Bode Routine Shows Op-Amp Output Z - Page 1
Forum section: electronics / projects

author: mawyatt
author_rank: Super Contributor
timestamp: July 18, 2022, 08:07:03 pm
content: Using the Bode Plot routine in many new DSO allows one to investigate many things, including open loop and closed loop systems and various threads have shown many examples.Here's another useful use, to show the Inductive nature of an Op-Amp output impedance in a closed loop configuration. The plot shown was created with the Siglent SDS2104X Plus and AWG (SDG2042X) and the DUT is a standard OP-07 in a Unity Gain configuration powered by +-15V.The setup is very simple with just a single 1000 ohm resistor connecting the AWG Output to the Op-Amp output. Bode Plot CH1 is connected to the AWG output, and CH2 is to the Op-Amp output, so the Bode Plot is across the 1K resistor. This configuration creates a Bode plot that shows results in dB 1000 ohms, so 1 ohm shows -60dB on the scale.As you can see from the plot the rising output impedance of the OP-07, starting around -90dB (~32 mill ohms) and -60 dB (~1 ohm) @ 12.5KHz and the phase rising from around 0 degrees at 10Hz and lingering around +90 degrees for the mid part of the graph.Anyway, another good demonstration of how Bode Plots can be utilized with these modern versatile DSOs!!Best,

author: Someone
author_rank: Super Contributor
timestamp: July 18, 2022, 11:32:46 pm
content: Appears to be noise limited at the lower frequencies, you'd expect the increasing open loop gain to produce much lower output impedance down towards 1-10Hz.

author: mawyatt
author_rank: Super Contributor
timestamp: July 19, 2022, 02:54:44 am
content: Yes the output impedance is very low due to the available open-loop gain at low frequencies with negative feedback driving down the output impedance. As the frequency increases the available gain drops and thus the output impedance rises as shown producing an inductive behavior.Here's a LTspice simulation from 1KHz to 10MHz with the default OP-07 model (believe this is a behavioral model with limited fidelity), and a Ti OP-07 we have (from old stock so questionable). Disregard the double ohms symbol in the LTspice plot it's a issue with the plotting routine. Also note the "noise" at the low frequency end of the SDS2104X+ plot as you mentioned, the new 12 bit SDS HD version should do better.The sim plot shows ~ -68dB(Kohm) at 10KHz which is ~0.4 ohms, the Bode Plot shows -61.8dB(Kohm) at 10KHz which is ~ 0.8 ohms. The OP-07 has about 40dBV gain at 10KHz, so this indicates something like 40~80 ohms open loop output impedance which seems reasonable (AD data sheet indicates 60 ohms nominal).Best,

author: Someone
author_rank: Super Contributor
timestamp: July 19, 2022, 03:23:42 am
content: There will be an interesting balance of the measurement source impedance, as you want both sides of the measurement to be well above the noise floor. Can the application/analysis on the scope account for the series combination? or are you assuming the impedance under test <<<< source impedance.

author: mawyatt
author_rank: Super Contributor
timestamp: July 19, 2022, 03:55:55 am
content: Source impedance doesn't matter much since the input signal is sampled at the DUT input resistor and the Bode Plot is computed as Vout/Vin magnitude and phase, although one could match the AWG output to 50 ohms, but won't make much difference in these test. The scope is set to 1Meg input impedance on both Input and Output, so little loading, and samples are directly across the 1Kohm resistor feeding the AWG output to the Op-Amp output. So one channel input "sees" 50 ohms in parallel with 1Meg and 1K and samples the voltage across these, and the other channel "sees" ~1K in parallel with Zout.As a simple test can use a 1 or 10 ohms resistor in place of the DUT to check things out, and you get -60dB(Kohm) and -40dB(Kohm) at ~0 degrees phase respectively.Best,

author: Someone
author_rank: Super Contributor
timestamp: July 19, 2022, 05:09:08 am
content: From what I understand you have:AWG (50 Ohm) -> probe A -> 1k Ohm resistor -> probe B -> Dut -> GNDDisregard probe loading for now.The measurment of the "input" at probe A is the voltage across both the 1k source impedance + the Dut, and the scope calculates B/A, which is not the input current to the Dut (that would be A-B). The "correct" impedance plot should be B/(A-B)

author: magic
author_rank: Super Contributor
timestamp: July 19, 2022, 08:32:11 am
content: That's pretty coolCan it plot open loop gain of OP07 too?BTW, you can't expect "infinitely" low output impedance at low frequencies, at some point ohmic resistance of connections comes, unless careful Kelvin sensing it set up.

