I'm fairly new to electronics and I am having a bit of a problem understanding which path to take. I am needing to construct a receiver stage that accepts a differential input signal. So I have been reading about and planning to use an op-amp wired as a difference amplifier. One of my questions is in powering the op-amp... and also understanding enough about different op-amps to make a good selection. I have +12VDC power and am uncertain whether I should use a voltage inverter IC, construct a stage to produce -6 and +6 or what. Then I started reading about single-supply opamps, but I haven't found anything that fully explains them (at least enough to make a choice). I know what "rails" are but what is the difference in "rail-to-rail", "not rail-to-rail" and "outside the rails" which are some terms I found on various chip makers sites. The last part of my confusion come from that fact that I intentionally want to clip any negative portions of the output of the opamp. One place I read said that I could use ground as the negative supply to the opamp to accomplish this, but another book I have said that if the input signals go outside the limits of the opamp power supply, the opamp can lock-up. Which is it and where can I go for a clear understanding? Thanks all and sorry for rambling.
I'll put off giving you advice about the application till we clear up some of the opamp generalities.
All these terms relate the voltage range of input and output signals to the supply voltages (rails). All opamps have specific limits on what the input voltages (there are two input terminals) can be and still have the input pair function as a differential amplifier. This is called the common mode voltage (the voltage the two inputs have in common while the differential amplifier amplifies their difference) range. There is also a different input voltage range (related to the supply rails), called absolute maximum voltage limits, that prevents damage to the device. The opamp does not actually care if its rails are on one side of signal common or straddle it, as long as the applied input voltages stay within the absolute limits. It will function as intended as long as the input voltages stay within the common mode voltage range, with respect to its supply rail voltages.
Ordinary opamps have a working common mode voltage that reaches to within a few volts of either supply rail voltage (regardless of what those voltages are with respect to signal common). Single supply types typically work from at, or slightly below, the negative rail voltage to within a few volts of the positive supply rail. Rail-to-rail types have a common mode input range that equals or slightly exceeds the voltage range between the rails.
The output of an opamp is also limited in its voltage swing by the voltages applied to the supply pins. Ordinary (not rail to rail types) typically can swing their outputs within a few volts of either supply rail. Those designated as single supply types can swing their output voltage to within millivolts of the negative rail under some load conditions, but still swing to within only a few volts of the positive rail. Rail-to-rail types can swing their output voltage to very nearly either supply rail (under some load conditions).
So you need to explore your range of input signal voltages and what you can do to bias them in different ways, and your requirement for output voltage swing, before you are ready to try to fit those requirements to a combination of opamp types and available (or practical) supply voltages.
This is a separate problem until you decide how this clipping is to be done. The two general ways involve saturating the output (which may have recovery problems and input over voltage problems, when the negative feedback opens up during saturation) and using some nonlinear feedback that abruptly changes the gain as the voltage passes through some particular voltage, but the opamp continues to function as a closed loop process the whole time.
A definite possibility for some opamp designs.
Start with the signals and work out to what you need to do with them, to set some boundaries on the problem. Try to keep as much flexibility in this part of the process as you can imagine, to not rule out some good solutions before they are explored. Then you are in a position to begin considering which of the three general kinds of opamps and their required supplys fit each way the signals could be processed.
Even if you write all that much, there are many points open which permit to give you a reasonable advise. First the input: where does the input come from? Impedance of the driver stages. What kind of signal? frequency and voltage range, how much common mode signal to be suppressed? DC accuracy needed, amplification needed? Then the output: What is the next stage(impedance), what range of voltage, do you need swing to gnd or have a reference voltage available? Then the desired function: how precise has to be the rectification and the clamping of the negative portion? These are not theoretical questions, but each refer to specific circuit constallations. I even doubt a differential amplifier will do the job, rather look for an "Instrumentation Amplifier".
Hi, Seware. I think I understand what you want, although your description is a little hazy. You have a single 12VDC supply and two input signals, both of which go below GND. You want to use an op amp to amplify the difference between the two signals, but you only want the positive excursions of the output signal -- in other words, you want the output to act like a rectifier of the difference between the two signals, with unspecified gain.
I don't think there are any rail-to-rail op amps which can amplify the difference of two signals, both of which are below the negative rail of the single supply. One thing you might do is use an op amp which has an output which can go down to the negative rail with a pulldown, and do something like this (view in fixed font or M$ Notepad):
This looks a little like the difference amp you know from school. There are a couple of changes. The voltage divider R1 and R2 sets up a DC value which should bring your input signal up out of the weeds so it's in play. You sum that DC level with your input voltage to provide a composite input your difference amplifier can work on. Note that you're going to have to get out your DC electronics book to figure out appropriate resistor values to get a composite Ri at a DC voltage level that's useful to you. Remember also that, to keep it simple, Ri and Rf for V1 should equal Ri and Rf for V2. Then you can use your standard difference amplifier equation (if you don't remember, see National Semiconductor AN31 for op amp basics).
