EEVblog #1318 - What's State-of-the-Art in µCurrent Opamps?

Part 1 in designing a better uCurrent.
Is there a better chopper/auto-zero amplifier than the MAX4239 used in the uCurrent?
A look at the exciting world of parametric searching for components. Follow along as Dave looks for a new state-of-the-art opamp. Success not guaranteed!

#opamp #design #tutorial

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[EEVblog]

UPDATE: The ADA4528 with a nominal 2.5uV offsett actually looks like it has a tighter spread than the MAX4239, and unity gain stable. But lower bandwidth.

[johnwettroth]

Hi Dave, I retired as the managing director of product definition for standard products at Maxim a few years ago. My group and I defined thousands of products like this for over 20 years and I had a hand in the 4239 parts as well as many, many others. There were a couple issues that came up in this video that I can shed a bit of light on from the chip company side. Although Philbrick made precsion amps in 50's with tubes, Intersil, the spiritual father of Maxim invented this category in CMOS chopper amps in the 70's with the ICL7652. Maxim's second sourced these parts and made improved second sources early in its life. Here are a few points that may give you a better handle on how chip companies approach high performance analog. Because of the way that these parts work, the nominal offset is zero. What gets in the way of this is real third order effect like thermocouple action in the leads and package stress. Maxim is somewhat unique in that it can and does do post package trim (zener zapping) vs. just laser trim a the wafer level. Parts that are perfect at wafer end up being a couple of uV post package due to stresses. The other thing about very high spec parts like this (100 nV) is testing. In order to get good yields, test max limits are generally set higher, we called this the threshold of pain. The typicial values and histograms are there to give you a feel for what you're really going to get. In difficult to test specs, you're balancing how many seconds you're spending on a million dollar tester- the cost get significant. Some specs llike leakage currents in analog switches will often have a max spec of 10 uA though will generally be in the femto amp range. We would release a different external part number that guaranteed a spec like this. You can get an idea of the real spread by asking a vendor to make you 10, 000 (a common minimum) with some spec tightly tested and they don't balk, it means the parts will yield if the customer is willing to pay the delta in test cost (plus NRE).. Maxim routinely does this for big customers with precision requirements like a test equipment company. The histograms in the data sheet is taken from the first few wafer lots as the test guys and the design guys dial things in- this process is called correlation. On a little amplifier parts on big wafers, there be 5000 die per wafer so you can generate a lot of test data pretty quickly. The voltage range/dynamic range issue is driven by modern processes. Maxim has a lot of boutique processes but most analog part in the market are made on kind of vanilla cmos processes which are generally digital and low voltage. These are generally 100-200 micron processes. If you see cmos parts with +-5V or +-15v supplies, they are fabbed on old 2 micron plus processes or a specialty analogy process. Maxim, TI and Analog and a very few others keep these old processes running for precision analog. The economics of doing 130 nm analog on 300 mm wafers makes parts really cheap even though it takes some real design chops to do it. Your comments about automotive are somewhat on target, they can be robust but mainly this designation has to do to with consistency of supply, change notices and paper. Sometimes specs are relaxed to improve yields also. Automotive guys can work with anything but they don't like surprises. Hope this helps a little.

[theondono]

In a project we had to measure a massively resistive sensor (>10GOhm) with a very small output (single digits uVolt range), which meant that we had to use the opamps with the lowest current offset available.

After testing several of them trying to find a balance between current bias (which also had some effects on the sensor), current offset and voltage offset, we opted for choosing the ones with lowest current metrics and placing a mosfet across the input. Before measuring we would short the inputs and measure the offset.

Goes to show that sometimes it’s better to rethink your architecture than to try to find the specs you wanted.

[steverobbins]

Here are a couple tips for good amplifier design.
1. It's always best to use opamps as inverting amplifiers if you can. That way you don't get common-mode input voltage. Also, Vos sometimes varies over the common-mode input range which adds distortion.
2. You could improve accuracy and lower cost by moving the second stage inside the feedback loop of the first stage. That way the second stage doesn't need to be precise. You could use a high GBWP second stage with relatively high Vos and cheaper 1% resistors. Those errors will essentially be integrated out by the first stage. This approach is called a composite amplifier.

