Dual rail LM358 op amp alternative suggestion - Page 1
Forum section: electronics / beginners

author: lk.dgironi
author_rank: Regular Contributor
timestamp: December 21, 2023, 10:32:12 am
content: Hello,I'm working on a PCB that contains a bipolar to unipolar converter, based on the LM358.The power rails are +-15V. The input is +-10V. It is converted to 0V to 10V. Resistors are 0.1%, 5V is a reference V.The first stage of this converter is the other side of the LM358 in voltage follower configuration, to use this just as a buffer. The output goes to another LM358 to adjust the gain.This converter works.Current required at the output stage is low.Bandwidth is low, the input changes at a rate of 10kHz max, usually less than 1kHz.I'm wondering if there's something "better" than the LM358 (possibly pinout compatible) for this kind of circuit, or the LM358 is enough for my requirements.Thanks!

author: Jwillis
author_rank: Super Contributor
timestamp: December 21, 2023, 11:37:45 am
content: Depending on your supply voltage and package you could use just about any dual op amp. The pin outs would be the same. You could use a TL082 dual op amp. Very popular op amp that's pin compatible. I'm sure there are others. Depends what parameters you require the most in your circuit

author: lk.dgironi
author_rank: Regular Contributor
timestamp: December 21, 2023, 11:53:59 am
content: It's a standard SOIC-8.I don't want to use a FET one. I was thinking about an NE5532, dual rail +-30V compatible, pinout compatible, pretty common. Should work I suppose.Problem is that I don't know what to look at to find a "better" oneNE5532 has less input offset voltage compared to LM358, what else?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2023, 11:57:10 am
content: The LM358 has the normal pinout for dual op-amps. With a +15 V supply there are many OP-amps to chose from. If precision is an issue I would consider the OPA2202, that is still reasonable affordable, though not available in a DIP case.The more direct upgrade and also single supply (not needed here) is the LT1013 - however this one has a different pinout in the SO8 case.Even with just 1 or 10 kHz the LM358 may already be a bit slow and with the added delay on cross over can be an issue.

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2023, 03:47:57 pm
content: What is it that you want to improve? Do you need lower noise, higher precision, higher bandwidth, lower power?Quote from: Kleinstein on December 21, 2023, 11:57:10 amI got caught by that once when I did a board layout. The LT1013 has a different pinout in the SO-8 package simply because the rectangular die is too large and must be rotated 90 degrees to fit. Later dual operational amplifiers, even from Linear Technology, had smaller dies or were square to avoid this problem.

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 21, 2023, 04:48:09 pm
content: There are literally dozens and dozens of better, much better opamps than the plain vainilla LM358.However, be prepared to pay anywhere from 5X to 20X from what a LM358 costs.Thus, you will also have to specify cost constraints, if any.

author: Zero999
author_rank: Super Contributor
timestamp: December 21, 2023, 08:13:55 pm
content: Is it necessary to use an op-amp?How low is the output current?Those are low value resistors you've chosen there.How accurate and stable is the 15V supply? Can it be used as a reference?If the output is driving a high impedance, then I would consider using a potential divider. The values shown below will give an input impedance of R2+R1||R3 and an output impedance of R1||R2||R3.If the 15V supply isn't accurate enough, then can you use a 10V reference? If so, make V1 = 10V, change R1 to 10k and omit R3.

author: lk.dgironi
author_rank: Regular Contributor
timestamp: December 22, 2023, 08:13:33 am
content: Thanks all.The circuit use op amp also for adjustable gain, peak detection and things like that. Unluckily I can not share all the schematic.Power is not a constraints, also costs, if less than 2USD each.I expect lower noise and higher precision. Even if I'm quite happy with the LM358, just wondering if any better in those terms.For something low cost I'm looking at the NE5532. What do you think about this?The LT1013 looks interesting, also in another project I'm using the OPA2188 for signal conditioning like this.

author: magic
author_rank: Super Contributor
timestamp: December 22, 2023, 08:22:31 am
content: NE5532 will certainly work here, with much lower noise and somewhat better guaranteed offset.LT1013 has weird pinout in SO8 and similar noise to LM358 AFAIK, but much lower guaranteed offset.LM358 has another issue, which is crossover distortion. This doesn't affect the output channel which always sources current, but it will affect the input buffer which sometimes needs to sink current from the 5V reference through the voltage divider. You may see a glitch near +3~4V input level.

