Not being an analog guy, I'm having trouble choosing an op-amp from the vast array available. I just want to use it as a voltage follower to handle a slight impedance mismatch between a sensor and an ADC.
5V supply with output swing from at least 0.25 to 4.5V. Low-bandwidth application (DC to 300Hz) Would like a dual-gate 8-pin SOIC or (T)SSOP if possible.I realize this question sounds stupid, but is an LM358 suitable for this application?
No. The input common mode voltage range of the LM358 only extends up to
1.5V below the rail - 3.5V with a 5V rail. A 4.5V output voltage doesn't seem too realistic either.You might want to look at the Motorola (now Freescale?) MC33201. It's particular virtue is a realtively low output impedance, which is handy when you are driving nasty capacitative ADC inputs.
-- Bill Sloman, Nijmegen
An amp I used successfully for driving nasty capacitive loads with rail to rail in and out (within 100mV of the rail for both), rated at 2.5V to 30V supply (typical 5V) is the LM2861. I was using this as a unity gain buffer, which is your application.
Cheers
PeteS
For the OP - unity gain is usually the worst case for capacitive loading, as it typical (as Bill alluded) on an ADC input. There's an excellent app note about it from National (marketing their designs, of course, but hey, that's business)
Cheers
PeteS
Interesting. There was one thing in the datasheet that made my brain hurt a bit though--maybe I've been doing device work too long, but I'm puzzled by this passage from Page 15:
The output stage Figure 1 is comprised of complementary NPN and PNP common-emitter stages to permit voltage swing to within a VCE(SAT) of either supply rail. Q9 supplies the sourcing and Q10 supplies the sinking current load. Output current limiting is achieved by limiting the VCE of Q9 and Q10; using this approach to current limiting, alleviates the draw back to the conventional scheme which requires one VBE reduction in output swing.
Q9 and Q10 are the output transistors. But how does limiting VCE perform current limiting? Maybe they meant VBE? I can think of ways to do that, but VCE?
How do you IC guys do current limiting in a bipolar RRO amplifier?
Cheers,
Phil Hobbs
No, for reasons Bill described. You might be able to get the output swing with a pullup, but the input stage can't handle signals close to the +ve (ObJohnLarkin: 'positive') rail. So, forget it.
Last time I had this requirement for a moderate quantity design, I picked a Microchip MCP602x series part for its low max Vos** (and a number of other factors), but there are many, many parts which will meet your stated requirements.
** Note that with RRIO opamps the Vos can behave a bit strangely as you transition between the two input stages (Vos shifts suddenly by scores of uV with CM input voltage (and the temperature sensitivity changes in the case of the MCP602x series)). If your ADC is ~10 bits you can virtually always ignore this, because it's quite a bit less than the quantization error (~5mV in this case if Vref = 5.0V/10bits).Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Microchip has some parts made just for this type of application. I use one of their MCP6271 for a little signal conditioning on one of our products. Just go to microchip.com and take a look. Should be several that they list as rail to rail input and output.
Jim
The main reason I was looking at the 358 is that I bought twelve reels of them a while ago, and still have six and a half left. It's the only dual op-amp in an 8-pin package that I have in my home lab, in fact. I have a regulated +12V rail in my application, and for that matter a rather wiggly +/-10V available at very low current. I had some crazy plan to power the 358 off +12, and bias the input up off the 0V rail, but I kind of stopped there.
I've downloaded the datasheets for the parts suggested by everybody, thanks for the input. I happen to have a friendly Microchip rep keen to get her parts into my work, so it will PROBABLY wind up being the uChip part....
I think this factor can be ignored in my application. The sensor value is never going to be static - it wanders constantly. The software filters the input; it maintans a running average of sixteen samples in a ring buffer. If "this" sample differs from the average by more than a threshold, a second ring buffer is populated with a copy of the current ring buffer, and the new value is put in the second buffer. If, after sixteen further samples, the average of buffer 2 is further away from the original average than 1/16th the difference between the first maverick and the original average (i.e. if a trend is detected), the second buffer is copied into the first buffer and it goes back to "quiet" mode. I'm not quite sure what to call this sort of filter, but it works very well in the application.
The software that needs the sensor reading only sees the filtered average.
I think I didn't explain this fully... it's a DC effect.
"Suddenly" refers to the slope of Vos vs. Vcm (offset voltage vs. common mode voltage) not Vos vs. time (or imagine you were increasing common mode voltage at certain rate). But it's well under a mV (100 or
200uV, IIRC) in size, so unless you have a lot of bits or are dithering with noise or something like that to get more effective bits it will be well hidden.Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Well, you didn't say anything about having 12V available (and we assumed you didn't). In that case, you probably can use the LM358, but you'd have to clamp the output so it doesn't cause problems with the ADC. The LM358 also has a rather large maximum Vos (that means a DC offset error! perhaps several bits), but aside from that it works almost down to the - rail. You could get a -0.6V with a diode and resistor and a +12V from your regulated supply if it was necessary to get down to the rail. If your ADC can tolerate a couple of K you could use the second amplifier as a clamp using a diode and the 5V supply as a reference. If you only need to go to 50mV or so then you wouldn't need the - supply, ground would do.
The CMOS parts from Microchip are nice, and not insanely expensive, but they cost a lot more than an LM358 bought in bulk (probably under a dime). You do get 500uV Vos, almost no bias current, and few external parts for the money. OTOH, there are some applications where the bias current is actually an advantage (detect an open connection).
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Not all the RRI opamps exhibit this effect. See the National's range of CMOS opamps.
For example:
-- Thanks, Fred.
Yes, DS011714-60 and DS011714-61. Much better.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
"Spehro Pefhany" a écrit dans le message de news: snipped-for-privacy@4ax.com...
From the application information of the 6484 datasheet :
The LMC6484 incorporates specially designed
wide-compliance range current mirrors and the body effect to
extend input common mode range to each supply rail.
Complementary paralleled differential input stages, like the
type used in other CMOS and bipolar rail-to-rail input amplifiers,
were not used because of their inherent accuracy problems
due to CMRR, cross-over distortion, and open-loop
gain variation.
They do not explain further. I wonder how they did this. Over the top for each supply rail, with just one diff pair!
-- Thanks, Fred.
They've stopped supplying even simplified schematics. It's gotten me into trouble at least once.
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