Active Rectifier Bandwidth and Dynamic Range - Page 1
Forum section: electronics / projects

author: mc172
author_rank: Frequent Contributor
timestamp: July 03, 2022, 12:20:56 pm
content: I'm trying to half-wave rectify some signals up to ~50 kHz with an active rectifier but I'm a bit stuck. I built a quick simulation in LTSpice and I got what I expected to see at low frequency, up to around 1 kHz but beyond that, there's significant distortion (it looks a bit like a leading edge mains dimmer) and as a result the usable dynamic range is perhaps two decades (100 mV to 10 V) at 50 kHz.I've attached two pictures showing the input vs output at 1 V p-p and at 100 mV, both at 50 kHz. I've also attached the simulation file. The green trace is the input and the red trace is the -(output) (the output with a minus sign in front of it to get it to line up with the input signal for comparison).I ended up trying various op amps and saw that the LT1022 was quite high speed (8.5 MHz) with high slew rate (26 V/us) as I have read that slew rate can be a problem with active rectifiers. It's also JFET input.I have also read in various app notes such as Microchip AN1353 that a termination resistor is required to minimise glitches that overshoot in the opposite polarity of the rectified signal, hence R3 in my simulation. Omitting this resistor does produce some significant glitches.I've done some reading and have found that the book by Clayton/Winder "Operational Amplifiers" (5e) mentions that the slew rate limits how fast the op amp can overcome the forward voltage of the diode, but I can't see how this can be a problem with a 50 kHz signal and an op amp capable of 26 V/us. The measured max. slew rate in the images attached is about 2.5 V in about 32 us, or about 0.078 V/us, which just so happens to be in the glitch region.I also read in AoE that the op amp output takes a certain amount of time to recover from saturation, but the addition of the second diode (which they talk about and is present in the circuit I'm looking at) improves this massively as the output never saturates so I'm a bit confused.Is there any way to get more bandwidth and more dynamic range out of active rectifiers, specifically half-wave? Am I doing something wrong? Any help appreciated, thanks in advance.

author: Ian.M
author_rank: Super Contributor
timestamp: July 03, 2022, 01:11:51 pm
content: Low capacitance schottky diodes will help, as their much lower Vf drop reduces the voltage range the OPAMP output has to slew through as the input crosses zero and the feedback switches from diode to resistor and visa versa, and their lower junction capacitance reduces the low-pass filtering effect of the reverse biassed feedback diode's junction cappacitance effectively in parallel with the feedback resistor. However their performance is fundamentally limited by the GBW, delay and slew rate of the OPAMP.

author: David Hess
author_rank: Super Contributor
timestamp: July 03, 2022, 02:21:09 pm
content: That is how it is done. Older higher performance designs used an externally compensated LM318 or NE531 which allowed them to play games with the compensation to get even higher slew rates. They also AC coupled the operational amplifier to eliminate offset voltage error.The LT1022 is one of the best choices because of its high slew rate, but the ancient LM318 or NE531 were faster. There are modern parts which are much faster; I wonder how well an LTC6268 would work.What could be *really* fast is to drive the diodes with the output from a transconductance stage, so the slew rate is only limited by current and capacitance. Did anybody ever try making a precision rectifier with an operational transconductance amplifier?Switched precision rectifiers are very fast, but have more limited precision. See attachment below from Linear Technology application note 13 - High Speed Comparator Techniques.Quote from: Ian.M on July 03, 2022, 01:11:51 pmThe old high performance designs I am familiar with used microwave schottky diodes, but I am not convinced that is the best way to go with modern fast operational amplifiers. I think the base-emitter junction of a small signal transistor might be better for lower capacitance, lower leakage, and cleaner recovery; they are plenty fast.

author: iMo
author_rank: Super Contributor
timestamp: July 03, 2022, 03:45:50 pm
content: Works better with 1N5711..

author: exmadscientist
author_rank: Frequent Contributor
timestamp: July 03, 2022, 08:29:22 pm
content: Fast precision rectifiers are dreadful circuits, very difficult to get any kind of bandwidth out ofFor the standard options, I'd recommend taking a look at the Burr-Brown app note (now TI SBOA068,https://www.ti.com/lit/an/sboa068/sboa068.pdf). Any time I have compared circuits, I have always found the ones in that app note come out ahead. Figure 5 is usually the winner if it suits your signal ranges; it does quite well indeed for most circumstances. You'll need a fast op-amp, no surprise; to give you some idea of what works, I've had some success with things like ADA4891 for a precision-ish, definitely-cheap rectifier. Note that they consider 20kHz to be high bandwidth for one of these, and that's still more or less true! (Don't bother with them trying to push differential amplifiers. I mean, sure, do bother if you need very high precision and are prepared to pay for it... otherwise, don't.)If you can design out the precision rectifier, that is always worth considering. Tricks like replacing rectify-then-compare with window-compare can pay off here.And if none of that seems to apply, that's when you get to play with the exotica!

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 04, 2022, 01:34:02 am
content: MC, Did you try reducing your R2 to say 500 Ohm?There is diminishing improvement doing that because the loop gain is A * R1/(R1+R2)You are presently at A* 0.5 and could go to, say, A * 0.95, however this also reduces the wanted signal level and you might lose fidelity anyway in the following amplifier.I seem to remember something about this from 50 years ago and the old Burr Brown book is dog eared at that page.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: July 05, 2022, 10:03:59 am
content: The HMPS-2820 diodes are excellent for this kind of application. The site has a Spice model. The only negative is that they are very small.

author: Wolfram
author_rank: Frequent Contributor
timestamp: July 06, 2022, 09:56:02 am
content: The slow turn-on of your ideal diode circuit seems to be caused by a combination of diode forward voltage and capacitance. For the ideal diode to turn on, the op-amp output needs to slew from forward-biasing D13 to forward-biasing D14. This slew rate is given by how fast the diode capacitance is charged by the input current, i.e. the input voltage divided by R33. The four main options you have are;1: lowering the diode Vf to reduce how far the voltage has to slew2: lowering the diode capacitance to increase the slew rate3: reducing R28 and R33 to increase the slew rate. Observe the input impedance to make sure the preceding stage can drive it4: increasing the op-amp GBWThe 1N5711 suggested by imo addresses points 1 and 2 excellently. In combination with changing the op-amp for an LT1360 and reducing the 10k resistors to 1k, the turn-on time of the circuit at 100 mV input amplitude is reduced to around half a microsecond. Going much beyond this is probably best done with a better circuit topologyHow does this circuit fit into what you are trying to do? Understanding more of the system makes it a lot easier to give useful suggestions on how to best solve the problem.

author: mc172
author_rank: Frequent Contributor
timestamp: July 06, 2022, 12:08:41 pm
content: Thanks for your replies everyone. I've been busy with other things so am only just getting round to looking at this briefly. I will attempt some more simulations with all of your suggestions and hopefully form a more comprehensive response at the weekend.Wolfram, I'm interested to hear if there is any better circuit topologies that I can look into, do you have any more information please?As for how it fits into the circuit and what I'm trying to do, I'm trying to measure forward and reflected power using peak detectors, one for each half of a complex power signal. If there's a better way of doing it, I'm all ears!

author: David Hess
author_rank: Super Contributor
timestamp: July 06, 2022, 02:24:34 pm
content: Quote from: MegaVolt on July 05, 2022, 10:03:59 amThe HP HMPS-2820 diodes are equivalent to the diodes Tektronix was using in this application in the 1970s.I did not mention them because as far as I know, Broadcom/Avago discontinued them.