author: blackdog
author_rank: Frequent Contributor
timestamp: July 19, 2022, 12:01:20 pm
content: Hi mawyattLast week I also started testing for making Bode Plots with Siglent equipment.And they are exactly the same types of devices you use.This is a link to a Dutch forum that I write on a lot, this is a link to a Dutch forum that I write on a lot, but that's really irrelevant to why I'm responding to your topic.https://www.circuitsonline.net/forum/view/158847#highlight=bode+plotSo when the Siglent equipment came on the market that could do Bode Plots I bought a Siglent scope a month ago,two Siglent generators I already have, the SDG 20242X and the SDG 1032X, these mainly for the very nice squarwave.There is not a whole lot on the web on how to properly measure to say opamps with apparatus that can make Bode Plot.Most of what is available relates to injecting a small signal into the feedback resistors via a transformer.I have not found much to e.g. as you indicate to e.g. the output impedance of an opamp to immediately.I think it really helps if you present a measurement here on the forum that you make a diagram showing how things are connected to the generator and the scope.This is the simple example I showed how I connected my first tested filter, the picture is stolen from Siglent America :-)On the Dutch forum I also want to show more of what you can do with the Bode Plot function.Except for measuring linear power supply, my knowledge of this is minimal and I would like to learn more here.Kind regeards,Bram

author: mawyatt
author_rank: Super Contributor
timestamp: July 19, 2022, 01:05:02 pm
content: Quote from: Someone on July 19, 2022, 05:09:08 amSince we are talking about Op-Amp output impedance or Zo, then a little analysis will show that the result is:Zo = R{V2/(V1-V2)} = R/(V1/V2 -1 ) and since V1/V2 >> 1, thenZo ~ R(V2/V1) and V2/V1 is what the Bode Plot shows with the mentioned connection.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: July 19, 2022, 01:12:46 pm
content: Quote from: magic on July 19, 2022, 08:32:11 amYes see this post, just a simple DIY isolation transformer:https://www.eevblog.com/forum/testgear/diy-transformer-for-use-with-bode-plots/As the frequency gets lower the more available Op-Amp gain makes it difficult to measure, this imposes a significant stress on the instrument Dynamic Range and measuring technique and where the SDS2104X+ does a nice job, the new HD version with core 12 Bit ADC is even better as has been shown by others.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: July 19, 2022, 01:25:39 pm
content: Quote from: blackdog on July 19, 2022, 12:01:20 pmAgree we should have included a simple diagram to show the measurement technique, see below.Best,

author: magic
author_rank: Super Contributor
timestamp: July 19, 2022, 02:26:40 pm
content: Quote from: mawyatt on July 19, 2022, 01:12:46 pmFor open loop gain of opamps specifically, there is a simple passive circuit which gives good results. I have used it with a manually controlled sine generator and an 8-bit scope; some range switching and averaging was necessary but it covered 0~120dB and down to 1Hz or less.I imagine that with a 60dB dynamic range ADC and FFT techniques, the sig gen could be replaced with a wideband white noise source.

author: mawyatt
author_rank: Super Contributor
timestamp: July 19, 2022, 03:59:33 pm
content: Quote from: magic on July 19, 2022, 02:26:40 pmThat's also a good technique for use with Op-Amps. The big value with the ISO Transformer technique is the ability to measure Open Loop Response of things that don't have a common ground reference, or with lots of larger interfering signals like SMPS do.Some of the DSOs like the mentioned Siglent types scale the inputs amplifiers/attenuators to achieve a larger dynamic range than the basic ADC, check out the threads on the SDS2104X Plus and SDS2104X HD byPerforma01andrfloopthat show impressive DR from these DSOs.https://www.eevblog.com/forum/testgear/siglent-sds2000x-hd-12bit-(published-for-chinese-domestic-market-only)/https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/Best,

author: mawyatt
author_rank: Super Contributor
timestamp: July 19, 2022, 09:58:05 pm
content: Here's an interesting result that shows the advantages of using an external buffer to a precision Op-Amp to reduce the overall output impedance.First shows the output impedance of a TI OP-07 in a unity gain configuration as mentioned before using a 1K input resistor and 1VPP AWG signal level.Second shows an added 2N3904 emitter follower buffer to the OP-07 output with a 1K to Vee emitter bias resistor. Note the reduced output impedance at 10KHz and 1MHz of the buffered output, over a 20dB or a 10X reduction!!Best,

author: mawyatt
author_rank: Super Contributor
timestamp: July 19, 2022, 11:01:40 pm
content: So why does the Op-Amp output impedance matter as shown above?One important issue is with Active Filter Stop Band Rejection like the 2nd Order Sallen-Key shown below. This is a simple unity gain 2nd Order SK LP Filter utilizing 10K and 10nF and a TI OP-07 Op-Amp.The first plot shows the Bode Plot response with the Op-Amp in a unity gain configuration, note the ultimate stop band rejection. The second plot shows the same configuration except with a 2N3904 Emitter Follower output buffer mentioned above, note the improved Stop Band Rejection after ~50KHz.Also note the passbands are identical as is the 10KHz response, so the improved output impedance only affects the stop band rejection well beyond the filter corner frequency!!Edit: Added LTspice simulations for the Sallen-Key LPF, including the buffered output version. Note the 10KHz readings agree spot on, are somewhat close at 1MHz but begin to deviate past 1MHz. Various reasons include the model fidelity and the Proto-Board used for the circuit construction with long jumpers as no attempt was made to keep things close and tidy. However the simulations show the same improvement in Stop Band performance past ~50KHz with the added 2N3904 Buffer.Best,