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OPA454: Output of the OPA454 (Tina-Ti app)

Part Number: OPA454

Hi,

i wish to use the OPA454 as an non-inverter op-amp.

The input voltage coming from DAC (0V ~ 2.5V) 

the gain in +40.

the supply voltage for V+ is 100V

the supply voltage for V- is 0V (GND)

I'm running the design with TINA-TI and for some reason I get this result:

but when I put the V+ to 95V and V- to (-)5V it work as expected:

So before i start the layout i would like to know if this device can be used for my application.

Thanks 

  • Hello Itay,

    The OPA454 minimum common-mode voltage VCM is specified as (V–) + 2.5 V for linear operation. If you set the V- pin to 0 V and the input voltage is 0 V to +2.5 V as you have in the first example, the input signal falls below the the lower limit of the VCM range. When you move the V- pin down to -5 V, the 0 V to +2.5 V input then resides above the minimum VCM limit and linear operation is attained. You will need to use dual polarity supplies such as you have in the second case if you want to use a ground referenced input voltage, or the input voltage will need a dc level added that brings the input voltage range into the linear VCM range of (V–) + 2.5 V, to  (V+) – 2.5 V.

    If the closed-loop gain is set to +40 V/V and the maximum input is +2.5 V, that would result in a theoretical output voltage of +100 V. The amplifier cannot swing any closer to the supply rail than about (V+) – 1 V, for Io = 1 mA. Make sure you observe the Voltage Output Swing from Rail (Vo) in the Electrical Characteristics table.

    Be sure to properly bias the OPA454 E/D and E/D COM pins in the actual circuit. The datasheet provides the necessary information about biasing these pins.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • In reply to Thomas Kuehl:

    Thank you Thomas for the fast reply.

    Unfortunately I can't use dual supply voltages or a bias DC voltage for the input signal.

    I guess I can't use this op amp for this design.

    Can you recommend on other TI op amp that I can use?

    I don't need high speed or big BW.

    The main characteristics are to be able to get 110 volt in the VCC pin. (Without needs of bias or negetive voltage)

    Thanks

  • In reply to Itay Kalfa17:

    Hello Itay,

    Since you don't have dual supply capability, are you open to level shifting the 0 to +2.5 V input level to a range that is within the OPA454 linear VCM input range of (V–) + 2.5 V, to (V+) – 2.5 V? Ultimately, what output voltage swing range do you require from the OPA454; something such as + 3V to +107 V for example?

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • In reply to Thomas Kuehl:

    Hi Thomas,

    My input voltage (input signal to the op amp) can vary only from 0V to 3.3V (using the DAC from the MSP430F5636). The supply voltage for the op amp is 100V (V+) and GND (V-).

    Will it help if I use first stage amp (with low gain) and then go for the high gain ? 

     

  • In reply to Itay Kalfa17:

    Ok, lets say that I can arrange 3.3V to this board - so I can use LDO to reduce it 2.5V.
    What should I do with the E/D and E/D Com is not clear from the data sheet.

    thanks
  • In reply to Itay Kalfa17:

    Hello Itay,

    I have investigated ways to sum a positive offset with the 0 to +3.3 V input signal, but a problem arises because the the summed  offset voltage voltage is multiplied by the closed-loop gain. That results in the OPA454 minimum output voltage being tens of volts, which likely isn't a usable solution.

    I have developed one promising circuit based that is based on applying the OPA454 as a difference amplifier. The circuit relies on gaining up the input 0 to +3.3 V signal with an OPA196 RIRO op amp. Its output is applied to one input of the OPA454 difference amplifier, while a high reference voltage is applied to the other input. The output phase is inverted from the 0 to +3.3 V input signal so a second OPA454, connected as a gain of -1 V/V inverter.

    The circuit hasn't been optimized, but as it stands it does result in an output that swings from about +2 V to +98 V, with a 0 to +3.3 V ground-referenced input. You can see the preliminary circuit below. Note that I used a sine wave input so any clipping would be evident in the simulations.

    If this appears to be of interest I can further refine it for a more practical circuit. Right now it is using a number of voltages sources that could be reduced to something powered off the +100 V supply.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • In reply to Itay Kalfa17:

    Hi Itay,

    The E/D and E/D COM pins are discussed in the OPA454 datasheet Feature Description section, 9.3.6 ENABLE and E/D Com. The Electrical Characteristics table on Pg. 6 provides the voltage level requirements for E/D and E/D COMM pins. See the section of the table, E/D (ENABLE/DISABLE) PIN.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • In reply to Thomas Kuehl:

    Hi Thomas,

    Thanks for your help, I really don't want to use a full bridge and PWN for my application, so i decided to create -3V to the design. 

    I have built the follow circuit:

    The scope looks good for me, I really want to know what do you think about this set-up (I don't need more then 10mA input current). thank you again for your help.

  • In reply to Itay Kalfa17:

    Hi Itay,

    I think it is a wise decision to use the dual supplies for the OPA454. Doing so allows you to overcome the Vcm issue, and the output will be able to provide a true 0 V output. Ten milliamps of output current shouldn't be any issue for the OPA454.

    Regarding your circuit, be sure to use the power supply decoupling capacitors at each OPA454 supply pins. Also, since the E/D pin does draw some input current the 1 MEG resistance may be a bit high in value. The 1 MEG is used to bias the E/D pin from a +50 V supply. You can float the E/D pin and it will pull high, but see the notes in datasheet section 9.3.6 ENABLE and E/D. I suggest connecting the E/D pin directly to +3.3 V with a small capacitor connected from the pin to ground, to filter noise.

    Additionally, observe note (1) on Pg. 3, "PowerPAD is internally connected to V–. Soldering the PowerPAD to the printed-circuit board (PCB) is always required, even with applications that have low power dissipation." In your case the PowerPAD must be soldered to the PC board pad that is biased to-3 V.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • In reply to Thomas Kuehl:

    thanks you Thomas,

    That was really helpful!

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