author: the_cake_is_a_lie
author_rank: Contributor
timestamp: December 22, 2023, 08:26:24 am
content: So a voltage gain of 1 + (5k/10k) = 1.5. Slew rate of 0.3V/µs and 0.6 MHz gain bandwidth product is limiting. The slew is really just what it is at 25°C or 300K at unity gain and is probably the higher of the positive and negative slew rates. Input of +/-10V means a Vpeak of 10V.min slew rate = 2 pi f Vpeak = 2 x pi x 10000 x 10 = 0.63V/µs so you need to upgradeThe reality is you want a slew rate at least twice the bare minimum to avoid the opamp chopping up and lowpass filtering the waveform. I'd also say to have at least 5x the minimum gain bandwidth product to increase gain and phase accuracy. 0.6 MHz gain bandwidth / 1.5 leaves 400 kHz so that's a solid 40x margin at 10 kHz max frequency. NE5532 or (JFET input) TL082 are fine. Nearly every opamp out would be fine here.You say you don't want to use a FET opamp but there are circuits where BJT opamps don't work well, perhaps this one. I'm not sure you intend to add a DC offset to the positive input when you say it's a reference, but if you do, a JFET input or CMOS opamp would be better with effectively infinite input impedance since you can add a parallel resistor to get the precise voltage divider and therefore precise DC offset that you want. Else you can use a summing amplifier but that'd force an inverting opamp and maybe the inverted phase isn't acceptable. Can use another opamp or a quad amp to invert the phase back.I'm surprised you need relatively expensive 0.1% tolerance resistors. Perhaps you don't and 1% are good enough.

author: lk.dgironi
author_rank: Regular Contributor
timestamp: December 22, 2023, 10:29:37 am
content: Thanks again.So now i know the "preferred" min slew rate.The 5V positive input comes from the REF195.The input to this circuit comes from a voltage follower configuration op amp (the other channel of the LM358), and it goes to an inverting configuration op amp (LM358 again) with settable gain by a trim pot.0.1% are not that much cost, considering I'm using 5K 0.1 on many other place of this circuit. Maybe 1% will be enough, I can give it a try on the next PCBA iteration, which I'll test with the new selected op amp.NE5532 is my low cost/jellybean alternative right now.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2023, 12:18:27 pm
content: The NE5532 is low noise, but it has quite some input current and needs quite some power. A step in between is the RC4558.The Ref195 is a relative part already.Chances are one can add the offset also with the inverting amplifier part. So one can likely get away with 1 OP-amp less.

author: lk.dgironi
author_rank: Regular Contributor
timestamp: December 22, 2023, 01:03:18 pm
content: Power consumption is not a problem, but the RC4558 could be a good compromise.As for the REF195 I may try. Have you got a suggestion for that?Attached the simulated circuit for the falstad simulator (https://www.falstad.com/circuit/circuitjs.html), if needed.

author: Zero999
author_rank: Super Contributor
timestamp: December 22, 2023, 01:25:29 pm
content: Quote from: lk.dgironi on December 22, 2023, 08:13:33 amIf the LM358 is providing adequate performance, then why not just stick with it? It's one of the cheapest and most widely available op-amps. It doesn't make any sense to use something more expensive and less easy to get hold of.Quote from: the_cake_is_a_lie on December 22, 2023, 08:26:24 amThat's only if the main component of the signal is 10kHz. If it's mostly low frequency, with only a small 10kHz component, then the slew rate requirement is lower.

author: the_cake_is_a_lie
author_rank: Contributor
timestamp: December 22, 2023, 07:59:59 pm
content: Quote from: Zero999 on December 22, 2023, 01:25:29 pmIt depends on your design philosophy. Like if the input signal is 5V or less and/or 5 kHz or less 95% of the time, then LM358 could be fine. If you have it sitting around then test it out. I'd rather design in terms of worst possible case so am saying go up a min slew rate of 1.2V/µs. The good news is that every audio-specific opamp designed since 1975 is fast enough. 1.2 to 12V/µs is in the same price tier as 0.3V/µs.Also, the slew rate is a somewhat dodgy parameter. TheLT1113 datasheet, for instance, demonstrates slew rate declines with increased temperature and gain bandwidth usage on page 8 and consequently lists both a guaranteed, minimum slew rate and a typical slew rate at unity gain. You want more than the minimum.Quote from: lk.dgironi on December 22, 2023, 10:29:37 amThanks for explaining. Yeah, probably best to give us the simplified diagram like you did. NE5532 is the most used audio amp in the world because it's cheap yet still high quality and has been used in audio equipment in decades. It's in every preamp I found listed for sale on eBay. I have a few sitting around. The criticism by Kleinstein of high input and power draw is valid but may not matter in your case. Another upgrade over NE5532 but at 3x the cost is LM4562.So 10x 1% resistors cost 5 cents and 0.1% cost 40 cents. I see what you're saying, nice to reuse the same component and hit bulk pricing. At hobby level, 35 cents is irrelevant. If you were to sell for profit then it starts to matter.There is a trick to use multiple 1% (or 5%) resistors to achieve better tolerance. Like use 4x at 4x smaller value in series or 4x of the same value in a 2x2 series and parallel grid. The tolerance improves because you increase the sample size to push the average value closer to the expected. Perhaps most people are familiar with the standard deviation formula that has sqrt(n) in the denominator, where n is the number of resistors. You'd also tick down the thermal increase in resistance but I doubt that's relevant.

