What OP AMP spec should I be checking to assure this behavior

Question

Recently, on the advice of others on another thread, I tried to migrate a dual supply OP amp circuit to single supply version. I needed a quad OP AMP, and the logic was that since my final circuit only needed to provide 0 to about 3.5 VDC, an LM324 would do just fine. Well that was easy to try because I had some on hand, but the result wasn't so hot. It turns out that my circuit must drive the equivalent of about 10K tied to +V, as in this simplified test diagram...

The result with the LM324 was that the output under these circumstances was a dismal 650 mVDC, a very LONG way from the hoped output of near zero.

This makes sense... the LM324 uses ordinary bipolar transistors for its output stages and that's about the best you can hope for. So I poked around and found a Texas Inst. quad OP AMP, the TLC274. It uses FETs for all its internal circuitry and its data sheet actually DOES mention its output includes the negative supply rail.

Unfortunately it certainly won't go to the negative rail with any load. In the example shown the best the TLC 274 could do was about 90 mV. Granted that is hugely better than the LM324, but its still a LONG way from zero. To be practical, I'd like the output in the situation depicted in my schematic to be 20 mV or less. Had I stuck with my earlier dual supply OP amps, of course this would not be an issue. But a negative supply is not available without a charge pump oscillator or similar trick requiring more parts so I'd like to stick with a single supply OP amp if I can. I'm just at a loss as to what OP amp specification I should be looking at so I don't keep running into his same problem. TI is pretty fast at sending samples, but ask them for support on a question like this and you can expect to wait a month.

Addendum:

The answers I received here have really enlightened me, and I want to thank those of you who chimed in. I have pored over the specs of several so called "rail to rail" op amps and have requested a few samples, but what I've really learned here is that "rail to rail" is a relative term, and a single supply OP amp may not be the best choice for my situation.

It is indeed a stretch to expect even the best CMOS drive OP-AMps to have an effective resistance low enough to ensure a 20mV or less output against a 1mA pull up load, and even more a stretch to think it would be sufficiently consistent between parts. The truth is that if I want precision control down near zero, a dual supply OP-AMP is a much better choice, if only because "0" really is not "0" with a dual supply, but only a "reference point" between +/- V supplies. Such a reference point can always be adjusted or trimmed as close to zero as necessary.

So since I don't have a V- supply available in my case, I have two choices. Either find a way to alter the circuit I am driving to eliminate the 10K pull up to +10VDC, or revert to a dual supply OP-AMP. To make this work I'll need to add a second OP amp package configured as oscillator and charge pump, to develop a negative supply for the second package. I've done this before and found that just a few volts of V- supply is usually sufficient to build up a good DC amplifier with good accuracy around 0 volts, with a little attention given to the source of the virtual "0" reference.

In any case I now understand that for such a precision DC amplifier, with good drive around 0 volts, it is nearly futile to expect a rail to rail OP AMP to automatically be a good solution. Its not the conclusion I'd hoped for, but win or lose its always good when you come away more educated.

Answer 1

You need to look for "rail-to-rail" op-amps and even then there will only be a few that will do 20mV (with a 1mA sink current) such as the AD8605 - not cheap but pretty good all round and beats the LM324 into pulp on most things providing it runs from less than 6V.

Here is linear technology's search engine with a few parameters also selected to give you a few to look thru

Going back to your idea of using an oscillator and small charge pump try looking for a single schmitt trigger in an SOT23 package - TI and Fairchild both make them. There are ones at TI that can output 30mA and can oscillate at tens of MHz. A couple of 1N4148 diodes and smallish capacitors should give you the negative rail you need.

---

Why can't one op-amp in a quad package generate a negative voltage (via a charge pump) to power the quad package's negative rail. If each op-amp takes 1mA unloaded then one op-amp has to supply 4mA but, in supplying 4mA this one particular op-amp now takes 5mA and therefore the total supply demand is now 8mA and of course this spirals out of control. On the other hand, one op-amp can be an oscillator and this oscillator can be buffered by a transistor that takes power from the positive rail independently of the op-amp. This will work so maybe consider that.

On a slightly different (but related) note, you can add an NPN BJT as an emitter follower to the output of an op-amp and use feedback from the emitter. What this gives you is worth trying. For a start you need an emitter resistor that won't drop more than 20mV when a 10k is pulling it up to 5V (a current of 0.5mA) therefore the emitter resistor can be no-more than 40 ohms. The other nice thing about this (if you can live with the extra current) is that the op-amp only needs to lower its output to ~0.7V to get ~0V at the output. This means the LM324 is now easily going to manage this situation. Here is an example: -

So, it's the 40 ohm resistor (Rload) that pulls down towards 0V to give you the low end of what you need. Going back to the original neg voltage generator, now that the op-amp is buffered from supplying its own current this method can be used to feed a charge pump.

Answer 2

In this case, there is no single parameter that you will find a datasheet to tell you if a given op-amp will meet your needs without digging a bit deeper.

The specification you are interested is, broadly speaking, output voltage swing. More specifically, VOL is usually the name for the minimum output voltage. However, the way this number is specified can vary between manufacturers and across product lines. A good datasheet will explicitly state the load that corresponds spec listed in parametric table. There may be several different numbers listed for different loads. If you are lucky, there may even be an IV curve of the output as it nears the rails.

The load conditions that are tested and specified will generally correlate to the largest load the amplifier was designed for. Consequently, you should focus on op amps that accommodate large load currents, and then look carefully at the output swing parameters listed in the datasheet.

All that being said, 20mV from the rail at 1mA is a lot to ask - you might want to investigate other options. The AD8605 seems to be up to the task... the next closest thing I turn up on a quick search is the LTC6087 which will pull down to 25mV at 1mA (typically).