Ground sensing single supply opamp recommendation - Page 1
Forum section: electronics / beginners

author: hirada
author_rank: Regular Contributor
timestamp: January 17, 2021, 10:44:54 am
content: After a morning @Mouser, I am wondering, is there such a thing as an opamp, that, with a single supply of +15V, can swing to ground down to single digit mV?Or, in other words, is it generally possible with a single supply to build a voltage follower that is capable of following down from 5mV and up? Within no worse than 1mV of accuracy? For rather low frequecies of <50kHz.No need to go up to full postive rail.Edit: 50kHz offset AC, of course. Or varying DC, if you prefer. Just to avoid confusion. No negative output expected from a single supply.

author: ledtester
author_rank: Super Contributor
timestamp: January 17, 2021, 11:07:35 am
content: Would the venerable LM324 work?https://www.onsemi.com/pub/Collateral/LM324-D.PDFFor input the common mode range includes the negative supply.For output, the datasheet states a the typical output voltage low limit is 5 mV for Vcc = 5V. Don't know what it would be like for Vcc=15V.

author: hirada
author_rank: Regular Contributor
timestamp: January 17, 2021, 01:05:14 pm
content: At least that is an excellent suggestion to start testing with. In addition 40mA typ output current, makes it really look like a promosing candidate.Thanks very much!

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2021, 03:29:14 pm
content: The LM324 (quad), LM358 (dual) and LM321 (single) are reasonable suggestions, but bear in mind it can't sink much current, before the output voltage rises considerably. Refer to figure 11, page 10 of the data sheet.https://www.ti.com/lit/ds/symlink/lm358-n.pdfIf that's an issue, there are more modern alternatives, such as the NCS20072.https://www.onsemi.com/pub/Collateral/NCS20071-D.PDF

author: hirada
author_rank: Regular Contributor
timestamp: January 17, 2021, 04:13:30 pm
content: Thanks for the heads up the recommandation, indeed I have not had a look at the sinking capabilities. But unless I misread the datasheet, the ncs2007x have a low output swing of 10mV typ, max 100 - 150mV max (@5V, for comparisson). That is considerable higher than the LM324 with specified 5 - 20mVAnd at 10V, we are already up to 22 - 300mV. Now, 10V is not specified for the LM324, so we have to test and see. But generally, those values seem a little high for a voltage follower down to single digit Millivoltages. Or did I get something wrong?

author: rstofer
author_rank: Super Contributor
timestamp: January 17, 2021, 05:11:17 pm
content: In terms of the LM324, if you look at the "Functional Block Diagram" on page 11 of the datasheet, you will see why the output is never going to get down to the negative rail or up to the positive rail (whatever they are). There is a totem pole output so there is going to be at least some voltage drop across the device.https://www.ti.com/lit/ds/snosc16d/snosc16d.pdfOn page 7 the spec for VOLis 5 to 100 mV with V+ = 5V and the Output Voltage graph (Figure 11) doesn't include 0V output. 5 mV might actually work but the output load has to be fairly low. The thing is, 5 mV isn't guaranteed.Even a nominal Rail-To-Rail Input-Output op amp like the TLV2462 can't really get anywhere near 0V output - see VOLspec on page 7. 500 mV!https://www.ti.com/lit/ds/snosc16d/snosc16d.pdfRail-To-Rail is a marketing term, not an engineering reality.If you really want to get to 0V output, I think you may be forced to provide a negative supply. It doesn't have to be a large number, you could use +5 and -1 if you wanted.General information re: Rail-To-Rail op amps:https://www.ti.com/lit/an/sloa039a/sloa039a.pdf

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2021, 05:45:01 pm
content: Quote from: hirada on January 17, 2021, 04:13:30 pmThe data sheets are confusing, because they're not comparing the output swings, under the same conditions. The NCx2007x specifies the negative voltage swing with a 10k resistor between the output and positive rail, giving an output current of 0.5mA, at 5V and 1mA, at 10V. The LM324/358 data sheets only specify the low output voltage swing of 5mV to 20mV, when its output is connected to 0V, via a 10k resistor. If the LM324/358 output is made to sink 20µA, the lowest output swing will be around 200mV.The LM324/358 and NCx2007x have different output stages. The LM324/358 is an ordinary class B output stage with a 50µA current sink to pull it down nearer to 0V. If the output has to sink an appreciable current, the voltage will rise. The NCx2007x has a CMOS output stage. When the output is at either rail, it looks like a low value resistor, so the voltage rises with the current, in a linear fashion.The NCx2007x can sink more current than the LM324/358, with a lower output voltage. Refer to the graphs of the output voltage vs current sunk reveal this: page 15, figure 16 of the NCx2007x and page 9. figure 11 of the LM324 data sheet.https://www.onsemi.com/pub/Collateral/NCS20071-D.PDFhttps://www.ti.com/lit/ds/snosc16d/snosc16d.pdf

