Single Supply Precision Rectifier Design and Optimization
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- čas přidán 19. 01. 2016
- An introduction to single-supply precision op-amp rectifier circuits, their inherent limitations, and optimizations to reduce output distortion at higher frequencies. Blog post with schematics, references, and additional reading: www.analogzoo.com/?p=1361
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In the first half-wave rectifier circuit 0:27 has a problem. When the input voltage goes negative, the resistor divider will try to apply up to 1/2 the negative voltage at the inverting input. The input's esd protection diodes will clamp the voltage on the inverting input. These esd protection diodes are not designed to withstand repeated (or continuous) conduction. For a 10k input resistor, and low voltage waveforms, you might be able to get away with this.
Pulling current out of an opamp's input pin by applying a voltage that is below the ground rail, can cause strange circuit behavior, and can negatively affect the opamp's lifetime; Keep the input voltages within the power supply rails. I've done my share of electronics failure analysis, and have seen circuit boards designs with with op-amp circuits that have voltages applied that are outside the power supply rails. These circuits have much higher failure rates than circuits that keep the input voltages within the supply rails.
The only exception to this being over-the-top op-amps which are designed to transition between input topologies and prevent reverse breakdown of the input transistors at the expense of drastically increased input bias.
The best video I have ever seen. Thank you!
Excelente vídeo. Thank you!!!
If you do this circuit with Unity Gain Difference Op Amp it. The resistors are precision which helps alot with resistor matching.
Great explanation ... Thanks
Very nice strategy 👍
Very nice video! U are great!
Very Nice, keep the work up!!
+TecKonstantin Thanks, and thank you for sharing that circuit!
I have modified this as an effects creators for electronic music. Replace the 10k feedback from inverting to output with a 1M pot to adjust gain and all sorts of distortion (which I want) is created.
Thank you for a good presentation.. Could the BJT and 74HC4066 be replaced by a fet, eg IRLML2803?
Good video, thanks. I have a question: in the circuit at minute 18:10, what type of diode is connected to the comparator LM393? How can the comparator output go low if the voltage between its two inputs is always greater than zero?(because of that diode)
Is there any way to include the input bias compensation resistor in your circuit?
Does the audio source needs to be isolated or can I rectify the audio coming from my computers jack directly? Thanks.
this is best for adc 16 bit 20:00, you can measure 0-5/65535 and 0 to -5v/65535 but will indicate negative, so i assume that i can measure -65535-0-65535, so an arduino will read from dac 0-65535 and will detect if is negative or positive, im i wright?
Hey! So I used this circuit in our single supply envelope detector for a compressor. Works perfectly! I might want to try achieving the switching with a transistor alone rather than a transistor and a chip. Why didn't you do so?
I didn't try it but I suspect that if you just use a BJT to pull the non-inverting input to ground you will get some distortion there. A FET switch will probably be better, and the 4066 is simple and cheap so I just went with that.
Nice video and well explained, thank you but can I just query something with you, it appears at 19:00 onwards when you explain the the circuit on the board, you say the second opamp output will track positive on the positive going input waveform and this will turn OFF the inverter logic (transistor) but you have drawn an NPN transistor which will be turned ON with a positive input waveform........the circuit wouldn't work as described.. correct me if I'm wrong or am missing something here.
+Michael Hawthorne The explanation perhaps wasn't the clearest but it should work as drawn. When the input is signal positive the comparator output is high, the transistor is on, pulling the input to the HC4066 low, so the analog switch is open and the opamp runs as a non-inverting amplifier. I don't see why there is a need for the transistor inverter - just swap the inputs to the LM393 and connect the output directly to the HC4066 input retaining the 3.9K pullup resistor.
+Michael Hawthorne TMM is correct, what I meant by "OFF" was that the logic output of the transistor inverter would be off (i.e., a voltage low state); for this to be true, the NPN transistor must of course be on, as you pointed out.
+TMM In theory, the transistor inverter is not necessary as you could use the comparator in an inverting configuration. In practice though, I've found the LM393 to be very prone to oscillation in an inverting configuration with a high source impedance (10k, in this case). However, the only other comparators I had on hand had longer propagation delay times (which would further limit frequency response), and the usual methods for preventing oscillation would have similar effects (hysteresis would cause a discrepancy in the zero-crossing detection, and capacitor compensation results in additional comparator delay). The inverter was a quick and easy solution that added minimal additional delay.
