Ultra low power op-amp part suggestion - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: August 29, 2018, 08:29:28 am
content: Hi,I have searched Digikey and TI and analog, But it seems that they lack this common mode voltage in their parametric filters, So I need an op amp with these specs in mind,It should be a single supply, rail to rail input output, Iq per op amp should be under 100uA, gain bandwidth should be more than 200Khz, and input common voltage should go beyond supply rails at least 2V,I prefer 4 op amp per chip options to be available and it should support standard industrial temp rang of -40°C ~ 85°CDo you happen to know some good candidates?

author: jbb
author_rank: Super Contributor
timestamp: August 29, 2018, 08:55:20 am
content: Beyond the rails input common mode is an unusual feature.Can you re-design your circuit to:Get the common mode inside the rails, at which point you have many more options?Get the rails higher?Only run the opamp part of the time (may require input switching too) so that the average current draw is reduced?

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: August 29, 2018, 09:20:18 am
content: ThanksQuoteActually no, it's used for current sensing

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2018, 10:05:12 am
content: Post a schematic.What's the power supply voltage?What's your budget?Have you considered using a current sensing IC?It's likely you don't need such an exotic, possible unobtainable op-amp. There are other options for current sensing.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: August 29, 2018, 10:59:33 am
content: Thanks, One other possible solution would be to use an ultra efficient boost converter to boost the op-amp voltage

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 29, 2018, 01:18:11 pm
content: You'll probably have better luck with amps classified as current sense amplifiers. The INA138 for instance seems to fit the bill although it's only a single amp in a SOT23 package.There's also the ISL28005, rail-to-rail, also in a single amp SOT23 package.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 29, 2018, 05:32:18 pm
content: Quote from: ali_asadzadeh on August 29, 2018, 09:20:18 am1. Actually yes.You can always divide down the input voltage, until it lies within the CM range, and then amplify as needed.The topology is the standard diff amp circuit, with two extra resistors (equal values), one from each input to GND. (The +in normally has a resistor in this position, so use whatever the equivalent parallel value is. Or just use two in parallel anyway, it'll be easier to get the value right.)Obviously, this reduces gain so you need to adjust the feedback resistor values, and it also exacerbates resistor errors. Generally speaking: you really don't want to be doing a current sense divider if you can avoid it.So,2. Use a current sense amp. Purpose made, usually pretty economized on supply current, reasonable CMRR (certainly better than you can get with resistors at the same price) and okay frequency response.3. Regarding supplies, it's very easy to charge pump a little extra rail, especially if it's fractional mA. Throw in a 555, a coupling cap and two diodes. (One of the few times I'll actually use a 555, to be honest.)Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: August 29, 2018, 05:55:36 pm
content: The usual high side current sensing amplifier does not need input beyond the rails. It just needs an input up the positive rail. There are special current sense amplifiers for this. There are also quite a lot of low power 5 V Rail to Rail OPs, that could work. There might be a problem with input offset however, and low frequency noise could be a problem, as the shunt voltage is usually small. Also check that the N/P input stage cross over is not just in the common mode range used, as the performance is often really poor in this range, that is often at around 1 V below the positive supply.The circuit shown looks a little like the current sense amplifier, though not correct.It might require separate OPs instead of a quad version, if the high sides are a little different, but this is not a problem, as the new OPs are usually available in SOT23-5 or similar and a quad version is less popular anyway.

author: ogden
author_rank: Super Contributor
timestamp: August 29, 2018, 05:57:23 pm
content: Quote from: ali_asadzadeh on August 29, 2018, 08:29:28 amCheck TI current sense amplifiers. I am not sure about GBW, but otherwise many qualify your requirements. For instance what aboutina138?

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: August 30, 2018, 06:12:14 am
content: Thanks for your hints,for other cases I would certainly consider the current sense amplifiers, But for some reasons I think boosting the positive supply rail a few volts would be better in my case,Quotemy hole current budget is about 1ma , So I would not think 555 would do the job, do we have something way more efficient,with ultra low Iq

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2018, 10:44:39 am
content: They make CMOS 555s, you know?...Tim

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2018, 07:09:55 pm
content: Why does the op-amp need an insanely low power supply current? Isn't there another 5V rail with a higher current output available? What voltage is the ADC running off?Microchip make some op-amps with<100μA quiescent current per amplifier and rail-to-rail inputs who's common mode range exceed both rails by 300mV.MCP6411http://ww1.microchip.com/downloads/en/DeviceDoc/20005791B.pdfMCP6404http://ww1.microchip.com/downloads/en/DeviceDoc/22229D.pdfNo doubt Linear Technology make some better op-amps, but I haven't checked.

author: mikerj
author_rank: Super Contributor
timestamp: August 30, 2018, 11:10:20 pm
content: You didn't specify price, so I will assume expensive parts are fineCheck out the|LT1636.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: September 01, 2018, 05:59:17 am
content: QuoteBecause it's used in an energy harvesting designThanks for the tips

author: JS
author_rank: Frequent Contributor
timestamp: September 02, 2018, 01:44:34 am
content: Quote from: ali_asadzadeh on August 29, 2018, 10:59:33 amI don't know what the schem is supposed to do but it won't do it, as it is it will just sense the supply voltage, invert it beyond the negative rail with a log conversion, so the output is trying to go below the negative rail.For the high side current sense the differential amplifier doesn't need any extra resistors to ground, just the usual from +input. Using a resistor from the -input to ground will just increase the noise gain and won't change a thing for the signal operation, common mode or anything else, just add noise. It usually needs to have a lower gain than what you will normally put into it, to ensure the +input is between the rails, but that's it.JS