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OPA615: Transient: Effect of 5 mV @OPA615

user5163865
Genius 3570 points
Part Number: OPA615
Other Parts Discussed in Thread: TINA-TI

Hi,

I have some questions about OPA615.

Our customers are considering OPA615, but they have some concerns.

Q1. According to the datasheet, Transient: 5 mV. How does this specification affect the customer system?

      Will transients multiplied by gain appear in the output after entering hold mode and sample mode?

Q2. Is it no problem if the duty ratio of sample mode and hold mode is not 1: 1? Customer's requirement specification is extremely long hold mode( sample=5% ,hold =95%).

Q3.  Customer inputs a signal of 50 MHz to 500 MHz. Also change OTA to low bias OPAMP to minimize leakage.

      Therefore, it is necessary to know the closed loop bandwidth of SOTA. Figure 30 is the result of measuring with the following circuit?

      I tried AC analysis with TINA and found the same result.

Could you please support me?

Thank you.

Hiroshi

(Gain=1, always sample mode,)

  • 0
    Genius 3570 points

     

    Sorry, Test result and circuit is below.

  • 0 in reply to user5163865
    Guru 140200 points
    Hi User,

    a complete schematic would be helpful.

    If you have a sample and hold circuit, then the charge injection must be taken into account. This is explained on page 15 of datasheet of OPA615.

    Kai
  • 0 in reply to kai klaas69
    Genius 3570 points

    I'm sorry for the late reply.
    I tested the DC input according to your advice.
    As a result, DC shift appears. But it looks bigger than the calculation.

    Chold=47p, ΔQ=40fC(OPA615 spec)

    ΔV=47/40 = 1.175mV But,  measured TINA-TI : 4.17mV

    In addition, customers measured on the evaluation board.
    Variations can also be seen in the DC shift.

    The customer's request is within 1 mV of error including transient.

    Can TI provide a means to solve this problem?
    They must quickly decide whether or not this IC can be used.

    I am sorry to trouble you because I do not understand enough about this IC.

    *Simulation result

    Test circuit

    Customer test result

    Hold cap = 100pF, Output OPAMP:LTC6268(Low bias OPAMP)

    Thank you.

    Hiroshi

  • 0 in reply to user5163865
    Guru 140200 points
    Hi Hiroshi,

    what happens if you decrease the amplitude of hold _control signal? TTL-level is enough. You don't need a 0 to 5V swing. So, you could decrease the signal swing to 0...3V.

    What happens if you increase the settling time of this control signal a bit? Do you need extreme steep edges?

    Nevertheless, I think that an error of less than 1mV including transient is unreaslistic for this sort of circuit. Even an error of 5mV would be great.

    Kai
  • 0 in reply to kai klaas69
    Genius 3570 points

    Hi, Kai-san

    I am grateful for your advice.

    The improved circuit has DC offset of about 1 mV. As you pointed out, I think it is difficult to realize this result with actual circuits.

    * Hold Cap : 76pF, AMP Gain :x2, Output Attenuator : 1/2, -->Total gain : x1

    But I will try it on the evaluation board.

    Thank you for your support !

    Hiroshi