LT6018 Noise characteristics - Page 1
Forum section: electronics / projects

author: moffy
author_rank: Super Contributor
timestamp: March 12, 2025, 09:57:39 pm
content: The LT6018 low noise opamp has some interesting noise characteristics :https://www.analog.com/media/en/technical-documentation/data-sheets/6018fa.pdf1. It has a very low 1/f voltage noise2. The noise current for an unbalanced input is 3pA/sqrtHz @ 1kHz, while for a balanced input it is 0.75pA/sqrtHz @ 1kHz, a 4:1 difference. I am not sure how this happens apart from maybe a correlated noise source, like maybe the current source for the input diff pair, but I am only speculating.Can anyone add some light to this conumdrum? It would be appreciated.

author: TimFox
author_rank: Super Contributor
timestamp: March 12, 2025, 10:08:13 pm
content: Op amps with input bias current calculation do so by adding current from two correlated sources to the two amplifier inputs.You get a reduction in DC bias and offset, but increase the random current noise.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 12, 2025, 10:22:08 pm
content: Noise in the current source for the differential pair is definitely a candidate for the correlated current noise.The interesting point with the LT6018 is that they actually give separate numbers for the correlated and not correlated part. In many cases they only give the correlated part.

author: David Hess
author_rank: Super Contributor
timestamp: March 13, 2025, 12:48:18 am
content: Quote from: moffy on March 12, 2025, 09:57:39 pmThis part is an improved LT1128 using super-beta input transistors operating at like a milliamp of collector current. Combined with a very low base spreading resistance, voltage noise and flicker noise is very low.QuoteThe input stage includes bias current compensation and because of how the compensation currents are derived, the input currents become correlated. See the LT1028/LT1128 datasheet for a more detailed schematic and description about what is going on.

author: moffy
author_rank: Super Contributor
timestamp: March 13, 2025, 01:44:06 am
content: Quote from: David Hess on March 13, 2025, 12:48:18 amThanks for the detailed explanation, I will have a look at the LT1028 datasheet.P.S. I miss the great innovations from Linear Technology when they were at the height of their game, even though their parts have always been a bit expensive.

author: moffy
author_rank: Super Contributor
timestamp: March 13, 2025, 01:59:32 am
content: From page 11 of the LT1028/1128 datasheet:"When the source resistance is unmatched, the total noiseversus unmatched source resistance plot should be con-sulted. Note that total noise is lower at source resistancesbelow 1k because the resistor noise contribution is less.When RS > 1k total noise is not improved, however. Thisis because bias current cancellation is used to reduceinput bias current. The cancellation circuitry injects twocorrelated current noise components into the two inputs.With matched source resistors the injected current noisecreates a common-mode voltage noise and gets rejectedby the amplifier. With source resistance in one input only,the cancellation noise is added to the amplifier’s inherentnoise."

author: Andree Henkel
author_rank: Regular Contributor
timestamp: March 13, 2025, 12:30:04 pm
content: OP 2. balanced input current noise vs. unbalanced input current noise.This is a question I had for long time.To my understanding basically all / most OPV circuits are of unbalanced type.Can you give examples of balanced type OP circuits?or are the balanced type noise and unbalanced type noise two noise terms that happen simultanously and have to be added up according to root(inb2+inu2) ?

author: Andree Henkel
author_rank: Regular Contributor
timestamp: March 13, 2025, 12:41:32 pm
content: ok think I got it.take example inverting amplifier. minus input sees parallel of feedback and input resistor.plus input just connected to ground.so calculation (of voltage noise at output from current noise density) is in,unbalanced* the parallel combination.but if I put a matched to the parallel combination at the plus input.the noise calculation changes to in,balanced* the parallel combinationIs this correct?of course I get the additional noise term for that resistor, but may still be improvement in overall noise in some cases at low resistor levels.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 13, 2025, 02:34:00 pm
content: The noise calculation is a bit more complicated:The unbalanced case should be Ok and simple with input resistance times current noise. The balanced case is a bit tricky. The idea for the balanced case is getting only the non correlated current noise from the 2 inputs and thus the geometric sum of In_uncorr*R from both unputs and thus 1.41*In_uncorr*R. The unbalanced case sees the geometric sum of the correlated and uncorrelated noise part.It is not so clear how they "measure" / calculate the current noise for the balanced case. There seem to be an issue with quite some datasheets:E.g. for the OP27 they at least give the procedure how they calculate there: add the resistors on both inputs and multiply with the noise current. This kind of an odd way, as the more sensible model is 2 uncorrelated noise sources for the 2 inputs plus one correlated noise source. Using the sum of both resistors gives a current noise that is only 71% of the uncorrelated part of the current noise.I don't know If they did the same mistake also for the LT6018 with the balanced part.Another complication is that with the inverting amplifier the noise gain is larger by 1 than the gain from input to output. To get the noise at the output one would need to multiply with the noise gain, not the signal gain.The effect of current noise scales linear with the resistors, while the thermal resistor gain only goes up with the square root. So resistor noise is more relevant compared to current noise at small resistors. Low resistors help even more with the current noise than the resistor own noise.

