OpAmp to uniform amplification of signal from DC to ~100MHz - Page 1
Forum section: electronics / beginners

author: gashtaan
author_rank: Regular Contributor
timestamp: March 23, 2024, 01:39:59 pm
content: Hi,I'd like to observe current consumption of various MCUs in sub-microseconds scale. The current consumption of MCU would be let say from 10uA to 30mA. My idea is to use 10R shunt resistor and amplify voltage drop with some high-bandwidth (~100MHz) operation amplifier with 10x gain to the level measurable with an oscilloscope. The problem is I want to see both fast changes in consumption on MCU cycle granularity and constant consumption during longer time periods too (i.e. I don't want to filter the DC component out). What I've seen there are OpAmps that performs great at high frequencies but they performs bad at low frequencies and vice-versa. Is there any OpAmp that would satisfy my needs or perhaps different solution?I was going to use BGA2803 for it (I've seen it's being used in similar project), but I'm not sure if it's usable to amplify DC too. In the datasheet they say "General purpose low noise wideband amplifier for frequencies betweenDC and 2.2 GHz" - but there are DC blocking capacitors in application information schemes and in Fig.9 is obvious that the gain drops significantly for low frequencies.

author: jonpaul
author_rank: Super Contributor
timestamp: March 23, 2024, 02:06:36 pm
content: use TEK crrent probe and amp, wide range capability, 50..100 MHz BAlt: 4 term series R and wideband diff probeSee app notes eg Linear Tech, on diffeerential amps.j

author: gashtaan
author_rank: Regular Contributor
timestamp: March 23, 2024, 04:29:19 pm
content: Thanks for advice. I think that differential amps are overcomplication for this and differential probes are too expensive. I want to put shunt on low-side, I hope there is an easier (and cheaper) way to do it with one or two ICs and some passives around. Am I too naive?

author: David Hess
author_rank: Super Contributor
timestamp: March 23, 2024, 07:19:02 pm
content: Let me describe how I would make a high side measurement with your requirements. There are two methods I would consider.The first involves the AD8129 difference amplifier which is almost ideal and more than fast enough. If its offset and drift are not good enough, then I might add a fast differential preamplifier made from a couple of current feedback operational amplifiers. The AD8129 could also be useful for a low side measurement because its common mode rejection will remove common mode noise from a remote measurement.Alternatively there is a2 or 4 transistor circuitfor measuring high side current. An operational amplifier could be added to remove DC errors, but I am not sure how to apply it to a low side current measurement.Aren't parts like the AD8007 fast and precise enough for your application?

author: dobsonr741
author_rank: Frequent Contributor
timestamp: March 23, 2024, 07:49:22 pm
content: Are you doing security research, or just want to analyze/optimize for low power?If the later, you won't see much sub microseconds detail in a real circuit. A good industry solution example is TI's EnergyTrace,https://software-dl.ti.com/ccs/esd/documents/xdsdebugprobes/emu_energytrace.htmlTheir sampling is much lower, 250KHz:https://www.ti.com/lit/ug/spruie1a/spruie1a.pdfThe patent behind ishttps://patentimages.storage.googleapis.com/30/8a/a1/b9bbd09052c5e5/US20140253096A1.pdf

author: gashtaan
author_rank: Regular Contributor
timestamp: March 23, 2024, 08:15:05 pm
content: I would like to avoid high side measurement as I need to look on other signals from/to MCU on other channels, so same ground is convenient for me.Looking on AD8xxx datasheets, I'm not sure how to use it in low side... I'll study it more, seems interesting.Quote from: David Hess on March 23, 2024, 07:19:02 pmI don't need much precision in absolute numbers, I just need to see how current changes on the specific time interval. Now I'm considering different approach: use high-bandwidth opamp (like BGA2803) for fast current changes together with some basic opamp with low-pass filter to amplify lower frequencies. It hardly would be the same ratio between them, but it should be enough for my purpose.

author: gashtaan
author_rank: Regular Contributor
timestamp: March 23, 2024, 08:27:43 pm
content: Quote from: dobsonr741 on March 23, 2024, 07:49:22 pmIt's the former, the security research. It would help me to capture precise timing of some actions inside MCU.