author: Zero999
author_rank: Super Contributor
timestamp: December 22, 2023, 08:41:51 pm
content: Quote from: the_cake_is_a_lie on December 22, 2023, 07:59:59 pmMy point was that bandwidth alone is often not a reliable measure of slew rate. For example audio never gets to 20kHz at the full voltage swing, which is why the tweeter in a speaker cabinet has a lower power rating, than the woofer. If the waveform is known (if in doubt, look at it on an oscilloscope) then it's easy to know the slew-rate.

author: shapirus
author_rank: Super Contributor
timestamp: December 22, 2023, 08:53:18 pm
content: Quote from: the_cake_is_a_lie on December 22, 2023, 07:59:59 pmThis only works if the distribution of the actual values of the resistors that you have is normal. In practice, however, it is not uncommon to buy, say, 20 resistors with 5% tolerance and see that all of them are, say, 3% off the nominal value, but within less than 1% deviation between each other.

author: MarkT
author_rank: Frequent Contributor
timestamp: December 23, 2023, 11:08:18 am
content: Quote from: lk.dgironi on December 21, 2023, 11:53:59 amNo guarantee that's going to work - the LM358 has better input offset, doesn't have its inputs strapped together with protection diodes, and doesn't need good decoupling, and its bias and offset currents are large - its expressly not a precision DC opamp.No idea why you think a FET opamp is a bad idea - so long as you meet the specs. Sounds like your circuit is effective a precision DC application, so audio opamps usually won't be a good fit due to their lax input offset voltage specs (no benefit in ac-coupled circuitry!). Anything better than the LM358 will be considerably more expensive as the LM358 really is the definition of a jelly-bean part.

author: shapirus
author_rank: Super Contributor
timestamp: December 23, 2023, 12:24:00 pm
content: I don't see why something like TL082 wouldn't work.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 23, 2023, 01:04:30 pm
content: A fast OP-amps has more chances to oscillate. The TL082 is no that fast, but it does not like capacitive loading very much. There is also a good chance the TL082 shows more drift than the LM358.Another point may be the sensitity to ESD that could be a point where the LM358 is more robust. With some 1-10 KHz signal frequency it is somewhat questionable that the LM358 is fast enough.Drift wise the NE5532 is actually not that bad, even though it is an audio part. At least the Phillips DS gives a max offset of 4 mV and thus less than the 7 mV for the LM358.A relatively low cost and low drift FET OP-amp would be the TLV9302. In most aspects it should be able to replace the LM358, though I would expect more 1/f noise.If cost are an issue it would be more the reference where one could save a bit (e.g. Ref5050 or ADR02 instead of the REF195 - depending on the pins used connected it may work on the same FP).The inverting stage that follows can also do the scaling and offset shift, possibly even without loading the reference much. With a different FP one would have more freedom with the reference.

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2023, 02:02:50 pm
content: The TL082 would work, but will be less accurate the LM358, as it has a greater input offset voltage.My go to part would be the OP07, because it has a low input offset voltage, bias current, is cheap and widely available, but the original poster doesn't want to change the package and might need the other op-amp in the package for something else.

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 24, 2023, 01:36:38 pm
content: As others have mentioned, if the LM358 is working fine for you a very simple and cost effective upgrade would be to employ the “A” version, which has a tighter offset level, and a couple of improvements on other parameters.Additionally, not all LM358s are created equally. From subjective experience, I can tell you that in a medium-high volume product (25k per year), we would always see fewer parametric shifts when using opamps from TI as compared to STMicro. I would think that opamps sourced from far east companies would have an even larger parametric spread, although they would still meet parametric specifications.