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OPA228: Bias and temperature drift Questions

James Azotea
Prodigy 60 points
Part Number: OPA228
Other Parts Discussed in Thread: LM741, TL1431, OPA227, OPA197

I am using an OPA228 as a comparator.  I use 1 OPA228  with a bias voltage of +15V on the +V pin and grounding the -V pin.  I use a second OPA228  with a bias voltage of -15V on the -V pin and ground the +V pin. Each sub circuit compares and outputs a positive or a negative voltage with a singe bias supply. This gives me an easy way to compare voltage level from an AC input voltage. The simulation model seems to work fine. I remember LM741 opamps could run off a single positive or negative bias voltage which was great for making positive and negative voltage regulators. 

Can I bias the OPA228 to operate in the manner above? Please advise.

Does the low voltage drift of the OPA228 make it a good candidate for a wide temperature range comparator using a TL1431 voltage reference?

Thanks

Jim

  • 0
    Guru 140200 points

    Hi Jim,

    if this means that the first OPA228 with positive supply voltage will also see negative input voltages and the second OPA228 with negative supply voltage will also see positive input voltages, then this seems to be no good idea. Even if the OPA228 would survive this torture, leaving the common mode input voltage range can slow down the operation.

    Why not powering both OPA228 with a true bipolar supply voltage of +/-15V?

    Kai 

  • 0
    TI__Guru 64640 points

    Jim,

    Even though some op amps may be used as comparators, OPA228 is NOT one of them because of the back-to-back input protection diodes that will disturb tripping threshold voltage set by your TL1431 reference voltage when inputs are taken more than two diodes apart – see below.

    Most bipolar (IB>1nA) input op amps like OPA228 require protection of its input transistors against reverse biasing of Vbe junction and for this reason if you want to use op amps as comparator it must be CMOS or JFET input op amp (non-chopper), which in most cases do not require such input protection.  How do you know op amp is CMOS and JFET? Look for IB of around few pA @25C.

    As far as operating any op amps on single supply, it is perfecting fine for as long as you are aware of the input/output voltage range limitations but as Kai alluded to why not use dual +/-15V in order to avoid non-linearities, which in case of OPA228 are 2V from either rails; this means on 15V single supply, OPA227/8 input and output linear range is 2V to 13V (see below).  

    All in all, if you are really set on using op amp as comparator, you could use OPA197 with rail-to-rail input and output ranges and no back-to-back input diodes - see below.  But why not to use much faster comparator for a given quiescent current?