author: jonpaul
author_rank: Super Contributor
timestamp: March 23, 2024, 10:57:12 pm
content: See the decades or R&D on cryptology use of CPU, IC power for security.Paul KOCHER founeded Cryptography Research now part of RSA.See "differential power analysis"He had a floor full of latest TEK and Agilent top range equipment.Jon

author: David Hess
author_rank: Super Contributor
timestamp: March 23, 2024, 11:24:30 pm
content: Quote from: gashtaan on March 23, 2024, 08:15:05 pmThere are several ways to combine a slow precision operational amplifier with a high speed operational amplifier to get the best performance of both.Given what you are doing, ultimate resolution will be limited by noise in the operational amplifiers and drift in the current shunt as its temperature changes do to self heating. The advantage of a part like the AD8129 which I mentioned is that it operates like an instrumentation amplifier so it removes the effects lead resistance and common mode errors. It translates a remote differential measurement to a single ended measurement at the point where you want to measure.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: March 24, 2024, 03:00:55 am
content: In case have not seen the open sourced work for from New AE:https://www.newae.com/products/nae-cwlite-arm

author: gashtaan
author_rank: Regular Contributor
timestamp: March 24, 2024, 05:12:38 am
content: Quote from: dobsonr741 on March 24, 2024, 03:00:55 amI know this project, but I don't see it's doing something similar to what I'm asking here about.

author: gashtaan
author_rank: Regular Contributor
timestamp: March 24, 2024, 05:37:26 am
content: Quote from: David Hess on March 23, 2024, 11:24:30 pmWhat I care is the interference mixed into weak signal from current shunt, I bet no opamp is so bad that it'll make worseI just want to put opamp(s) and shunt close together to amplify the signal to the level that the interference will not be so significant and then measure it. Drift in the current shunt also don't bother me, as I said, I'm not interested in absolute numbers.Anyway, AD8129 seems interesting, I'll study it more. I noticed that NewAE used it in their differential probe too... it's not very suitable to measure low-side shunt though.http://media.newae.com/datasheets/NAE-DIFFPROBE_datasheet.pdf

author: Calvin
author_rank: Regular Contributor
timestamp: March 24, 2024, 08:02:38 am
content: Hi,for a Diode leakage testing device we designed in our company (range from 1µA to ~10mA) we used a ADA4523-1 chopper OPA in the configuration as I/V converter (inv input in).We can truely measure down to 100nA and even below.Post amplification with variable gains was added to boost the signal to a sufficient level for an ADC.Bandwidth is of course not as high as the thread opener asks for, something about 10kHz-100kHz could be possible though.But similarly to many scope probe circuits (like the P6042A current probe, et al), one could maybe add a a parallel HF-branch and join both outputs again?regardsCalvin

author: David Hess
author_rank: Super Contributor
timestamp: March 24, 2024, 07:15:22 pm
content: Quote from: gashtaan on March 24, 2024, 05:37:26 amThe AD8129 also works fine for a low-side shunt, but the question then becomes why bother. Its input noise of 4.5 nV/SqrtHz is about as good as most 100+ MHz parts, but its differential input has the advantage of removing common mode noise and errors. This allows more than one ground connection for measurements without introducing common mode noise, and that by itself reduces noise of the measurement.The alternative of using a non-inverting amplifier at the shunt works fine also, but requires the connection for the measurement to be directly at the shunt, and no other connections be made for lowest noise. This can work fine if the ground connection to the shunt is used as a single point ground for *all* test connections.Other than the AD8129 I am not sure what I would use. Any fast current feedback amplifier will do the job. I would look for one with high gain, low noise, and offset null capability but the later is not common.

author: gashtaan
author_rank: Regular Contributor
timestamp: March 24, 2024, 07:58:12 pm
content: Thanks for the valuable insights. Maybe it works fine, but at least that NewAE diff probe needs very specific positive and negative power supply in low-side configuration. To me it's very inconvenient, I would like to have only 5V and 3.3V power rails on the board. I thought it's limitation of diff amps, but maybe it's just the property of the probe. Am I understand it wrong?Quote from: David Hess on March 24, 2024, 07:15:22 pmPretty please, can you give me some example of such amp?

author: KE5FX
author_rank: Super Contributor
timestamp: March 24, 2024, 08:14:20 pm
content: Offset nulling isn't really a thing with CFB amps, at least as far as I understand it. Their inputs are (intentionally) very different.