author: exe
author_rank: Unknown
timestamp: January 17, 2021, 06:32:08 pm
content: There are so-called "rail-to-rail output" and "output-to-ground" opamps.I wonder if adding an emitter follower with a load resistor would make it swing very low to ground. I think I've seen people adding a diode to opamp output to lower the minimum voltage (inside the feedback loop, of course), worth trying that.Or, just keep loading output of lm358 until it is close-enough.

author: rstofer
author_rank: Super Contributor
timestamp: January 17, 2021, 07:26:42 pm
content: Here's an op amp touted as Rail-To-Rail Output and yet VOLonly gets within 12 .. 20 mV and only with no load.https://www.analog.com/media/en/technical-documentation/data-sheets/ada4523-1.pdfI just don't see a device that can get to within single digits of mV on the output. But there are hundreds of devices so maybe...

author: eblc1388
author_rank: Frequent Contributor
timestamp: January 18, 2021, 07:05:18 am
content: I remember seeing a EDN design idea back in Aug 2004 edition which uses a simple two transistor current source at the output of the LM324 to sink 4mA constantly. Doing so would enable the output to go down to 4mV or less.The title of the design idea is "Current source enables op amps output to go to ground". However, when I visited EDN site today, I can only see the article description but failed to view any images by clicking on the direct links or clicking on the PDF version link. Sadly, without schematics, the EDN Design Idea archive would lose 95% of its value.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 18, 2021, 09:07:28 am
content: The usual current sinks have a hard time going down all the way. So this would not really help much.Best chance to come close to ground at the output is a single supply or RR OP that can sink / deliver relatively high currents. Some thing like the TS912 / TS922 are relatively good in this respect.For the input side there are many OPs that go all the way to ground and a little below (like some -100 mV). For the output side one often design the circuit that one does not really need less than some 100 mV.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2021, 09:20:35 am
content: Quote from: eblc1388 on January 18, 2021, 07:05:18 amI did a similar thing with a TL072. It works quite well, but don't bother simulating it, using TI's SPICE model. It won't work.https://www.eevblog.com/forum/projects/better-ltspice-tl072-modelI imagine it could be made to work closer to the rail, with the transistor working in reverse, which would have a lower gain and much lower saturation voltage, but it would limit the maximum voltage to around 5V.

author: ledtester
author_rank: Super Contributor
timestamp: January 18, 2021, 09:36:08 am
content: Quote from: Zero999 on January 18, 2021, 09:20:35 amFor the forum thread try this link:https://www.eevblog.com/forum/projects/better-ltspice-tl072-model/msg999066/#msg999066

author: hirada
author_rank: Regular Contributor
timestamp: January 19, 2021, 07:13:51 am
content: Thanks very much. Again, very educative. Especially for the heads up with reading the datasheets. Made f.e. also the TS272 look worse.Baseline is, asides any advanced tricks, there is no real way of doing a voltage follower that works from (really close to ) ground up with a single supply.

author: fourfathom
author_rank: Super Contributor
timestamp: January 19, 2021, 07:32:14 am
content: Have you specified the source and sink current needed at this output? You can always use a generic opamp to drive the gate of an NFET (source to ground, drain to resistor or current source, and to output). This gives an inversion, so you need to swap + and - opamp inputs. A decent NFET will have a few Ohms (or tenths) of RDSon, so can sink at least some current and drive the output voltage down to the millivolt region.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 19, 2021, 01:01:52 pm
content: The TS272 and TLC272 have a special problem working near ground. They are Ok with the input near ground or the output near ground, but they don't like both near ground at the same time.If it needs to be close to the ground side the AD8532 may be an option (about 2 Ohms to the ground side - so some 1 mV up to 500 µA). However this OP has has a maximum supply of 5.x V.Adding an additional discrete inverting stage to the output can make the stability tricky.