author: moffy
author_rank: Super Contributor
timestamp: March 13, 2025, 09:17:48 pm
content: The interesting structure that seems responsible for the correllated bias compensation and noise is a dual collector transistor or it's equivalent, there was a discussion on this forum previously about the OP27/37 bias compensation, and the equivalent schematic showed a dual collector PNP as the bias compensator.

author: TimFox
author_rank: Super Contributor
timestamp: March 13, 2025, 09:22:28 pm
content: Both sides of that dual PNP are driven from a measure of the common-mode (average) of the two NPN input transistors' base current, hence the correlation.

author: moffy
author_rank: Super Contributor
timestamp: March 13, 2025, 09:28:28 pm
content: Quote from: TimFox on March 13, 2025, 09:22:28 pmThat's what I thought. I originally tried to imagine a device that would create two correllated currents and finally decided on a dual collector PNP. Then I did a search on the topic and came across the forum post confirming the idea. Never knew about the concept before this.

author: TimFox
author_rank: Super Contributor
timestamp: March 13, 2025, 09:34:57 pm
content: I remember when PMI introduced the OP-07, which added the PNP input bias cancellation to the existing OP-05, around 1975.I'm not sure if that was the first such device.The OP-05, in turn, was based on the Fairchild uA725 precision amplifier.Edit: the OP-05 had bias cancellation. The OP-07 added factory trimmed offset.

author: David Hess
author_rank: Super Contributor
timestamp: March 14, 2025, 06:24:43 am
content: Quote from: TimFox on March 13, 2025, 09:34:57 pmThe OP-05 has the same schematic and input bias cancellation. The OP-07 added on-die offset trimming, and the PMI schematic even mentions it.QuoteBoth designed by George Erdi who came up with the cross-coupled quad layout for differential pairs.

author: moffy
author_rank: Super Contributor
timestamp: March 14, 2025, 08:15:21 am
content: Having looked at a number of Super Beta transistor input opamps it looks as though they generally have a lower 1/f corner frequency for voltage noise than the non Super Beta opamps. They don't have as low 1kHz noise as the best low noise opamps, but they also appear to have superior input current noise as well. e.g. OPA2210(Super Beta) vs OPA2211.Is this correct and if so can someone explain the reason?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 14, 2025, 08:42:15 am
content: A main reason for the current noise is the input base current. With higher current gain one naturally gets less input current and thus also less current noise.A second point is the balance between current noise and voltage noise. Here one has choices, somewhat like having more amplifiers in parallel or higher current in the transistors. With 4 times the amplifiers in parallel one gets half the voltage noise, but also double the current noise.Which currentlevel to use is a design and market decision. There is the tendency to make the amplifiers really good with one feature to set them appart from others a bit more. So the super beta one tend to prefer lower current. There should be nothing to prevent you from making a lower voltage noise super beta amplifier. The transition from normal to super beta transistors is a smooth one: higher gain usually comes with lower voltage rating. At some point one accepts that one needs extra bootstrapping to reduce the voltage seen by the transistor and can than go very high in the gain. This makes it a bit step like for the amplifier design.Why the 1/f noise is often better with super beta types is a tricky question and I don't know. There is the tendency that one wants / needs rather clean material for the highest gain and this helps with low 1/f noise too.

author: David Hess
author_rank: Super Contributor
timestamp: March 14, 2025, 01:56:57 pm
content: As far as I know, all of these precision operational amplifiers are super-beta types. They only advertise super-beta for the lowest input bias current ones. I am not aware of any compromise with super-beta transistors for noise, just breakdown voltage. Their big advantage is good beta at low collector current for low input bias current, but they work just fine at high collector currents.For Linear Technology, but this applies to others, they have a whole series of parts with almost identical designs operating at different collector currents, covering a bunch of ratios of current to voltage noise. For Linear Technology, this was the LT1012, LT1001, LT1007, and LT1028/LT1128. PMI had the OP-97, OP-07, and OP-27.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 14, 2025, 02:40:33 pm
content: The internal circuits shown for the LT1007,OP27,LT1028 in the data sheets don't show a cascode stage to limit the voltage for the input transistors. So this circuit would not work with super beta transistors with a maximum CB voltage in the range of 3-5 V. I know the circuits are often simplified, but I don't think they would leave out that much of the circuit.For the product of voltage an current noise the LT1001,LT1007 are comparable and quite a bit worse than the known super beta types like LT1012 or OPA202,OPA205. Of cause one has to take the data for the current noise with a grain of salt.

author: TimFox
author_rank: Super Contributor
timestamp: March 14, 2025, 02:43:35 pm
content: Quote from: David Hess on March 14, 2025, 06:24:43 amMy mistake on the 05 to 07 upgrade: I corrected my post.

author: David Hess
author_rank: Super Contributor
timestamp: March 15, 2025, 09:55:02 am
content: Quote from: Kleinstein on March 14, 2025, 02:40:33 pmYou are right. I thought they did.QuoteBut the LT1001 does show the cascode configuration.I wonder how much current noise comes from the tail current. Maybe it is much smaller than the current noise of the differential pair? Otherwise matching source resistance would lower the apparent current noise and I have never seen this except where input bias current cancellation is present.