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OPA454

Other Parts Discussed in Thread: OPA454, TPS63700, TLV1702

Dear Sir/Madam, I am facing problem with recently purchased OPA454 ICs. While testing the circuit of Fig. 57 of the datasheet, OPA454 is not coming as expected. I am using following values of components: R1=1KOhm, R2=89KOhm; V+=90V, V-=0V, E/D Com.=0V, E/D=3.3V, Rp=1MOhm. By varying Vin from 0 to 1 Volt I am getting output maximum upto 300mV which varies inversely proportional to Vi. Please look into the problem.

Regards,

Sandeep Kumar

  • Hello Sandeep,

    You have configured the OPA454 for single supply operation with V+ equal to +90 V, and V- equal to 0 V. That is fine, but you must observe the common-mode input voltage range (VCM) and output swing ranges (Vo) relative to your selected power supplies.

    The OPA454 minimum VCM is (V-)+2.5 V. Since you have set V- to 0 V, VCM min. is + 2.5 V. Your Vin level of 0 to +1 V falls below the minimum and outside the linear CMV range. Also, Vo minimum is specified as (V-)+ 1 V. The output cannot move off the rail until the input CMV is large and positive enough to drive the output off the rail.

    If you need the input range to be 0 to +1 V, the V- pin must be sufficiently negative relative to the input level. Operating the V- pin at -2.5 V, or -5 V would solve the problem. If you have a positive, low voltage available on your board a negative supply can be generated using a dc-to-dc inverter such as the TPS63700:

    http://www.ti.com/lit/ds/symlink/tps63700.pdf

    The V+ supply can remain at +90 V because the OPA454 supply-to-supply maximum is 120 V.

    Regards, Thomas

    PA - Linear Applications Engineering  

  • Hello Thomas,

    Thanks a lot. It solved my problem. I used V- to be -2.5V.

    Regards,

    Sandeep

  • Hi Thomas,

    In non-inverting configuration to achieve a gain of 11, which combination of Rf and Rin is better: Rf=10KOhm & Rin=1KOhm OR Rf=100KOhm & Rin=10KOhm. Both can provide gain of 11. Which one is better and why? This is with OPAMP IC OPA454.

     

    Regards,

    Sandeep

  • Hi Sandeep,

    Gain performance wise you shouldn't see any difference between one set of feedback/input resistors, or the other. Then, their selection comes down to secondary effects:

    • The OPA454 is a high voltage operational amplifier and the output can swing close to +/-50 V (depending on supplies). The voltage across the feedback resistor can be large and the resistor's power dissipation must be considered (P = E2/R). The 100 K feedback resistor would have 1/10 the power dissipation compared to the 10 k.
    • When its comes to input voltage offset contribution due to input current flowing through the feedback/input resistors, the lower it will be. Therefore, the 10 k/ 1k combination is preferable.
    • The thermal noise generated by the resistors will be lower for the 1 k/10 k set, vs. the 10 k/100 k set. The thermal noise for either set of resistors is based on their parallel combination; 1 k || 10k ( 909 Ohms), and 10 k || 100 k (9.09 k Ohms). Then apply en = √(4KTB).

    I hope this helps you decide.

    Regards, Thomas

    PA - Linear Applications Engineering

     

  • Hi Thomas,

    Thank you very much for clearing the doubt. So it can be concluded that lower value resistor combination is better provided the power dissipation should not exceed the specified power rating of resistors.

    Regards,

    Sandeep

  • Hi Sandeep,

    Yes, that is a good plan.

    Regards, Thomas

    PA - Linear Applications Engineering

  • Hi,

    The "OPA454" Op amp is used as a comparator in  my design. The inverting terminal is connected with 0.7 volt and non-inverting terminal is connected with 0.4 volt with open loop configuration.The V+(POWER SUPPLY) pin is connected with +28 volt and v- pin is connected with ground. The E/D pin is connected with 5V supply through the 4.7K pull up resistor and SF pin connected with V+ through the 3M pull up resistor.


    Now the output of the op-amp is +26 volt. If inverting terminal connected with 0V and Non-inverting terminal connected with +0.4 volt the output goes to zero.Let me know,is there any issue in the op-amp act as a comparator.


    Regards,
    Rajkumar.M
    DATAPATTERNS PVT LTD
    CHENNAI

  • Hello Rakumar,

    Operational amplifiers are sometimes used as comparators, but they are not optimized for that application. They are intended for linear operation and have defined input and output regions of operation.

    You are using the OPA454 with a single +28 V supply, while the inputs are at voltages less than 1 V above the V- potential which is 0 V in your application. Therefore, the inputs are at voltages that fall below the minimum common-mode input voltage (Vcm) rating. The data sheet Electrical Characteristics table lists the minimum, negative Vcm as (V-) + 2.5 V. The inputs are being driven well below the low end minimum. The only way to bring the amplifier operation into the linear Vcm range with the intended input voltages is to move the V- voltage from 0 V, to an appropriate negative voltage a couple of volts below 0 V.

    Is there any possibility to use an actual comparator? The TLV1702 is usable with a single supply up to +40 V. Its Vcm range is from supply rail, to supply rail. Thus, for you can compare two voltage that are very close to 0 V. You can see the TLV1702 data sheet here:

    http://www.ti.com/lit/ds/symlink/tlv1702.pdf

    Regards, Thomas

    PA - Linear Applications Engineering

  • Hello Thomas,


    Can I get comparator with same footprint of OPA454.

    Regards,

    Rajkumar.M

    HDD

  • Hello Rajkumar,

    Single comparators are not nearly as common as dual and quad comparators. That really limits what is available. When I review TI's comparator offerings I don't find a single, that drops into the OPA454 footprint and can handle the higher voltage that you are using in your application.

    Regards, Thomas

    PA - Linear Applications Engineering