+20dB Wide Band 50Hz(60Hz) Notch Amplifier - Page 1
Forum section: electronics / general

author: blackdog
author_rank: Frequent Contributor
timestamp: May 22, 2018, 06:34:30 pm
content: Hi,This is the first LAB amplifier I have built in one of the beautiful Fischer boxes.My starting point was to make a LAB amplifier that would give 20dB gain and that would suppress as much as possible the230V powergrid 50HZ frequency that we have here in the Netherlands.I would like to have these specificationsInput 100K or higherBandwith about 1Hz to 1MHZ at -1dBPower: 2x 9V batterySome gain switch posibilityNotch attunation >50dBProtection of +-60V on the in and the output connectorsReasonable low noiseNo squarewave abberationsthis amplifier is designed te drive max. 1M 50 Ohm coax cable and do not terminate it!And with these requirements, this has become the schematic for this notch amplifier.Sorry, i still have to make the front for this amplifier, making it will not take much time now, because I have gained a lot of experience with the other box.But it is also good to show here, how much impact it has if you make the front nice and tidy.For this amplifier I chose the beautiful OPA140 amplifier series of TI, these have a low noise, Fet input, good square wave reproduction and quite large bandwidth.The first opamp is a single version and the opamps used for the Notchfilter are the dual version, that is the OPA2140.Adjustment of the amplification and attenuation all happen around the first opamp. This is actually nothing special, a textbook circuit.And then we get the notchfilter, this type was developed by Henry P. Hall and below a link to explanation of this Notchfilter by: Kenneth Kuhn.http://www.bramcam.nl/NA/NA-Noise-Measuring-Amp/Kenneth-Kuhn_the_hall_network.pdfFor people who live in the area where one has 60Hz as mains frequency, scale the capacitors down in the notch filter, that is the easiest.I have chosen the Q of the filter in such a way that it is easy to adjust and remains stable enough.I also selected the capacitors so that they are as equal as possible.That's a bit more important than exactly 150nF, 3x 148.5nF is also good.Because there is also a frequency trimpot available.Some pictures...The frequency response around 1MHz.Red shows the C9, 35pF trimmer cap respons.Wiring around the BNC connectors and switches.Where necessary, the components are floating in order to keep paracitic capacities low.At the bottom right you can see the 8-pin IC socket, which is for the input amplifier, the OPA140.Also visible is that I regularly use the space in the IC socket for disconnecting capacitors or as here one of the resistors needed for the OPA140.In the middle is the OPA2140 visible which is used for the Notch filter.Here i use the nice Hameg HMF2525 as a 50Hz source for trimming the 50Hz Notchfilter.And the Fluke 8920A (A beautiful instrument) tels the Notch dept, more than 51dB.Just for fun!Measuring the freecuency respons with my Rigol-815 SA, the rippel around marker-1is due to the fact that the SA was not yet properly heated, so this is not due to the amplifier to be measured.This is the noise from the amplifier when the input is terminated with 50 Ohms, at a 22Khz bandwidth.The gain here is 20dB, so the noise at the input is about 1.3uV in this bandwidth.And the last noise measurements, lower than 10Hz and if I have it right,something of -3dB at 750Khz from the Audio Precision Analyser, 93.3uV and that is 9.33uV at the input.Doing input protection testing, 60V power supply en a box with some dendering relais, simple and effectiveSimple setup of the "distroyer" boxThis is how its build.Of course I tried to draw the Notch on graph paper, but I didn't quite succeed...And this is the Notch displayed on the Audio Precision, it is difficult to make a goed picture of the display, sorry for that.Shoot at it!Kind regards,Bram

author: Kosmic
author_rank: Super Contributor
timestamp: May 24, 2018, 05:16:21 pm
content: I was planning to build something like that to measure linear dc converter noise. So this is really interesting to me.I hope you don't mind if I reuse your low battery indicator designIt's a pretty good idea.Thanks for sharing with us Bram.

author: blackdog
author_rank: Frequent Contributor
timestamp: May 24, 2018, 06:58:46 pm
content: Hi Kosmic,No problemo, use what you can.Big parts of this schematic is wat i have learnt from others:we are standing on the shoulders of those who came before usProtection remarksI have set up the protection at the output in such a way that it allows abuse for a short time.R13, which is within the loop of the opamp that controls the output, is installed there for extra protection of the opamp.This opamp with the pins 5, 6 and 7 is used as a simple 1x buffer to ensure that the output remains nicely 50 Ohm over almost the entire frequency range.One of the features of the OPA140 opamps series is the relatively low output impedance when there is no more loop gain.Therefore you can't just replace this opamp for say a TL072 if you want to have the same nice frequency range and low distortion as I do with this schematic.R17 and R18 make the 50 Ohm output impedance to drive the coaxial cable neatly.If you would like to be very precise in this output impedance for the lower frequencies, then you can take for R17, twice a 22 Ohm resistor parallel.R18 of 39 Ohm must be a 1-Watt resistor but not a wire wounded type, which are too inductive.I use 1W metal film types for this.This resistance dissipates most power in case of error situations at the output.Also look at the way the protection components are drawn in the diagram, this is done especially so.C13, C14, D3, D4 and the two TVS diodes, all mounted close together at the output BNC.Always think about how the currents will flow in case of an error, so take this into account when constructing the circuit.At the input I have chosen for the standard protection that you also see in many other devices.R7 of 221 Ohm has been added extra at the +input of the opamp to give some extra protection in case of coarse overloading of the input.So, what am i talking about, look at the pictures below from some testing i dit to make ik clear, i measure the voltage over the diode.The low inductance test setup, the D.U.T is a BAV23.Generator settings, 50KHz and 5% Duty Cycle.Looks almost normal..But lets speed up the timebase a little...And now at 200nSec, Que! 2.5V w.t...But it gets worse....I regularly say that you need to be familiar with the instruments you use! That, of course, also applies to me to!Now the scoop is set to "Refresh" and 4Gsample/Sec. BrrrrrrrrLets zoom in a little, 2.75V Peak!Don't think too easily about diode limiting, especially if the edge is fast of the signal you want to limit.And of course shoots at it!Bram