[Blowcrafter]

5:42 Dave perfectly explains why I never finish my projects... I agree 105% :D

[stcoso]

When you searched in the mouser catalog you had the "op amp" search key active. That's why you've found so few parts. There's the AD LTC1050 that may suit (Vos 0.1uV typ, 5uV max, 2.5MHz, 18V)

[mikemike]

Rochester Electronics works with major semiconductor manufacturers to continue to supply their obsolete and end-of-life parts that the original manufacturers don't want to be bothered with. They stock billions of original chips and wafers and package the chips as needed. When the original chips are no longer available, they can make more - for a price - using the original masks or by recreating the design via reverse engineering. They've recently become a Digi-Key Marketplace supplier, but they've been around forever and are headquartered in a lovely resort town not far from me.

[stephenbell]

Rochester Electronics is a US based firm and have been around for a long time. They tend to specialise in buying up obsolete stock and EOL products.

[ccfb]

My worst words:
"Whoa, I got an idea!"
Explains my unfinished projects...

[MikkelKroman]

I think the reason you're noticing you're missing parts is because you first search for "opamp" and then click the category - maybe try clearing the "opamp" search criteria. I noticed you did it with both mouser and digikey.

[mr_gerber]

The MAX4288 with 0.1µV is a database entry issue. According to datasheet, Input offset voltage is Typ 0.1 MILLI volt, not micro.

[p_mouse]

Actually, I made it a Friday morning routine once a month to have a look at the 4-5 biggest manufacturers to see what new flavors they have. Cost you maybe an hour or two.
I'm actually blown away how little knowledge people have on a professional level what's actually available. Sometimes come with horrible old and close to obsolete parts.
I even had whole discussions about certain ideas that people thought it wasn't possible, yet there was just an easy of the shelf part available.

[faultylee]

EXACTLY!!! I get sucked into the rabbit hole every time I go look for parts too, lol

[randomviewer]

I'm really surprised nothing new has come around. I was actually getting a bit worried that something had, because I had just purchased one of your uCurrents a few weeks ago.

[denisdrozdoff]

"Generally temperature wouldn't change that much"
Cries in MIL-STDs

[miltoneduardososa]

@EEBblog I've been waiting for a video like this for a long time, thank you so much!

[HardDiskSpeaker]

Thank you for how to seek the specs and how to find a information from datasheet's various graphs, numbers, lines etc! 😎👍

[yaghiyahbrenner]

Bring back fundamental Fridays, we watch more of your old content than the new stuff.

[mikemike]

I always loved reading new product announcements and poring over new datasheets and app notes. I learned a lot that way. Now, searching for parts often seems like 99% of the job and is definitely more tedious and less rewarding. Digi-Key's and Mouser's parametric searches, while better than most, are really not that well designed. Now I spend hours looking for parts that need to meet multiple environmental specs and conform to multiple standards. Try finding a waterproof panel-mount USB-C jack or a power cable that meets both UL and CSA standards for use in low-voltage LED lights suitable for wet locations or . . . You get the idea. Not nearly as much fun as reading the first RCA datasheets for 4000-series CMOS or the first datasheets for the Intel 4004, 8008, and 8080, the Motorola 6800, the AMD 2900 series, etc. Those were the days. I miss my bookcases full of trade magazines and data books. And I miss designing and building circuits instead of writing code that does the same thing.

[Roodster]

Rochester Electronics is one of those suppliers that tend to buy up parts that are going obsolete, yet are notoriously difficult to deal with. An option to buy from them through digikey would have been a god send 5 or so years back (I mean, "email us a scanned credit card, both sides" after agreeing to direct bank transfer and two months of back and forth of various checks, nda's and so on, what a waste of time that was)