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Electronics Forum / Basics / November 2006Supply dependent voltage reference?
Hello once again! I'm reading about Texas Instrument's Single Supply Opamp Circuits reference guide. I was wondering if there is a way to create a half-supply voltage reference that is completely dependent on the input supply voltage. The reason why I'm asking is because if the supply voltage drops, then I want the voltage reference to also take this into account. For example, if I start at 5V, then my initial half-supply voltage would be 2.5V. But in time if the supply voltage drops to 4.8V, then I want the half-supply to automatically adjust to 2.4V. Any recommendations? Thanks! > Hello once again! > [quoted text clipped - 12 lines] > > Thanks! A voltage divider with equal resistors from the supply you want to reference from will do it. If you need low impedance, then use this as an input to a voltage follower, and use the output as your reference, or (for occasional transients only) put a 0.1uF cap across the resistor to ground. Cheers PeteS > Hello once again! > [quoted text clipped - 10 lines] > > Any recommendations? That's the way it always used to be done. A potential divider followed by an op-amp is how I've done it. You may also want to provide some supply ripple filtering on the 1/2 Vsupply reference too. Graham Thanks all! I was actually thinking about the voltage divider setup, too. But it didn't occur to me that can connect the divider output to a voltage follower opamp. Initially, I thought that using the voltage divider alone may pose some loading issues if I'm pulling too much current. Is this a right thought? > Thanks all! > [quoted text clipped - 3 lines] > alone may pose some loading issues if I'm pulling too much current. Is > this a right thought? You are correct. Keep in mind that any loading (by taking current) presents an effective resistance to ground inversely proportional to the loading. One thing; unless you are using an opamp designed for it, do NOT put a large amount (0.1uF is large) of capacitance on the output of the follower; it will probably oscillate due to loop instability. Cheers PeteS Thanks, PeteS! > You are correct. Keep in mind that any loading (by taking current) > presents an effective resistance to ground inversely proportional to the > loading. Ahh.. It took me a few minutes to grasp this, but now I see it makes perfect sense. > One thing; unless you are using an opamp designed for it, do NOT put a > large amount (0.1uF is large) of capacitance on the output of the > follower; it will probably oscillate due to loop instability. Loop instability is still somewhat new to me. Thanks! Another question popped out regarding this topic. If I were planning on using the voltage divider & voltage follower setup as my half-supply source, what op amp parameters should I check? Will this have a similar fan out characteristics that CMOS or TTL devices have? > Thanks, PeteS! > [quoted text clipped - 10 lines] > > Loop instability is still somewhat new to me. An opamp in a negative feedback loop tries to produce an output that makes its two inputs have the same voltage. If the output is loaded by a lot of capacitance, changing the output voltage requires that the output drive a large current into the capacitor to charge it up. This can crank the output stage all the way to current limit conditions, which take a while to recover, once the correct voltage is achieved. But during that recovery, the cap is charged past the correct voltage, and the opamp has to suck current out pf the capacitor to get its voltage lowered back to the correct value. But if this process also hits the limit current, the voltage will overshoot that way, also, and the process repeats... an oscillation. >> Thanks, PeteS! >> [quoted text clipped - 22 lines] > hits the limit current, the voltage will overshoot that way, also, and > the process repeats... an oscillation. Well, that's one way of looking at it. An op amp, as opposed to a comparator, has an RC pole (which induces other things) internally, and quite deliberately, to roll the open loop gain off. Now that pole (sometimes known as the dominant pole) will have a -3dB point at perhaps 10Hz (or below). At this point, it will have induced -45 degrees of phase for a voltage style device. At 1 decade further (100Hz in our simplistic example), it will be at 90 degrees and will stay there. Add another -90 degrees and what was negative feedback becomes positive feedback. If that holds at the unity gain point, then you have an oscillator. This is a _very_ simplistic look at such things; a typical amp has multiple poles and zeroes, but the datasheet usually has guidance on the maximum output capacitance. Cheers PeteS > This is a _very_ simplistic look at such things; a typical amp has > multiple poles and zeroes, but the datasheet usually has guidance on the > maximum output capacitance. Using this datasheet guide, how would I be able to calculate or estimate the maximum capacitance presented by other opamps being driven by the half-supply source (voltage divider + opamp voltage follower configuration)? Is there a datasheet parameter that will tell me this? Also, after looking at the Single Supply circuit guide even more, I noticed that the author mentions the following about the voltage divider + voltage follower setup: "...but its performance deteriorates at low frequencies." Why is that? Here is a link to the PDF: http://focus.ti.com/lit/an/sloa058/sloa058.pdf Thanks! >> This is a _very_ simplistic look at such things; a typical amp has >> multiple poles and zeroes, but the datasheet usually has guidance on the [quoted text clipped - 4 lines] > by the half-supply source (voltage divider + opamp voltage follower > configuration)? Is there a datasheet parameter that will tell me this? The input capacitance of most active devices is specified in their datasheets. You also need to add the track / wiring capacitance, but most amps will be ok for any reasonable load. Note that not all amplifiers are stable at unity gain. This is covered in most texts on the subject. If you haven't read the underlying theory of feedback, now would be a good time to start :) > Also, after looking at the Single Supply circuit guide even more, I > noticed that the author mentions the following about the voltage > divider + voltage follower setup: "...but its performance deteriorates > at low frequencies." > > Why is that? As you approach DC, the output impedance of the amp approaches it's output resistance, which is somewhat higher than the dynamic impedance (such as step response). The output resistance of the amp will typically be a few 10s of ohms, the impedance (in terms of step response) a few milli ohms. Incidentally, this circuit would benefit from a feedback resistor (see my comments in the other thread) of about 50k rather than a straight wire; that would miminise input current offsets. > Here is a link to the PDF: > http://focus.ti.com/lit/an/sloa058/sloa058.pdf > > Thanks! Cheers PeteS > Thanks all! > [quoted text clipped - 3 lines] > alone may pose some loading issues if I'm pulling too much current. Is > this a right thought? If the current is higher, you used lower value (but still equal) resistors. It places more of a load on the power supply, but it takes more load to affect it. Michael > The reason why I'm asking is because if the supply voltage drops, then > I want the voltage reference to also take this into account. For > example, if I start at 5V, then my initial half-supply voltage would be > 2.5V. But in time if the supply voltage drops to 4.8V, then I want the > half-supply to automatically adjust to 2.4V. use a voltage divider, possibly several.  SignatureBye. Jasen |
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