Check this low cost part,TP5552 from 3PEAK, it's around 5uV but it's under 0.6USD for 10 units.
https://lcsc.com/product-detail/Others_3PEAK-TP5552-VR_C248604.html
I appreciate a large part of the video is to show ways to go about finding suitable parts and understanding datasheets and the actual devices chosen and specs are less important.
However I think your concentration on offset voltage is a case of not seeing the woods for all the trees. The MAX4239 seems to be a rather poor choice for the ucurrent - it's far too noisy at 30nV/sqrt(Hz). Given the ucurrent's specified 300khz bandwidth, input noise is 108uVpp! (This is inline with the 10mV output noise reported in these Forums). Offsets below, say 10uV, are lost in the noise.
None of these alternatives were available at the time the current was designed, so the MAX4239 may well have been the best choice for the circuit architecture chosen, but a better option would probably have been to use a hybrid amp using a low noise fet amp with a zero drift/chopper for DC stabilization.
For DC applications, such as using the current with a slow DMM, most of the noise is filtered out but again the MAX4239 has a LF noise spec of 1.5uV. Added to the 2.5uV max offset spec means that lower noise, but higher offset parts may be as good if not better.
Data sheets often have serious inconsistencies between typical values listed in the specifications and those shown in the typical graphs and some manufacturers have a reputation for supplying parts that almost never come close to their typical values.
The high bandwidth of the max4239 is of limited use because of the high noise - though there are more noisy ones. At higher frequencies the max4239 is not that bad for an AZ OP with supposedly some 30 nV/sqrt(Hz). Still a scope input amplifier may be lower noise for the higher frequencies, or at least comparable.
How much bandwidth does the uCurrent actually need?
How much bandwidth does the uCurrent actually need?
It needs to be fast enough for a multimeter's AC input ranges, which themselves are not all that flat above low frequencies.
John Wettroth
1 day ago
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.
John Wettroth
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.
So the offsets mostly come from package stress and thermocouple action in the leads. I wonder how the stress contributes -- changes in the thermocouple potentials or unbalancing resistances further into the wafer?
It's also really interesting to read about the voltage limits. This explains why many low-power opamps don't go above 5.5-6V, even though it would be really useful if they could go just a bit above 4 AA batteries (6.4V).
±75ppm/C is not ±15ppm/C but 0.28 vs $4 for Vishay Y14870R01000B9R
Searching only for in stock parts helps to get more common parts. However one may also miss some parts as sometimes even common parts run out of stock. If available have the usually stocked or similar tag helps.
One problem with the search function at Digikey / Mouser is that they sometimes mix typical and maximum values.
The definition of what the typical TC is also seems to be different. Some manufacturers seem to be a little more optimistic than others. I would have expected something like the RMS value of the TCs found. Over time the process may change (usually improve) a little.
The idea with the µCurrent is to have precision parts and not adjustment, thus the high accuracy parts and OP with lowest possible offset.
For a high BW use the noise could become really important.
For high performance / lowest noise the resistor steps are quite large. At the low end of the ranges the noise gets increasingly problematic. However more resistors would increase the costs quite a bit with the high precession parts.
Is there such a thing as a log autozero amp...
High dynamic range is also important for the µCurrent. Not sure if there is something clever that could be done here, for example a logarithmic mode? Is there such a thing as a log autozero amp...
High dynamic range is also important for the µCurrent. Not sure if there is something clever that could be done here, for example a logarithmic mode? Is there such a thing as a log autozero amp...
Dynamic range on the order of 26 bits with reasonable accuracy is feasible with a logarithmic design. I do not remember ever seeing an automatic "zero" logarithmic converter but it could be done using a pair or more of reference currents.
Since the output would be a logarithm, I am not sure it makes sense without a dedicated meter to show units and I have seen current meters which did exact this to show picoamps to milliamps without range switching.
High dynamic range is also important for the µCurrent. Not sure if there is something clever that could be done here, for example a logarithmic mode? Is there such a thing as a log autozero amp...
Dynamic range on the order of 26 bits with reasonable accuracy is feasible with a logarithmic design. I do not remember ever seeing an automatic "zero" logarithmic converter but it could be done using a pair or more of reference currents.
Since the output would be a logarithm, I am not sure it makes sense without a dedicated meter to show units and I have seen current meters which did exact this to show picoamps to milliamps without range switching.