difirentiator amplifier to detect 0.4v/ns slew rate - Page 1
Forum section: electronics / projects

author: calvingloster
author_rank: Regular Contributor
timestamp: September 22, 2025, 01:54:58 pm
content: Not sure if this is realistic but is it possible to use an opamp in a difirentiator circuit to output between ground and max rail voltage for slew rates ranging from 0.01v/ns and 0.4v/ns? Would there be an opamp fast enough to detect and react to these slew rates?

author: Marco
author_rank: Super Contributor
timestamp: September 22, 2025, 02:12:44 pm
content: How fast do you want to detect it? Obviously it becomes trivial eventually. What's the source impedance?

author: PGPG
author_rank: Frequent Contributor
timestamp: September 22, 2025, 02:24:42 pm
content: It is not clear what you exactly need.If at input slew rate is between 0.01V/ns and 0.4V/ns than such slope should be detected, but all slower and faster slopes should be ignored?OpAmp (better Comparator) will react with output change from ground to max rail voltage for any slew rate at input provided the new state at input lasts long enough for it to be able to change its output state.

author: calvingloster
author_rank: Regular Contributor
timestamp: September 22, 2025, 02:30:08 pm
content: I don't need to detect it very fast. There can be some delay till the circuit triggers that a fast slew rate was detected. As long as it's a constant delay.My bad I guess I want to detect slew rates above a certain threshold, and for this case it would be anything equal to and above 0.4v/ns.The source impedance is 50 ohms

author: calvingloster
author_rank: Regular Contributor
timestamp: September 22, 2025, 02:43:22 pm
content: The slew rates I gave are an approximation of a slow slew and a fast slew. I guess I only need to detect if the slew is faster than 0.4v/ns. I initially though I could use a difirentiator circuit to output a voltage based on the slew rate but I'm open to any other circuit too.

author: Marco
author_rank: Super Contributor
timestamp: September 22, 2025, 02:51:43 pm
content: 10s of seconds fast enough?

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 22, 2025, 03:17:58 pm
content: Sounds like you just want a logic output which detects tells you when the slew rate of the input is above some threshold (and doesn't depend on anything else about the input).In that case, you just need a differentiator circuit and then some comparators (one for positive and one for negative) with appropriate thresholds, then OR those comparators together.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 22, 2025, 06:02:17 pm
content: Quote from: calvingloster on September 22, 2025, 02:30:08 pmHow many volts, and what is the actual waveform?

author: calvingloster
author_rank: Regular Contributor
timestamp: September 23, 2025, 08:58:21 am
content: The voltage is 3.6V and I have attached a picture of two waveforms overlaid. The purple waveform is fast rising edge transition and reflection (the part I want to detect) on a 150m coax cable, the yellow is the same but just on a 250m cable. AS the cable gets longer it becomes harder to detect this reflection.The second picture I have somewhat guessed what the waveform might look like on an even longer cable. This reflection edge gets less and less noticeable and has a slower rise time.For shorter cables its quite easy to detect the reflection as the reflection constructive interference creates a sharp rise time. the problem is that a comparator set at one voltage will only work till a certain length. Hence why I though trying to detect slew rate instead.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 23, 2025, 11:16:27 am
content: Ok looks like this is some sort of TDR application?But those waveforms don't look like what I'd expect, you should be seeing fairly flat plateaus in between reflections. Could be an issue with the driver (not well matched to the cable) or with the cable (lossy, or high dispersion).

author: calvingloster
author_rank: Regular Contributor
timestamp: September 23, 2025, 11:18:59 am
content: Yes its for a TDR, When measuring short cables the waveform is nice and flat but longer cables it seems to be charging up like a capacitor. Could it possibly be that the cables are rolled up?Im using a SN74ACT04D schmitt inverter as a driver, I've used 3 outputs in parallel with 150 ohm resistor on each output.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 23, 2025, 12:17:44 pm
content: Quote from: calvingloster on September 23, 2025, 11:18:59 amI don't think that's relevant here, so long as it's coax with a proper shield.QuoteSounds reasonable. I'm not sure what the effective output impedance of ACT logic is, probably 20-50 ohms, so that should give an overall output impedance near 50 ohms. What does the output look like if your terminate it directly with 50 ohms (no cable)?What sort of coax cable are you using? Can you measure it with a VNA?

