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OPA454: Level shift

Part Number: OPA454
Other Parts Discussed in Thread: LM339, OPA462, TINA-TI

Hallo, I am trying to create an high voltage, high gain level shifting circuit.

This should be able to raise the waveform's amplitude to an higher level.

I've chosen the OPA454 to do the job, in its non-inverting amplifier configuration, but I'm experiencing some issues during my tests.

My needs are as following: given a digital input (different cases, from different MCUs, so 3.3 to 5.0 volts), square wave PWM, 50% duty cycle, variable frequency (1 kHz to 200 kHz, user selectable) and providing a source of 100VDC (10-100V, user selectable), I would like to have , as output, a PWM which have to be identical in the waveform (frequency, duty cycle, square shape), but with the sample amplitude as fed by the source (so 10-100V, as decided by the user).

The input voltage is varied externally to the op-amp, the same for the frequency and the DTC (which are controlled by the MCUs).

The desired output current doesn't exceed 40 mA.

I thought that it was simply to accomplish because I've previously and successfully done a similar thing with an LM339 (with lower voltages as a source, of course) but the OPA454 has the E/D + Status pins that I can't figure out how to manage correctly (if those are required).

The main, and dangerous, issue I have is the identification of the pin 1: there is NO dot on my SOP8 packages (Texas Instruments original op-amps)!!! On the chip there is a vertical line followed by the chip name, the brand (TI) and some other text.

The second doubt I have is related to the use of the pull-up resistor (1 MOhm, as per the specifications on the datasheet), between the V+ and the Status pins...

Finally the E/D (Enable/Disable) feature: is it mandatory? I've read, on the datasheet, that these pins can be left floating/unconnected...

Last, but not least, at the output, I need to connect an inductive load which has - as expected - huge back-emf spikes (this is by design and these spikes has NOT to be suppressed), so my concern is not to damage the OPA454 with such spikes.

Any help will be appreciated a lot, thank you in advance to everyone.

Fabrizio, Rome - Italy

fabrizioricciarelli@gmail.com

P.S.: I'm not an electronic engineering, only an enthusiastic hobbiest, dedicated to the alternative energies research

  • Hello Fabrizio,

    If you can provide your OPA454 circuit diagram with the power supply, input drive and output load information indicated that might help us come to resolution faster.

    I suspect that from your description that you may using a single polarity, HV supply? If that is the case and the input PWM is from 0 V to 3.3 V or 5 V peak, then the common-mode input voltage range of the OPA454 would be violated on the low end and the results won't be as expected. So please do clarify how you are powering the OPA454.

    The datasheet in Section 9.3.6, ENABLE and E/D Com, does explain that those pins can be left floating and the OPA454 will be enabled. There is an explanation how a fast, negative going pulse coupled to the E/D pin might affect if the enable/shutdown function causing it to go into shutdown. That might not be a common condition for many circuits. Connecting the E/D pin Com and E/D pin as indicated in the Electrical Characteristics table is a robust way to assure proper operation.

    The datasheet indicates that E/D common when not floated has a voltage range of (V-) to (V+) - 5 V. So it has quite the range that it can be biased at. For the OPA454 is to be in the enabled state the E/D pin requires a voltage of the E/D common + 2.5 V, to E/D Common + 5 V. To disable the OPA454 the E/D pin needs to be biased with a voltage equal to the E/D Common level, to a maximum of E/D Common + 0.65 V. I hope that makes it clearer.

    Do note that once you do have the OPA454 PWM circuit functioning correctly that you will not be able to amplify a frequency as high as 100 kHZ, to 100 Vpk-pk. The OPA454 has a slew rate of 13 V/us and that means for a 50 Vpk output signal the maximum frequency that can be sustained will be about 42 kHz. You can get closer to your maximum frequency goal with the new OPA462, which has a slew rate of 25 V/us. Here's a link to the OPA462 datasheet:

    http://www.ti.com/lit/ds/symlink/opa462.pdf

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Hallo Thomas,

    many thanks for your kind and absolutely fast answer!