author: Zero999
author_rank: Super Contributor
timestamp: January 19, 2021, 01:27:51 pm
content: Yes, adding a discrete inverting stage would be a nightmare stability-wise. What's worse is the opem-loop gain would depend on the temperatre, load impedance and voltage, so extensive testing would be required to esure it's stable, under all operating conditions. An emitter follwer would be better, if it's a non-inverting configuration.If the output stage is class B, not AB, then I don't see why it can't work down to 0V, in an application where it doesn't have to sink any current. There would be no current flowing through the low-side transistor, thus no voltage drop.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 19, 2021, 01:38:08 pm
content: Many of the OP still use BJTs for the output stage, and these have a saturation limit of some 20-50 mV. So one can not go lower than that, even with no load.There are a few OPs with FET base output, but this is mainly for low voltage CMOS types. Even than there my be a resistor for current limiting.

author: rstofer
author_rank: Super Contributor
timestamp: January 19, 2021, 04:36:54 pm
content: That AD8532 has some merit. We can get less then 10 mv output with as much as 1 mA of load current according to Figure 13.The VOLspec is for 10 mA and with that much current, the output voltage is much higher - like 100 mV. As long as the load current is low, it is possible to have very low output voltage.https://www.analog.com/media/en/technical-documentation/data-sheets/AD8531_8532_8534.pdf

author: TimFox
author_rank: Super Contributor
timestamp: January 19, 2021, 05:41:32 pm
content: The "mature" CA3130A cmos op-amp has a common-mode input range that goes below Vss, and a "typical" output (into 2 k load) of 2 mV, max 10 mV, with respect to Vss. Into an infinite load resistance, this is typical 0 V and max 10 mV. This is at Vdd = +15 V.https://www.renesas.com/us/en/document/dst/ca3130-ca3130a-datasheet

author: Zero999
author_rank: Super Contributor
timestamp: January 19, 2021, 10:39:05 pm
content: Quote from: Kleinstein on January 19, 2021, 01:38:08 pmThe saturation voltage is not a limit, if the lower transistor never conducts, because it's configured not to sink any current.Here's a class B op-amp design, roughly based off the LM358, but without the current sink on the output. R1 and R2 pull down the output to nearly 0V. The small output voltage, when the input is zero, is due to the bias current, which is lower than the real LM358, as the BC547B has a higher gain than the BJTs inside the LM358. This is obviously no good in an inverting configuration which has to sink current.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 20, 2021, 09:46:47 am
content: The BJT based class B output with a resistor to ground (so ideally working as class A, with the low side not active) still has a problem once the transistor pulling the output low turn on (e.g. on a transient) the transistor can latch up in this state - if turned on the output would never go lower than the saturation limit and thus would keep it on, as long as the the amplifier would call for less than some 20 mV.

author: Zero999
author_rank: Super Contributor
timestamp: January 20, 2021, 01:33:04 pm
content: Quote from: Kleinstein on January 20, 2021, 09:46:47 amYes, you're right, it's class A, rather than B. I agree, the transient response would be pretty crappy, but it's fine for low frequencies. If you run the LTSpice file I posted, you'll find, the output has no problem swinging from 4V to 100nV and back again. I don't see why it wouldn't work, if I built it in real life, all be it with poorer input offset, even with matched pairs for the input stage.How is the circuit I've posted prone to latch-up?

author: hirada
author_rank: Regular Contributor
timestamp: January 21, 2021, 07:57:13 am
content: @TimFox:Albeit quite expensive, the CA3130A has been added to my shopping list. You can't have everything, I guess, so let's see how it turns out. Otherwise I will need to reconsider going dual rail. Thanks for that recommendation.At least it may be helpful in practical understanding some of the other comments in this thread about specs and the limits of it for an opamp.@blueskull:I do not need to source 40mA, I would be happy with 20. And probably could even go down to 15mA. But that's about the limit. The 15V are set in stone. The 40 mA output current of the LM324 just gave an extremely confident headroom. My assumption: The larger the headroom, the higher the chances to actually meeting the spec. Sorry, if this lead to any misconception.As I am looking for a single supply buffer, that works from ground up, I do not need to sink any current and the gain is (ideally) one. Now while I do accept, that there may be cases, where a voltage follower may need to sink current, I did not sink about this scenario. Think.However, with the on following discussion, as far as I have been able to follow it, it was rather helpful for my understanding, how any why sinking current affects the specs. Especially in term of reading a datasheet. Or at least I hope so. Practice will showSo thanks all again for your time and the explanations

author: exe
author_rank: Unknown
timestamp: January 21, 2021, 09:29:22 am
content: Quote from: Kleinstein on January 20, 2021, 09:46:47 amI don't understand how latch up happens. Is this something specific to ICs, or any bjt push-pull stage can latchup?