author: David Hess
author_rank: Super Contributor
timestamp: March 25, 2024, 12:09:41 am
content: Quote from: KE5FX on March 24, 2024, 08:14:20 pmCurrent feedback operational amplifier with offset null are not common, but they do or did exist, for example:https://www.analog.com/en/products/lt1217.htmlhttps://www.analog.com/en/products/lt1223.htmlhttps://www.analog.com/en/products/lt1227.htmlQuote from: gashtaan on March 24, 2024, 07:58:12 pmCurrent feedback operational amplifiers require positive and negative supplies because of how they are constructed with diamond buffer elements. This is why they did not become available until complementary bipolar processes, fast NPNs and fast PNPs, became available.There are fast single supply operational amplifiers, but I think they are all voltage feedback. They could be used, but current feedback parts have advantages at high speeds.QuoteThe only single supply part that I know of which can give enough gain at 100 MHz might be theLTC6268-10, but I have never used it so maybe someone else can comment.Keep in mind that in order to be useful, the amplifier has to have enough gain-bandwidth product to provide useful gain at 100 MHz, so that means a GBP of many times 100 MHz, in this case 40 times.

author: KE5FX
author_rank: Super Contributor
timestamp: March 25, 2024, 02:02:22 am
content: Quote from: David Hess on March 25, 2024, 12:09:41 amInteresting, never ran into those before. Seems that the more-modern CFB parts don't even pretend to be any good at DC.QuoteThey can be used with a single supply like any other opamp, given a midpoint voltage. This one ought to be pretty flat from AF up to 100 MHz or so, if you keep an eye on the slew rate:QuoteGBP? CFB amps don't got no steeeenkin' GBP... at least not in the traditional sense.

author: David Hess
author_rank: Super Contributor
timestamp: March 25, 2024, 02:22:42 am
content: Quote from: KE5FX on March 25, 2024, 02:02:22 amTheir DC precision depends on matching between their PNP and NPN transistors so cannot be as good as the differential pairs in voltage feedback operational amplifier, but offset null still helps.QuoteSure, but this application requires operation down to DC. Level shifting could be used instead but would further compromise precision.QuoteGBP varies with the value of the feedback resistance so frequency compensation can be adjusted to match the gain. Bandwidth still drops as gain increases, but not as quickly as with an amplifier that has fixed compensation.Voltage feedback operational amplifiers can use external compensation to do the same thing, and some are available which are decompensated, or a fixed gain stage can be added within the feedback loop to do the same thing.

author: KE5FX
author_rank: Super Contributor
timestamp: March 25, 2024, 02:47:22 am
content: QuoteWell, yeah, it had better, or things will get oscillatey at some point.QuoteThe behavior is largely unrecognizable compared to a traditional voltage-feedback amp with a GBW product. It really cannot be described as a GBW product. When the available gain starts to decrease, it pretty much falls off a cliff. Trying to compensate it is another good way to build an oscillator.I have alotof bench time with these puppies.

author: gashtaan
author_rank: Regular Contributor
timestamp: March 25, 2024, 06:45:29 am
content: Quote from: David Hess on March 25, 2024, 12:09:41 amThat's what I afraid of. The need for negative power supply is definitely something I want to avoid.Thanks for the advice about gain-bandwidth product. Honestly, I don't know what does it meanI haven't seen such parameter in the datasheets...It's probably dumb, but will something like this work?Keep in mind that I don't need accuracy. I just need to see fast current changes on some "DC offset". When I observing current consumption of MCUs it mostly looks something like this:https://circuitdigest.com/sites/default/files/inlineimages/u3/Power-Profiler-Kit-2-Data-Logger-View.png

author: David Hess
author_rank: Super Contributor
timestamp: March 25, 2024, 12:09:40 pm
content: Quote from: gashtaan on March 25, 2024, 06:45:29 amYes, there are two path amplifier designs which would work but they are more complicated. The gain (and phase) of the two paths need to be matched where the transition from one path to the other occurs or the frequency and phase response can get weird.In your example a resistor needs to be added in series with the output of U7 with its resistance and value of C13 also controlling the transition.

author: gashtaan
author_rank: Regular Contributor
timestamp: March 25, 2024, 12:28:08 pm
content: Oh right, I naively added U7 buffer to avoid U5 feedback being affected by the output of U4, but it didn't ocurr to me that the same problem is still thereI thought I would experimentally set gain of the U5 to match the gain of U4 later, but that the signals would be at different phases didn't occur to me either.