+TMM Actually, the inverting comparator oscillation issue can probably be fixed by connecting the comparator's positive input to ground via a 10k resistor, rather than connecting it directly to ground. Then you can get rid of the inverting transistor completely.I probably should have tried that. :\
Also, an LM211 comparator would probably be a better choice for this application, but I didn't have any on hand.
At 21:01 you put up a caption stating "it is also using the same MCP601 op amps as all the other circuits in this video" - for a dual supply design. But the MCP601 is a single supply op amp. Please explain.
+Smudger Dave Any op amp can be powered from either a single or a dual supply. A "single supply op amp" has had certain characteristics that are typically advantageous for single supply operation - such as rail to rail outputs - emphasized when its internals were designed by microchip/TI/Linear/Analog/whoever. But when using the op amp in an application, the op amp itself doesn't know or care if you run it from a single 5v supply or a split +-2.5v supply, which is exactly what I did for the dual supply rectifier circuit in this video.
Can you explain precision bridge rectifier the same way.. Pls
An eligant way to solve the problem of saturation and a swinging diode drop. I must remember this idea for my future projects. However it just occured to me looking at this circuit, would it be possible to replace the NPN transistor and 4066 analog switch with just one N-MOSFET such as a $0.02 2N7002A
The FET would is configured as common source and will ground the op-amp non-inverting input when comparator output is high.
Can one simply connect a diode anode to op amp "+" and cathode to comparator output? Remove 4066 and BJT.
Great videos!!
I expect the diode would likely introduce some distortion. A p-channel JFET would probably be better suited to replace the 4066 (I just had a bunch of 4066's on hand), and the BJT is not required if you use the comparator in an inverting configuration.
Thanks, You're right!! Use of the diode, as I suggested, sucks. Becomes reverse biased when input goes neg. Won't clamp the op amp "+". Would the P-ch JFET be a problem when the source-gate bias reverses? Perhaps an N-ch and reverse the comparator? Is it ok to exceed the 4066 input pin limit of -0.5v? Seems to work well with your -1v demo.
I appreciate your videos, keep up the good work.
What's the best way to get rid of the diode glitch on a half wave single supply setup?
For single supply half-wave rectification, no diode is used. Unfortunately, it typically relies on driving the op amp to ground, which puts the op amp into saturation, and as demonstrated in the video, this is the cause of the glitching that you see at higher frequencies.
So there's no way of reducing the glitch on the half wave circuit?
There is, but to fix a problem you have to understand its source. As shown in the video, the glitch observed in the half wave rectifier circuit is due to the op amp's output saturating, delaying its response. You can get an op amp with a higher slew rate (and higher price tag), and that will help some, but doesn't address the core problem. A switching circuit is probably the most obvious approach, as demonstrated in the full wave rectifier circuit at the end of the video. That circuit should be easily adapted to a half-wave rectifier by using the analog switch to ground the input signal during the unwanted half of the signal. Of course, you would also have to ensure that your signal source is OK with that. If you can provide more specific details on your desired application and performance, then perhaps I can give a more specific answer.
Excellent video. I watched it because I was interested in overcoming the diode drop used in an RF detector circuit in SWR measurement circuits in amateur radio equipment. In this case I'm talking about a frequency of 30MHz for HF bands, and even higher for VHF. We use germanium diodes as simple detectors because of their low forward voltage drop, but with low level RF signals even 200mV is a huge drop, in addition to being non-linear with any current load at all. Clearly the slew rate is the key to achieving linearity of a rectifier using the circuit techniques you describe above. I would estimate that I need an opamp with a frequency capability of at ten times that of the frequency I'm monitoring. A Max477 from Maxim, for example is rated at 300MHz. Do you know of any circuits, devices or techniques to overcome diode forward voltage drop that one might use in this application?
Have you solved your frequency-limited problem? I am curious to know what you used to do so.
ого. на последней схеме уже достаточно много деталей... ну тогда можно сдвинуть входной сигнал до середины питания, сделать его инверсию, и переключать мультиплексором при пересечении этой половины питания... ну и результат сдвинуть обратно к нулю.
like: continuous flow of speech while the diagrams etc. are magically updating on the whiteboard. don't like: metal screwdrivers near a scope screen.
+Dennis Lubert Yes, fair point. I need to use something longer/thinner anyway, the camera tends to auto-focus on my hand or the screwdriver handle if they get in the frame. Glad you like the video editing! :)
Why wouldn't you just use two opamps? Both working with single supply. One is being used as just a simple buffer/voltage follower, the other one is being used as an inverter. After that it's a matter of summing the two. Maybe you can even do this with comparators. (you probably need to correct for some gain).
