleasant day !!

Description of the issue:

The reason I am here is to get some clarification regarding the output swing range of AD8397 full-swing Opamp. I have simulated a simple Voltage follower circuit in LTspice to check the maximum and minimum Voltage at the output. Below mentioned are the configuration and observations made.

  1.  I have used "ad8397.cir " model for simulation. I got this model from the product info page of AD8397
  2. Supply of +5V and - 5V are given to VCC and VEE respectively
  3. Input is a 1000Hz sine wave with 0 DC offset and 10VPP
  4. During the positive half cycle the output clips at 4.57 V
  5. During the negative half cycle the output clips at -3.64 V

Query:

  1. In datasheet, it is mentioned "Output swing to within 0.5 V of supply rails"
  2. But in simulation the observations made were different.
  3. Please let me know if the simulation result is wrong Or if is there something more I need to know.
  4. I have attached screenshots of the simulation result for your reference.

Thanks,

Sameep Pai K S



Added product number and category
[edited by: GenevaCooper at 2:03 PM (GMT -5) on 10 Feb 2023]

Top Replies

    •  Analog Employees 
    Mar 5, 2023 in reply to ZoltanDTI +1 verified

    Hello ,

    I have run some tests in the lab using your setup and here are the results:

    Dual Supply: Vs = +/- 5 V

    Input = 1kHz, 10Vpp

     

    Output:

     

    Single Supply: Vs = + 5 V

    Input = 1kHz, 5Vpp, 2.5 V…

Parents
  • Hi,

    I think it can be due to the input stage. Here is a sentence from the datasheet: "When operating the AD8397 in a unity-gain configuration, the output does not swing to the rails and is constrained by the H-bridge input." and another one: "To avoid overdriving the input and to realize the full swing afforded by the rail-to-rail output stage, use the amplifier in a gain of two or greater. "

    What is strange: the input common mode range is not specified in the datasheet. Anyway, I think one should follow the recpmmendation: use gain of at least two if you want to get the rated output swing.

    Zoltan

  • Hi Zoltan,

    Your swift response is highly appreciated.

    Thanks for pointing out that in order to get the rated rail-to-rail swing the opamp must be operated with a gain of at least 2

    But this Question initially aroused within us when we faced an Issue while simulating the Filter circuit for AD2S1210.To keep the story short I just simulated a voltage follower and raised the query. Below I have mentioned the Query and what we are looking for in detail

    • We are using AD2S1210  in our design. For the excitation Coil output of this IC, You recommend using a filter circuit
    • This was also put forth by you in circuit note CN0276 
      • In this, AD8692 is used in the first stage and AD8397 is used in the Second Stage
      • To get a TYP VPP of 4.60 V output from AD8397(This is the voltage level we are interested in), the AD8397 is configured at 1.28 gain
      • The same circuit is simulated and we observed clipping at ~1.6 V
      • The screenshots of the simulations are included below. Please have it for your reference.

    Query:

    • Can you please let us know if the observation made in the simulation is correct
      • if yes please confirm whether it is supposed to be clipped off at 1.6 V in the actual circuit and still have no effect on the performance of AD2S1210
    • Also, in an earlier question, I asked :
      • Can VCC and VEE be provided with different voltages?(Example :VCC=12V and VEE = -5V).
      • If Yes, would this configuration have any long-term reliability issues

    Thanks,

    Sameep Pai

  • Hi,

    Can you please let us know if the observation made in the simulation is correct

    I can only guess if it is correct, since the input common mode voltage range specification is seriously missing in the datasheet. So ask an ADI employee,   to update the datasheet or at least provide this information.

    Can VCC and VEE be provided with different voltages?(Example :VCC=12V and VEE = -5V).

    The opamp do not have a ground terminal, therefore only VCC-VEE matters. Of course, the signals are ground referenced, therefore the input and output voltages are limited by VCC and VEE. The clipping problem will probably disappear, if you apply negatíve VEE: Give it a try in the simulation.

    would this configuration have any long-term reliability issue

    VCC-VEE is well within the absolute maximum ratings, so it is safe to operate the opamp with these supplies. Another very strange fact that the input voltage range is not mentioned in the absolute maximum ratings. 

    no effect on the performance of AD2S1210

    I don't really understand what do you mean. What kind of effect could it have?

    Just a question: what is the minimum and maximum voltage at the AD8692 output? What should be the corresponding minimum and maximum at the output of the AD8397?

    Zoltan

  • Hello ,

    I have run some tests in the lab using your setup and here are the results:

    Dual Supply: Vs = +/- 5 V

    Input = 1kHz, 10Vpp

     

    Output:

     

    Single Supply: Vs = + 5 V

    Input = 1kHz, 5Vpp, 2.5 V DC offset

    Output:

     

    Based on the results above, at gain of 1, the part is still able to provide an output within 0.5 V of Supply Rails.  The statement "When operating the AD8397 in a unity-gain configuration, the output does not swing to the rails and is constrained by the H-bridge input", I think this is the reason why there is 0.5 V output head room/foot room from the supply stated on the data sheet. At gain of 1, the output should be still within 0.5 V of the Supply rails as can be seen on the results above. 

    I do agree with  that in order "To avoid overdriving the input and to realize the full swing afforded by the rail-to-rail output stage, use the amplifier in a gain of two or greater. "

    At gain of 2, the part should be able to provide a rail-to-rail output. Please see the simulation below:




    In conclusion, there seems to be a problem with the spice model when configured in Unity Gain (Gain = 1). I will be reporting this issue to the LTSpice Service Desk in order for it to get fixed.

    Regards,
    Paul

  • Hello,

    Just getting back on this. There is an existing FAQ which contains information about the Input Voltage Range of AD8397. Please see this link: AD8397 input voltage ranges datasheet Rev A - Documents - Amplifiers - EngineerZone (analog.com)

    I hope this helps.

    Regards,
    Paul

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