# When would you need a low bandwidth opamp?

This opamp has a unity gain bandwidth of 27kHz, which is by far the lowest I've ever seen. (I first misread the 7.7V/ms slew rate as 7.7V/$\mu$s, because that's the units most often used.)

27kHz looks very bad. Is there any reason why they would make opamps with these speciifications?

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There are all kinds of applications that don't need high bandwidth. For instance, amplifying a fixed voltage to provide a reference. Or amplifying a signal from a sensor for a slowly changing quantity like temperature. 27 Khz may be adequate for audio. The question is not when would you need the low bandwidth, but when can you dispense with the bandwidth in favor of optimization in other parameters. (Of course if all else is equal, why deliberately go with low bandwidth. An ideal op-ap has infinite bandwith and unlimited gain; if you had that, why use anything else.) –  Kaz May 19 '12 at 23:06
@Kaz - "27 Khz may be adequate for audio". Not hifi if you want to amplify it. Even at unity gain such a small bandwidth most likely will give bad TIM (Transient Intermodulation Distortion), which is more audible than harmonic distortion. –  stevenvh May 20 '12 at 8:19

That 27kHz is nothing. The LPV511 has a little brother, the LPV521, which has a gain-bandwidth product of 6.2kHz.

They're not making it low-bandwidth on purpose. There is no real advantage to the low bandwidth, though it improves stability.
The low gain-bandwidth product is a consequence of the low power design. The LPV521 consumes only 350nA. You already mentioned slew rate, and it's closely related to bandwidth. The LPV521 has a slew rate of 2.4V/ms. To change the output level of an opamp fast you have to pump current to the output drivers. That's not what this opamp is designed for. Lots of applications are very low frequency, DC to a few tens of Hz at most. A typical application shown in the datasheet is a current monitor for a battery operated device, which will probably be near DC.

Anyway, you'll have to pay dearly for such a bad opamp ;-). Seriously, even in large quantities the LPV521 costs more than a dollar, while you can get common opamps for 6 or 7 cents. It's that 500nW you're paying for. Try to find other opamps which will operate on a coin cell for 5 years or more.

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At school I learned that the Gain × Bandwith product is a constant. Reading this answer I wonder if this rule is limited by the slew rate. My feeling says slew rate is not affected by feedback and therefore limits (unity gain) bandwidth. –  jippie May 19 '12 at 7:40
@jippie - GBW is constant, that's correct. At high frequencies you'll have high dV/dt (for a sine highest when it crosses zero), but also at high gain dV/dt is high. You can't increase it by having both. If you decrease gain by applying feedback your bandwidth will go up, you'll allow signals with a higher rise time. Slew rate is indeed constant. –  stevenvh May 19 '12 at 7:52
@stevenvh In a dominant-pole compensated opamp, the maximum slew rate is limited by the input stage differential pair tail current, i.e. there is only so much current available (I = Cdv/dt) to swing the VAS stage compensation capacitor. –  Bitrex May 20 '12 at 8:43
The AD8553 has an even smaller bandwidth: 1 kHz. –  Federico Russo Jul 29 '12 at 12:59

I'll try a brief answer first and THEN look at the data sheet and costing :-)

It is most likely very low power and/or can run from low voltage. Low speed assists low dynamic power reduction and the lack of need to support high power and speeds allows lower power to be targeted.

You may also value the low EMI aspects.

OK - lets look at the data sheet ...

Phew! Got it right :-)
The first few paragraphs of the data sheet tell the tale. Very strong emphasis on low power for long life in battery applications and low voltage operation to suit battery power.

They say:

LPV511:

• Micropower, Rail-to-Rail Input and Output Operational Amplifier.
The LPV511 is a micropower operational amplifier that operates from a voltage supply range as wide as 2.7V to 12V with guaranteed specifications at 3V, 5V and 12V.

The LPV511 exhibits an excellent speed to power ratio, drawing only 880 nA of supply current with a bandwidth of 27 kHz.

These specifications make the LPV511 an ideal choice for battery powered systems that require long life through low supply current, such as instrumentation, sensor conditioning and battery current monitoring.

The LPV511 has an input range that includes both supply rails for ground and high side battery sensing applications.

The LPV511 output swings within 100 mV of either rail to maximize the signal's dynamic range in low supply applications.

In addition, the output is capable of sourcing 650 µA of current when powered by a 12V battery.

For perspective - 880 nA or 0.88 uA is somewhat less than a gnats breath.

880 nA for one year is 880 x 8765 / 1,000,000 mA/nA ~= 8 mAh/year.
Operated from 3 AA Alkalines of about 2500 mAh capacity and 1V/cell end point and with no shelf life consideration, you could run one of these for about 300 years. Or, in a real world situation with a say 5 year shelf life and half capacity taken by battery degradation and a 2500 mAh initial capacity, this is about 8 mAh/year x 5 years / (2500 x 50%)
= ~ 3% of the available battery capacity.
ie you could run 30 of these for 5 years from 3 good quality AA Alkaline cells or have much other circuitry and a few of these with a 5 year life. Or use a 3V nominal Li coin cell and a few others things and get good lifetimes.

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wow low bandwidth and terrible drive capability. I haven't done enough extreme low power analog to know if 880nA is a superior power consumption for an op-amp or not, but it doesn't sound fantastic at a first glance. –  akohlsmith May 20 '12 at 3:46
880 nA is somewhat less than a gnats breath. 880 nA for one year is ~= 8 mAh/year. Operated from 3 x AA Alkalines with no shelf life consideration, you could run one of these for about 300 years. Or, in a real world situation with a say 5 year shelf life and half capacity takenj by battery degradation and a 2500 mAh initial capacity, this is about 3% of the available battery capacity. ie you could run 30 of these for 5 years from 3 good quality AA Alkaline cells or have much other circuitry and a few of these with a 5 year life. –  Russell McMahon May 20 '12 at 4:36
That would then be 750 years for the LPV521. Great 6.2kHz bandwidth. See my answer. –  stevenvh May 20 '12 at 5:53
@RussellMcMahon I know what I was doing wrong; I was comparing to input current for op-amps which is almost zero. It'd be an interesting project now to do something useful with them! –  akohlsmith May 20 '12 at 16:58