    You're right: I'm using a single polarity configuration, 10-100V (hooked to V+) and 0V (hooked to V-) and, for the sake of simplicity, I wouldn't use a dual voltage rail (is there any valid alternative?).

    I can confirm that my PWM is usually in the range of 0-3.3V pk-pk / 0-5.0 V pk-pk (hooked to VIN).

    The ground of the MCU is hooked to the "virtual" ground of the OPA454 (which is, essentially, the opposite end of the 100nF capacitor hooked to V-).

    As preferred, and suggested, the robust way is always the better so I would love to understand how to implement correctly the E/D function (and the Status flag, if needed in some way).

    My intuition (but I absolutely may be wrong) is to connect in parallel the VIN pin with the ED pin so they will be fed at the same time with the PWM signal then, in the same way, connect in parallel the ED common with the ground of the PWM (the MCU's ground).

    My circuit diagram is identical to the one provided in the datasheet (ref.: www.ti.com/.../opa454.pdf page 27, Figure 72. Basic Noninverting Amplifier Configuration), but without the E/D, E/D common and status pins that I left unconnected.

    I connected a scope probe across the RL (I used a 5k resistor) to see the results.

    Unfortunately, varying the voltage input, I obtain only a STRAIGHT constant increasing voltage (50V RMS, 0V pk-pk), almost identical to the one provided at V+, NOT a PWM (0-50V pk-pk)!

    I really appreciated your suggestion about the OPA462 and I'm aware about the slew rate limitations but, to obtain higher voltage values, I've been intrigued by the voltage boost schematic (ref.: www.ti.com/.../opa454.pdf page 24, Figure 67. Typical noninverting application with output voltage boost)... does such configuration have frequency restrictions? I mean: the above mentioned voltage boost is able to output 195Vpp with a swing as high as 100kHz, like the single component?

    But let's go ahead step by step: can you suggest a reference, or provide a schematic with the correct connections to attain my needs? (with particular attention to the E/D and Status pins)

    If the dual voltage supply is mandatory, I can - with some efforts - implement one of the following solutions (changing the component's values as needed):

    www.google.com/url

    www.google.com/url

    Thank you for your precious time and sorry if I wasted it a lot!

    Fabrizio

  • Hello Fabizio,

    An issue you will run into running the OPA454 single supply is the common-mode input voltage (Vcm) range and output voltage (Vo) swing range limitations. The Vcm range for linear operation is (V–) + 2.5 to (V+) – 2.5 V. Therefore, if you use a single +100 V supply, the Vcm range is +2.5 V to +97.5 V. If you apply an input voltage of 0 V to +3.3 V, or 0 V to + 5 V, you see that any level below +2.5 V is outside the linear Vcm input range.

    Similarly, the Vo range limits apply as well. For example, from the Electrical Characteristics table Vo with RL = 4.8 kΩ, AOL ≥ 100 dB, and Io = 10 mA, is (V–) + 1 V to (V+) – 2 V. I think you can see that if the output is driven to its most negative swing level it will hit its rail at about +1 V above ground when a single +100 V supply is employed. When a negative supply is added such as -5 V, then the low end range for Vcm and Vo moves to -2.5 V and -4 V, respectively. If the negative supply is added the positive voltage must be reduced to keep the total supply voltage across the OPA454 to safe levels.

    "I've been intrigued by the voltage boost schematic (ref.: www.ti.com/.../opa454.pdf page 24, Figure 67. Typical noninverting application with output voltage boost)... does such configuration have frequency restrictions? I mean: the above mentioned voltage boost is able to output 195Vpp with a swing as high as 100kHz, like the single component?"

    Unfortunately, the bandwidth and slew rate of the OPA454 or OPA462 this very high-voltage circuit doesn't change. They are dictated by the Op amp ac electrical characteristics. I ran a simulation on the three Op amp HV circuit and the output was slew rate limited above ~40 kHz.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Hallo Thomas,

    thank you for your excellent explanation: you've been crystal clear (even for a noob like me).

    So I will go for a rail split prior to feed the op-amp.

    I think there is no need to make a perfectly balanced split...isn't it?

    I mean: 100V splitted into 55V/-45V equals, in functionality for the op-amp, to a 50V/-50V, right?

