Reach 0V with single-supply op-amps

The National Semiconductor LMC6484 looks as if it might come close to within 10mV of the negative rail if the output wasn't sinking much current - like a few uA.

If you've got a clock around that you don't mind loading (or buffering) a capacitative charge pump and a diode would probably do it. You've got to filter out the clock frequency ripple, but that usually just a couple of extra capacitors and a resistor or two.

-- Bill Sloman, Nijmegen

National's single-supply LM6132/6134 has a guaranteed maximum output low of 19 mV into a 100 K load, although that guaranteed max goes up as the output current (which you need to specify) increases, with a "typical" max low of 7 mV into 100K ohms. Might work?

What is the load? If it's a resistor to 0V then many CMOS opamps will do it. If the opamp output has to sink current then you need to say how much.

I posted a circuit here not too long ago that used an opto-isolator in a novel way to generate a negative current (not my idea, Bob Pease (RIP) I think.

The boring way would be a switched capacitor converter and RC filter if needed.

This, doesn't get much cheaper:

THe old LM324 would do 0 volts if I remember..

Jamie..

I notice Linear have just released an all-in-one version if you are feeling really lazy:

You need to develop the habit of refreshing your memory with actual data sheets

The graph at the top of page 8 - on the left - would have provided information that your memory clearly didn't retain.

-- Bill Sloman, Nijmegen

You really need to stop being a nasty old BASTARD. No one will come to your funeral... they'll all be out carousing and celebrating.

Which I do believe to be in error (they hire Slowman-types to do the clerical work on data sheets :-). Having used literally thousands of those devices, they'll easily get down to 50mV at 1mA load. ...Jim Thompson

Are you a circus wannabe left behind? What a wanker!

Goes to show just how much actual hands on you have with components. You're no better than the bible thumpers, just believe what the good book says, even though most of them create their own version of what you read. Sort of like you, read a data sheet and create your own version of it says and means.

Jamie

er

I was using the LM324 when you were in diapers.

The LM324 won't do 0V - as you claimed that it did. Without help it won't get as close to 0V as the National Semiconductor CMOS op amps that Rich Webb and I've identified earlier in the thread.

If one does what Marco Trapanese proposed, and pulls down the output with a negative supply, practically any single supply op amp will do the job, but pulling current out of the substrate can have interesting side effects and isn't to be recommended.

Referring people to data sheets is scarcely "bible thumping". The data sheets are written by the people who make the devices, and they can be sued if the devices don't perform as the data sheet says they will. You've got less recourse if you rely on the bible, or on opinions aired on sci.electronics.design. You may be of the opinion that your advice has the status of divine revelation, but few would share your confidence.

-- Bill Sloman, Nijmegen

This is advice that you might also take. I can't say that I'll care who comes to my funeral - it's not an occasion that I can expect to know much about.

Sadly Jamie was claiming 0V, which the LM324 won't make. The original poster - Marco Trapanese - would have settled for something under 10mV, which the LM324 may do if it isn't sinking any current. There are a couple of National Semiconductor CMOS op amps which will do better - Rich Webb and I identified one each, earlier in the thread, but your passion for kill-filling people you've disagreed with has presumably blinded you to this.

Il 29/08/2011 02:32, Bill Sloman ha scritto:

Thus, it could be done only if it's recommended in the datasheet, like OPAx330 does.

Marco

Il 26/08/2011 15:27, John Devereux ha scritto:

You're right. I forgot to mention. The op-amp output feeds the on-board ADC of xmega. The ADC acquires at 1 kHz.

Very interesting. Do you have any hint how to find the app-note where it's described in detail, if any?

Marco

Il 27/08/2011 13:59, John Devereux ha scritto:

Great! :)

Marco

As Jamie says, you can't always trust the data sheet. I once put together an Analog Devices circuit straight off the data sheet - wondering how the circuit got away with having op amps driving capacitors directly - and found that it didn't.

It was easy enough to put in the extra components that stopped the circuit oscillating and still let you rely the capacitors to keep the reference voltage clean, but it did make me more cautious. I'd never trusted Texas Instruments data sheets and application notes, but Analog Devices data had previously been pretty good.

You probably want to use a germanium diode or a Schottky with a very low forward drop to catch the current sink to prevent it from actually pulling current out of the OPAx330's substrate.

-- Bill Sloman, Nijmegen

OK I don't know much about that ADC - most of them can source or sink a current that increases with sample rate, and 1kHz is not very high.

I never saw it. James Arthur posted the original suggestion, the thread is:

(Sorry don't know how to post a concise link to the thread in google groups. If that link also exposes my amazon login and credit card details buy me something nice while you're there...)

Il 29/08/2011 16:57, John Devereux ha scritto:

Use tinyurl:

Marco