author: VEGETA
author_rank: Super Contributor
timestamp: January 21, 2021, 11:59:17 am
content: I think LM358 and LM324 are a good start. However, if you want modern and better parts while still direct pin-to-pin replacements... then you might look for LT1013 and LT1014 which are dual and quad op-amps respectively.https://www.analog.com/en/products/lt1013.htmlhttps://www.analog.com/en/products/lt1014.htmlThey are not so cheap thought but not expensive. LT1013 is about 2$ from Mouser, while LT1014 is about 5$.these options are a good start, but if they work then you need to optimize for cost and so on... you won't need that if it is a hobby or personal project.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2021, 12:31:34 pm
content: Quote from: hirada on January 21, 2021, 07:57:13 amDo you want to source or sink current?Sourcing current is easy, because the lower output transistor can be off. You also won't be sourcing the full current, at the lowest output voltage, unless you're using it to drive an extremely low load impedance. 15mA, with an output voltage of 5mV, would be a load impedance of just 300mOhm. If it has a class A stage, such as the CA3130A, it will be gauranteed to output 0V, as long as the output stage is not sinking any current, as no current will be flowing through the output stage.Sinking current is more tricky, because the lower transistor in the output stage needs to have a low enough resistance, to get below the required on voltage. As above, sinking 15mA, with an output voltage of 5mA, would require an on resistance of 300mOhm and no small op-amp IC can do that, because the output transistors will be too small. You might be able to use a ridiculously over-rated power amplifier IC, designed for Amps, but it's likely to have a poorer offset voltage. A discrete output stage, as mentioned above, could be added, but it would be very challenging to stabilse and avoid oscillation.

author: TimFox
author_rank: Super Contributor
timestamp: January 21, 2021, 03:21:57 pm
content: See Figure 17 of the CA3130 data sheethttps://www.digchip.com/datasheets/parts/datasheet/235/CA3130-pdf.phpfor a circuit to boost the output current by adding external CMOS inverters alongside the internal output stage. (Use unbuffered inverters for this circuit.) I remember that this trick was used in General Radio DigiBridges for the current-sense amplifier.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2021, 04:27:15 pm
content: Quote from: TimFox on January 21, 2021, 03:21:57 pmThere isn't any frequency compensation, so I'd be surprised if it's stable. The high gain of 255 will help somewhat, but be prepared for fun and games with oscillation.

author: TimFox
author_rank: Super Contributor
timestamp: January 21, 2021, 05:22:21 pm
content: Quote from: Zero999 on January 21, 2021, 04:27:15 pmTo clarify, I was referring to the op amp configuration, not the entire high-gain amplifier circuit. The CA3130 is not inherently unity-gain stable, and requires a compensation capacitor from pin 1 to pin 8 (the input to the CMOS inverter output stage) at unity gain. The output stage is a 4000-series inverter, which is a transconductance near the middle of the output voltage range, crossing over to an ON resistance near either supply pin. The circuit in Fig 17 adds three additional inverters (from a CMOS array) in parallel with the two output FETs (pin 8 to output at pin 6) to increase the transconductance and output current capability (not after the op amp, as often done with buffers and conventional op amps). The actual circuit in Fig 17 is for a higher gain inverting amplifier. I believe that an unbuffered 4009 CMOS hex inverter could be used, but I haven't tried it. The output stage is interesting, since the voltage gain depends on the load resistance (high-Z output in "active" region).

author: Zero999
author_rank: Super Contributor
timestamp: January 22, 2021, 11:03:59 am
content: Quote from: TimFox on January 21, 2021, 05:22:21 pmYou're right. I missed that the CMOS inverter is connected in parallel with the CA3130's internal CMOS inverter. Yes, the gain varying with the output voltage/current is an interesting feature of the CA130, which makes frequency compensation challanging.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 22, 2021, 01:16:39 pm
content: MAX44267? Dual op amp with built in charge pump. $5