author: gashtaan
author_rank: Regular Contributor
timestamp: March 25, 2024, 01:37:26 pm
content: Quote from: David Hess on March 25, 2024, 12:09:41 amI'm reading about that NewAE probe that uses AD8129 differential amplifier and it seems it can be used also with positive power supply, it just must be 2.25V higher than measured voltage. It even allow to measure DC bellow 20kHz (with lower gain, 3-5x instead of 10x). It must be used to high-side measuring in such configuration, but if I understand it right, it is possible to measure the output relative to GND (amplifier output will be somewhere between power-supply and GND?). It's starting to look very promising for me.https://rtfm.newae.com/Tools/CW501%20Differential%20Probe/

author: David Hess
author_rank: Super Contributor
timestamp: March 25, 2024, 08:04:27 pm
content: Quote from: gashtaan on March 25, 2024, 01:37:26 pmThe datasheet is clear; the AD8129/AD8130 inputs, all 4 of them, must be at least 1.5 volts away from each supply rail, and the output only reaches to within 1.1 volts of each supply rail. The differential probe level shifts the input into range for the AD8129, and uses a virtual ground for the output which comes out at pin 4 of the connector.A resistive level shift would defeat the purpose of using it in your circuit because it would increase noise, decrease sensitivity, and decrease bandwidth. There *is* a way to use the AD8129/AD8130, but it involves a much more complicated circuit with a dual matched transistor to sense the current and do the level shift, like the first example I gave.I always do these types of circuits with a separate bipolar dual supply so I can avoid the single supply issues which just add complication.

author: gashtaan
author_rank: Regular Contributor
timestamp: March 26, 2024, 06:35:06 am
content: Quote from: David Hess on March 25, 2024, 08:04:27 pmThe pin 4 of the connector is optional auxiliary middle level of measured current. The level can be controller either from this pin or by R3 trimmer or even connectedto the GND, what they encourage to do if the user want to measure DC with the same gain. But in this configuration +REF will not be 1.1V away from V- negative supply rail. It seems to be some hack that is not following the datasheet.Can you please point me where in the datasheet is mentioned that all 4 inputs including +VREF must be at least 1.5V away from supply rails?

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2024, 04:37:07 pm
content: Quote from: gashtaan on March 26, 2024, 06:35:06 amPage 5 lists the common-mode voltage range under input characteristics as +/-3.5(3.volts with +/-5.0 volt supplies for the AD8129(30) and a differential operating range of +/-0.5(2.5). Page 6 lists the output voltage swing under output performance as +/-3.6 volts with +/-5.0 volt supplies.There are sets of specifications for +5, +/-5, and +/-12 volt supplies which vary slightly. None of them allow anything like operation close to a rail on the inputs or output.

author: gashtaan
author_rank: Regular Contributor
timestamp: March 26, 2024, 05:41:38 pm
content: I did see that range on Page 5, but didn't know how to understand that it's meant for specific condition V+IN - V-IN = 0.In absolute maximum ratings there is:Input Voltage (Any Input) −VS − 0.3 V to +VS + 0.3 VOf course, I know it's not OK to push it to these limitsOK, I think I understand now. The documentation (NewAE probe) is somewhat misleading - they forgot to mention that configuration for measuring from DC applies only to double-ended power supply.

author: gashtaan
author_rank: Regular Contributor
timestamp: March 26, 2024, 07:43:41 pm
content: If I will measure voltage drop across shunt resistor at DC circuit, Vin+ vs Vin- difference would be normally positive, right? If I will use single-ended power supply (5V), set Vref to minimum 1.25V and choose maximal shunt resistance to measure 0-30mA to still keep voltages between allowed range, what will happen in case of short circuit (full 3.3V across shunt)? It will momentarily exceed allowed input voltage, but still be there enough room to not exceed maximum rating.

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2024, 10:37:22 pm
content: Quote from: gashtaan on March 26, 2024, 07:43:41 pmSome external input protection can be added to the difference amplifier to protect the inputs from over-voltage. The datasheet discusses this.

author: gashtaan
author_rank: Regular Contributor
timestamp: March 27, 2024, 05:49:57 am
content: Quote from: David Hess on March 26, 2024, 10:37:22 pmThere are discussed cases when power supply being lower than 4V or higher than 12V. Otherwise it is stated:"If the supply voltage is restricted to less than ±11 V, the internal clamping circuit limits the differential voltage and excessive supply current is not drawn. The external clamp circuit is not needed."That's why I've asked. If just harmless clipping occurs in cases of over-voltage in either input or output, it doesn't matter to me.OK, I buy the ICs and test it. I've bothered you enough