author: calvingloster
author_rank: Regular Contributor
timestamp: September 23, 2025, 01:16:57 pm
content: Im using RG58 that i got from the local electronics store.I don't have a VNA unfortunately and i'm not home now to try it with the 50 ohm termination. I have this picture with a 5m cable and the response is similar for 40m too. But longer cables have the weird slow increase in voltage.The yellow trace is the coax voltage

author: ejeffrey
author_rank: Super Contributor
timestamp: September 23, 2025, 02:39:16 pm
content: Yes you can do this, it's quite straightforward. To use a differentiator you just need an opamp with sufficient bandwidth, followed by a comparator.A differentiator with finite bandwidth is just a high pass filter with gain. If you don't need the gain you may be able to dispense with the opamp all together. A simple RC high pass filter followed by a comparator will provide a reasonable approximation of a true differentiator. For instance 10 pF + 50 ohm will give approximately 200 mV with a 0.4 V/ns slew rate. You can directly sense that with a fast comparator.The difference is that since the capacitor driving 50 ohm instead of a virtual ground, high frequency low amplitude signals will not have as much gain as is ideal. I.e., a 150 mV peak signal will never trip the comparator even if it has a very high slew rate.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 24, 2025, 11:48:58 am
content: Quote from: calvingloster on September 23, 2025, 01:16:57 pmAfter doing some numbers checking, I'm pretty sure the shape of these waveforms is expected for such lengths of RG-58. At 100MHz, typical loss is about 16dB per 100m, so that's going to cause a great deal of distortion.I threw together a very crude LTspice sim and here's what it shows for a 150m length of RG-58, if anything it's worse than what you're observing.

author: calvingloster
author_rank: Regular Contributor
timestamp: September 26, 2025, 09:42:04 am
content: Oh that's really cool, so how do TDRs measure long distance cables of 500 + Meters? I guess a more expensive cable would help a bit but you ultimately dealing with a varying signal

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 26, 2025, 11:46:16 am
content: A couple ideas:1. Reduce the the applied rise/fall times2. More complex waveforms can be applied, and more sophisticated methods for analyzing the waveforms are used (i.e. cross correlation).3. Use continuous AC, like a single port network analyzerAt some point you have to ask yourself, if a 500m length of cable has 80dB of loss, is there even a point to having 500m of that cable in one continuous length?

author: Terry Bites
author_rank: Super Contributor
timestamp: September 26, 2025, 12:36:48 pm
content: A differentiator cannot detect slew rate.It responds to the rate of change of the input. At points of inflection in the input signal the output sign flips.See this brief on differentiators.You can calculate the required GBW of the opamp. Its going to be very high for the time constants you'll be employing.Any stray capacitance at the ninv input will cause instablity so it could be a tricky build.Surley there are better established ways of implementing TDR?

author: calvingloster
author_rank: Regular Contributor
timestamp: September 26, 2025, 12:58:31 pm
content: Thanks @mtwieg. You make a very good point and have explained stuff really well thank you. Could you possibly explain how you simulated the cable in LT spice? Possibly share the circuit and values? I would like to mess around with different cables like lmr 400 and rg213 and different lengths and pulses in LT spice if it's possible to model them.

author: calvingloster
author_rank: Regular Contributor
timestamp: September 26, 2025, 01:08:36 pm
content: Quote from: Terry Bites on September 26, 2025, 12:36:48 pmThanks for the app note. Makes sense👌

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 26, 2025, 01:10:45 pm
content: I've attached the simulation file. It just uses a lossy transmission line model (LTRA) with rough parameters applied. Note that the LTRA model has constant insertion loss over frequency, unlike a real cable. I set my parameters to match RG-58 cable behavior at 100MHz.Making a more realistic model which accounts for frequency dependence is apparently challenging, see here:https://groups.io/g/LTspice/topic/lossy_transmission_line/50214222

author: calvingloster
author_rank: Regular Contributor
timestamp: October 01, 2025, 12:22:00 pm
content: Quote from: mtwieg on September 26, 2025, 01:10:45 pmAwesome thanks dude. I'm going to try this out tomorrow 👌