    The question derive from the many different techniques to do the split: the more precise - steady and stable - split requires a more complex circuitry.

    The simplest, compact way I see is the one made of a resistor divider and a couple of caps, like this one:

    It is not the better way, I'm aware of this, but it can be a start for my testing purposes.

    So ok, we have a dual power supply, now what about the E/D?

    Starting from my new splitted power source what I have to apply to such pins?

    Did you take in consideration my proposal to put in parallel the E/D pins with my PWM (as described in my previous post)?

    As usual, thank you so much.

    Fabrizio

  • Hi Fabrizio,

    Correct, the OPA454 power supplies may be unevenly split. For example, the supply can be +50 V/-50 V or +95 V/-5 V, or other combinations. Just keep in mind the Vcm and Vo ranges are relative to the supply levels you intend to use.

    The E/D voltage levels will be per the characteristic table levels which are referenced to the V- supply voltage level. Previously, I provided the example of what they would be if V- is set to -5 V. If you use -6 V for V-, then Vcm on the low end would be (-6 V) + 2.5 V, or -3.5 V. A 0 to 3.3 Vpk, or 0 to 5 Vpk PWM signal would now be within the Vcm range.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Good morning Thomas and thank you again for your patience!

    Assuming I would apply a -50V to V-, the Vcm end would be (-50) + 2.5 = -47.5V ... how a 0 to 5V PWM signal could be within the Vcm range??? Which is such Vcm range?

    You have to excuse me but, as specified in my first post, I'm only an electronic enthusiast, NOT an engineer and this is my first experience with a dual source op-amps (plus an E/D function...), that's why I asked for a simple schematic to refer to.

    May you, please, specify the exact voltage values do I have to apply to the E/D pins once I have: PWM = 0-5V pk-pk, V+ = +50V, V- = -50V ?

    Last important thing is: if the PWM changes from 0-5V pk-pk to 0-3.3V pk-pk AND/OR the splitted voltage varies - say, from +50/-50 to +30/-30 - AND/OR the frequency changes from 30kHz to 15kHz, DOES the value applied to the E/D pins HAVE to change accordingly or it HAS to remain unchanged?

    I can't figure out what is the element to which the E/D is related to...

    Many thanks in advance

    Fabrizio

  • Hi Fabrizio,

    If the OPA454 is powered from +50 V/-50 V supplies and the Vcm range is rated (V-)+2.5 V to (V+)-2.5 V, the Vcm input range is then -47.5 V to + 47.5 V. Therefore, when the applied input voltage is 0 V, the Vcm is at its mid-range level half way between -47.5 V to + 47.5 V. When the applied input voltage is at the +3.3 V peak,it is just that amount above the midpoint in the Vcm range. In either case, those levels are well within the OPA454 Vcm range.

    Regarding the OPA454 E/D operation I show a TINA Spice example circuit where the E/D com pin is set to V- (-50 V), and the E/D pin is biased 3 V above E/D com. In that case the E/D pin would be at -47 V. This would place the OPA454 in the enabled, output on state. Maybe this will better illustrate the E/D comm and E/D pin biasing required.

    I've included my TINA Spice file if you would like to simulate the circuit.

    OPA454_non_inv_01.TSC

    Regards, Thomas

  • Hi Thomas,

    as usual, many thanks!

    After having examined your schematic I've been able to test it successfully but, being unclear to me how to apply a voltage at VED, I decided to feed such pin the way previously announced: I've put the VIN pin in parallel with the E/D pin, fed it with my PWM signal, and it works like a charm!

    I suppose that the schematic symbols (absolutely new to me) made of an arrow and labeled as "AM" are ammeters... I derived my intuition from the fact that, in the graphic representation of the waveforms to the right, the values should be mA...

    My circuit simulation software is NI Multisim, so I'm not so familiar with Tina-TI (even if I made use of it in the past).

    Anyway, for the moment, you gave a great help to me: thank you Thomas!

    Now I will try to proceed with a voltage booster and, in case of any issues, I'll open a new post specific for it.

    You are a great professional, thank you again

    Fabrizio