The only other quirk is Rp, which is a pulldown necessary for most op amps that go down to the negative rail (or within a couple of millivolts).
This difference amplifier will obviously saurate at negative output (0V) as V1 exceeds V2, which I guess is what you're looking for. And you won't have to worry about amplifying inputs below the negative rail. You can start with 1/4 of an LM324, and work your way up in specification and expense as your problem demands.
If this doesn't fill the bill, or if you want more help, you'll have to throw a little more light on your problem:
What are the maximum and minimum frequencies of interest? You've got me half thinking this is RF the way you phrased this.
What's the maximum signal amplitude? If your signal is DC or low frequency AC, what's the maximum and minimum excursion of the voltage?
What's the output impedance of your signal source? Can your signal source source and sink current?
Is your 12VDC supply floating, or is it referenced to the input signals?
What's at the output of the gain block you want? What's the load?
What's an acceptable level of error or distortion?
And, as inevitable when posting before the second cup of coffee, a minor revision. Use two identical voltage dividers R1 and R2 for _each_ input as above, instead of sharing. Other than that, OK, I guess. Sorry for the confusion.
And, as inevitable when posting before the second cup of coffee, a minor revision. Use two identical voltage dividers R1 and R2 for _each_ input as above, instead of sharing. Other than that, OK, I guess. Sorry for the confusion.
And, as inevitable when posting before the second cup of coffee, a minor revision. Use two identical voltage dividers R1 and R2 for _each_ input as above, instead of sharing. Other than that, OK, I guess. Sorry for the confusion.
Or actually, if you're using a dual or quad op amp, it might be better to feed the voltage divider into one of the op amps set up as a voltage follower and save the two resistors.
Sorry for the lack of info. I'll try better. Basically I have a two-wire inductive pickup that is used for RPM count of a motor shaft. The AC voltage on the two wires are of nearly equal magnitudes and opposite polarity. Differential signal right? The voltage range I see on my oscope for each wire is +- 8 V. Freq ranges from 0 - 300 Hz (18000RPM). Pickup resistance is155 ohms +- 20%. I currently have available a 12VDC battery and a buck-boost circuit that outputs 5VDC. So my desired output of this stage is a TTL voltage edge on the positive pulses of the composite signal. My use of op amps is limited outside of simple signal amplifying (again an elec. newbie)... so reading all that I could find, I surmised that using a differential op amp setup would allow me to create a composite signal that I could then rectify to eliminate the negative swings. I just don't know enough about op amps to make an educated guess as to how best to power the thing, what type of opamp to use, etc. Do I require pre-filtering for transients or this what CMR is for? I'm sure there are circuits available out in www-land that I could study and learn from but I have not found exactly what I am looking for yet. If you need more info please ask and thank you for your time.
You're not looking for an opamp at all. You already have 8Vp-p. If your sensor has only two wires, and neither of these are connected to the case ground, then you can simply ground one of them, clamp the other to the
+5V rail and to ground with a couple of diodes, and drive the input of a schmitt trigger inverter, such as the 74HC14:
+5V | V DIODE (e.g., 1N4148) --- cathode ------- | |\\ | |--------+----+-----------| >o------- Out | sensor| | |/ | |---, V 1/6 74HC14 ------- | --- | | GND GND
This is assuming, of course, that your sensor is simply an inductor, and there's a magnet on the shaft.
The output will be "better than" TTL levels - HC parts give a practically rail-to-rail output.
I would have sworn that you post from Google Groups--but nope.
Folks on Usenet like to see a bit of the previous post included with (actually, above) the text of your post. This is called **context**. If you observe how most folks post, you'll see what I mean.
Most newsreaders blockquote the entire previous post for you so that you can easily snip out those portions which don't apply, leaving just enough to give context to your addition.
Rich Grise (pointedly) remarks on context here:
formatting link
The sub-thread was about Google posters; you can easily see why I figured that you posted from there.
The guidelines for Usenet are here: http://66.102.7.104/search?q=cache:8PaSp2kKbWoJ:
formatting link
*-top-*-*-message+do-not-*-*-*-original (Worth scanning--especially the parts I've highlighted.)
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.