In this circuit I guess you could also change the input of the comparator to inverter the circuit. Saves the transistor.
There's still the problem of op amp saturation recovery. For example, when the input goes negative, the non-inverting op amp will saturate, and it will take some time to come out of saturation when the input goes positive again. The same problem will occur with the inverting op amp when the input goes positive. These saturation recovery delays will result in "glitches" in the rectified waveform and limits the circuit's high frequency response.
You can certainly use the comparator in an inverting configuration and save yourself a transistor. However, many comparators (such as the LM393 used here) have their inverting input pin right next to the output pin, and especially with high source impedance (such as the input protection resistor in this circuit) capacitive coupling can cause unwanted feedback, resulting in "chatter" on the comparator's output. Careful circuit layout prevents this, but using the comparator in a non-inverting configuration is more forgiving of sloppy layout (i.e., testing the circuit on a breadboard).
I honestly never encountered any 'feedback' issues as the way you described. Even not in 300-400kHz Class-D switching circuits, where these kind of problems should be much more significant.
The reason why I am talking about alternatives, is because in practice, I now need to buy at least two different packages. Which takes up a lot of space. There are some comparator-opamp-in-one packages on the market, but the choices are very slim and limited (+ pricey). In the end it all comes down to practical choices. Nowadays it's not so difficult to find affordable fast high-speed opamps (BW around 200MHz) for less than a dollar. It would be interesting to compare these with this circuit.
With reasonable circuit layout and/or low source impedance, you probably wouldn't encounter the feedback I described. But larger source impedances and stray parasitics can cause nasty oscillation on the rising/falling comparator edges (it's pretty easy to replicate on a solderless breadboard, for example). ADI did an "ask the engineer" piece which talks a bit about it: www.analog.com/library/analogDialogue/Anniversary/5.html. I wouldn't expect the same problem on a printed circuit board or a dead bug solution with careful layout.
I didn't try any high speed op amps, but its not really the speed of the op amp that is the limiting factor in most cases, it's how fast the op amp can come out of a saturated state. The MCP601 op amp I used certainly isn't high speed (2.8MHz bandwidth), but even frequencies in the audio range get distorted when used in a typical rectifier configuration. Note in the 'scope waveforms that once the op amp output starts tracking the input again, it catches up quite quickly; it's the delay in tracking the input, caused by saturation, that poses the problem. Unfortunately, saturation recovery time is not specified by manufacturers, so you'd need to get a reasonable sample of op amps and test them to see how they perform. If you have an op amp that doesn't distort your waveform at the frequencies you need, you can just use one of the single op amp circuits presented in the beginning of the video.
You're absolutely right about making practical choices, and it all depends on your application and design. Do you really care about that little bit of distortion in the bottom of the waveform? If you're implementing a peak detector circuit, probably not. Are you digitizing the rectified waveform? If so, many PIC microcontrollers (even the really tiny ones!) have built-in comparators that you can use, no need to buy another IC. Are you optimizing for cost, size, power consumption, etc? You have to take it all into consideration of course and make the trade-offs that best suit your needs. This is just another circuit that may be useful for some situations, but certainly will not be optimal for others.
devttys0 fg
360p for everyone or is it just me?
+Евгений Жук (Nacky) No, not just you apparently; not sure how that happened, I definitely shot the video in 1080p!
+devttys0 Thanks for quick response, I thought there was a problem on my side. The vidio is great anyways.
+devttys0 when uploading youtube converts to all other resolutions, starting with the lowest. As long as it is doing that, only those low ones it has converted to are being displayed. Finally it will reconvert your resolution into its own format and only then it will be displayed. You can somewhat "fix" this by uploading it hidden, and then when its rendering is finished, unhide it. Oh btw. what do you think of enabling the feature of subtitles for others to add (in different languages)?
+Dennis Lubert I heard about that but thought that its long time gonsky, as everybody seems to use the hide-unhide trick.
+Dennis Lubert Interesting...I uploaded the video as private, then only *after* youtube said it was finished rendering did I add the video annotations and make the video public (which is what I always do). Odd that youtube said rendering was finished, but clearly had only finished for lower resolutions; it did seem that the rendering finished really quickly for this video! I'll have to watch out for that in the future...
I've turned on community subtitle submissions for this video (czcams.com/users/timedtext_video?v=1TLZBWx6BCc), I'll be sure to add it to my other videos as well!