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OPA541: Input Signal Conditioning for a Voltage Follower Configuration

Tim Herbison
Prodigy 145 points
Part Number: OPA541
Other Parts Discussed in Thread: REF200,

Would anyone know if the Burr Brown REF200 100 uA Current Source IC and .01 uF capacitor would be needed for conditioning the 50 ohm output of a Keysight 32210 Function Generator.

Planning to use it to amplify the current using an output of 10 hertz, 5V0-pk square wave.

Application is being used to drive a pair of Helmholtz coils which require a unipolar 1 Apk square wave of current.

Resurrecting an old design and am unsure if this signal conditioning is needed as a 555 timer was used previously.

  • 0
    Guru 140200 points
    Hi Tim,

    can you show a schematic?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 145 points

    Sure thing. Here is the schematic.

  • 0 in reply to Tim Herbison
    Guru 140200 points

    Hi Tim,

    can't see the schematic... :-)

    Can you post it again? Click the "Insert Code, Attach Files and more..." button. Then click the "Insert File" button in the header.

    Kai

  • 0 in reply to Tim Herbison
    Guru 140200 points
    Hi Tim,

    the REF200 looks a bit weird in the schematic. Its only effect is to shift the output signal by an DC offset voltage of 200µA x 50R = 5mV. Hmm, it should not be necessary in combination with the OPA541. Maybe it was used in conjunction with a different circuit?

    From my feel I would run the OPA541 with a small gain of let's say factor "2". This can be helpful when the output of OPA541 sees a complex load. And as the Helmholtz coil is an inductance, it's a good idea to use the protection scheme shown in figure 13 of datasheet.

    What is the 0R4 resistor for? Is it a current limiting resistor because the Helmholtz coil is very low ohmic? What is the resistance of the Helmholtz coil? And what is its inductance?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 145 points

    Thank you Kai.

    The previous has been added to the attached word file and depicts it use with a 555 timer, LCC110 photo diode, and  +5V, and -5 V rails.

    The Helmholtz coils will be the same and have a input resistance of 5.6 ohms and inductance of 6 uH.

    The resistor shown in the output is not shown correctly but is intended to be the current limit resistor RCL presented in the datasheet.

    Its value of 0.4 ohms is based on equation 1 of the datasheet using a 2 A limit.

    Figure 13 recommends fast recovery diodes but does not recommend a part number.

    Kindly recommend a fast recovery diode.

    Also please confirm the filter capacitors chosen.

    Their values are based on 50 KHz and 50 MHz conducted susceptibility requirements and the "Rule of One" commonly used for AF and RF electromagnetic compatibility.

    Also need a recommendation on heat sink.

    The note recommends a Waekfield-Vette 345 series I trust will be meet the needs we have at zero g and 30 C max ambient air and 1 A max and 5 V or 5 W max continuous current draw.

    Finally please confirm that the TO-3 package chosen is not recommended and confirm that a TO-220 package would is recommended.

    I have also added 100 KOHM and 50 KOHM feedback 1/4 W Input resistors and IN4003 diodes.

    7737.TI Tech Community Requested Schematic.docx

  • 0 in reply to Tim Herbison
    TI__Guru* 93105 points

    Hi Tim,

    Figure 13 recommends fast recovery diodes but does not recommend a part number. Kindly recommend a fast recovery diode.

    ES3BB-13-F

    Additionally, it is recommended that transient voltage suppressor diodes be added from each supply pin to ground as shown here:

    The TVS reverse standoff voltage (VR) needs to be somewhat higher than the supply voltage levels you intend to use. You can get more information about TVS diodes at this website:

    https://www.littelfuse.com/products/tvs-diodes/surface-mount/1ksmb.aspx

    Also please confirm the filter capacitors chosen.

    I presume you are speaking about the OPA541 power supply bypass capacitors? If so, 100 nF ceramic capacitors connected directly from the OPA541 +V and -V pins to ground. Larger capacitance 10 uF capacitors are recommended on those power supply lines as well, but they can be located more distant from the +V and -V pins.

    Also need a recommendation on heat sink.

    The datasheet recommends the Waekfield-Vette 345 series heat sink as you noted. That heat sink is recommended for high power applications where the OPA541 must dissipate many watts of power. A complete thermal analysis associated with the OPA541 operating conditions would be needed to see if something less substantial could be used for the heat sink.

    Finally please confirm that the TO-3 package chosen is not recommended and confirm that a TO-220 package would is recommended.

    The OPA541 in the TO-3 package is not recommended for new designs (NRND). That means that version of the OPA541 will cease being available at some point in the not too distant future. Any new design should use the OPA541 in the 11-pin TO-220 package.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

    Hi Tim,

    Figure 13 recommends fast recovery diodes but does not recommend a part number. Kindly recommend a fast recovery diode.

    ES3BB-13-F

    Additionally, it is recommended that transient voltage suppressor diodes be added from each supply pin to ground.

    Also please confirm the filter capacitors chosen.

    I presume you are speaking about the OPA541 power supply bypass capacitors? If so, 100 nF ceramic capacitors connected directly from the OPA541 V+ and V- pins to ground. Larger capacitance 10 uF capacitors are recommended on those power supply lines as well, but they can be located more distant from the V+ and V- pins.

  • 0 in reply to Tim Herbison
    Guru 140200 points
    Hi Tim,

    I find the original circuit with the 555 not all that bad. It has the advantage, that the opto provides a signal which truly swings from 0V to a reference voltage. If you use a function generator, on the other hand, you would need to adjust the offset voltage whenever you change the amplitude to get a true unipolar signal. This will become annoying after a while. :-) The REF200 isn't necessary, though. A simple pull-up resistor would also do the job.

    I would wire the OPA541 with a gain of +2. Use two 10k resistors to accomplish this, one connected from the -input of OPA541 to GND and the other from the -input to the output. Then, connect a 22pF phase lead capacitance in parallel to the 10k resistor which is connected from the -input to the output. This will stabilize the OPA541 by improving its phase margin.

    Figure 13 of datasheet recommends the 1N4003 for light inductive loads. I think this diode should work. Or use the diode Thomas has suggested.

    Tell us, whether you decide to use the function generator option or the 555 circuit in future.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 145 points
    Thank you Kai and Kevin.

    As you can see it has been a while since I have been a designer and you have been most patient answering my list of demands.
    The 555 circuit is being discarded as variable duty cycle is required by the project scientists. We looked briefly at its modification but it was too awkward and time consuming and also would be difficult for the astronauts to adjust.

    All the Best
    Tim Herbison
    EE
    Zin Technologies INC.