Impedance mismatch  what I can do ?      - Page 1
Forum section: electronics / beginners

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 20, 2011, 01:48:24 pm
content: I have lots of questions and I need some one to rescue me..I got one AC/DC current clamp and it haves BNC plug for connection with an oscilloscope.Today I read the specifications again, and find out that it has an Impedance of 100 ohm ..First question: does all the oscilloscopes haves 100 Ohm input Impedance ?I am connecting this clamp on my multimeter with 10 M Ohm Impedance.Second question :do I need some sort of Impedance transformer so to balance the 100 ohm Impedance with the input of the DMM that is at 10 M ohm ?Third and last question:how the Impedance mismatch could effect one circuitry with output of 1mV DC ?Ok that's it, do your best.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 20, 2011, 02:12:20 pm
content: Quote from: Kiriakos-GR on July 20, 2011, 01:48:24 pmclamp meter model and brand? bnc plug to oscilloscope for what?

author: jahonen
author_rank: Super Contributor
timestamp: July 20, 2011, 02:12:58 pm
content: No, oscilloscopes have usually 1 Mohm input impedance or optionally 50 ohm. BNC connectors themselves are either 50 or 75 ohms, 50 ohms being the most common type by far.However, for this application, you can just ignore it as there are no actual matching to be done. Just connect the clamp directly to the dmm via an adapter and it will work just fine. The output impedance (or, resistance) is probably mentioned for the DC loading purposes, 1 Mohm input will load the output to 1M/(100+1M) =~ 0.9999 of actual value when connected (usually completely irrelevant considering all other uncertainties).Regards,Janne

author: ejeffrey
author_rank: Super Contributor
timestamp: July 20, 2011, 02:23:44 pm
content: Most oscilloscopes have 1 Megaohm input impedance, some can be switched to 50 ohm.In this sort of application you generally don't want an impedance match, you want the probe to be much higher impedance than the source. This prevents the measuring device from drawing current from the source that would create errors due to the voltage divider effect.Impedance matching gives maximum _power_ transfer to the measuring device, not maximum voltage transfer. In RF work, this is often desirable, but not usually at low frequency.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 20, 2011, 02:50:24 pm
content: You are all correct, at the advanced characteristics found the 1Mohm equal or larger.Additionally it say 100 pF or lower.I tried to measure the input of the one multimeter with the capacitance meter of the other,and it does not work.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 20, 2011, 03:21:14 pm
content: The advantage of a 100 Mohm meter would be if you were measuring high impedance signals. If you have a signal source with 40 megaohm source impedance, a 1 megaohm scope or even a 10 megaohm DMM will look like a short circuit -- you are doing a current measurement, not a voltage measurement. Even a 100 megohm meter would cause a 30% error, but that might be tolerable in this sort of application.Places you might come across such a source is when measuring voltages due to leakage resistance or capacitance, or piezo-electric sensors.Similarly, if you want very high accuracy, high impedance is important. Your 100 ohm current clamp is much lower than 1 Megaohm scope, but it will cause a .01% voltage drop -- 1 part in 10,000. If you want 6 digits of accuracy, you would need a 100 megaohm meter, or you would need to apply a correction factor for the source impedance. This works fine, but requires accurate knowledge of both the source and meter impedances.As always, you have to be cautious about claims of really high impedance probes and ask "at what frequency". A meter might be 10 MOhm at DC, but stray capacitance often is the dominant factor. 300 picofarads has a 10 megaohm reactance at 50 Hz, and 30 pF is 100 megaohms. Well designed high-impedance measuring tools (like an electrometer with teraohms of input impedance) are carefully designed with guard electrodes and so forth to reduce these effects, but even so they only operate at low frequency.

author: jimmc
author_rank: Frequent Contributor
timestamp: July 20, 2011, 06:52:51 pm
content: As it works down to DC, I suspect that the Current Clamp has some form of feedback circuit using a coil around the magnetic circuit formed by the arms of the clamp. If for instance this winding had 10,000 turns then 1mA through this winding in the correct direction would null out the magnetic field of 10A flowing through the conductor the clamp is placed around. Passing the 1mA through a 100 ohm resistor will give a voltage drop of 100mV or 10mV/A. Connecting the clamp to a high resistance meter would prevent sufficient feedback current flowing for the device to work.(Hall effect sensors are not accurate enough to use directly but can easily detect a null.)To use with a 1M or 10M 'scope or DMM get yourself a BNC 'T' piece and fit a 100ohm resistor in a socket connected to one arm, the current clamp to the other arm and the 'scope to the third arm. For the DMM use a BNC to 4mm adaptor. Even with 100pF of input capacity the upper 3dB point will be at 16MHz.Jim

author: ejeffrey
author_rank: Super Contributor
timestamp: July 20, 2011, 07:19:29 pm
content: No

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 20, 2011, 07:57:37 pm
content: I like to thank you all for the info.The clamp is Hall type with lots IC's in it. (very modern one).Today I did allot of experiments with DC and AC measurements, and actually I calibrate it a bit.I had never use it on AC so far, and it took me some time to find out how it works.On the AC it gives an output in mV AC... By measuring DC Amperes it gives as output mV DC.All that I have to do is to turn the DMM mV mode to AC or DC.My problem was that in 10mV/A output I was getting some fast changing readings no matter what at DC.When it worked on AC everything was better and smooth.I will say more as soon I finalize my testing.All that I need are a stable output in mV mostly at the DC Amperes.If some one had for example the Fluke 400i clamp, we could possibly compare our findings.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 21, 2011, 12:06:48 am
content: Quote from: Kiriakos-GR on July 20, 2011, 07:57:37 pmpeople will not believe infaitharound here, without hard "mathematical" proof. if you do not mention the brand and model number of the claimed "modern clamp meter", you can tell all the bed time story as you like, nobody will believe

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 21, 2011, 07:43:21 am
content: Quote from: Mechatrommer on July 21, 2011, 12:06:48 amThe best bed time story is the one that helps you sleep.I have only one clamp up to date, and it is made in France.

author: jahonen
author_rank: Super Contributor
timestamp: July 21, 2011, 08:03:19 am
content: I have a Fluke 80i-110s, I have used it with a scope and DMM for both AC and DC, no problems.I have seen this same probe branded for many different manufacturers. Can't guess who is the original manufacturer.Regards,Janne

author: Ronnie
author_rank: Regular Contributor
timestamp: July 21, 2011, 08:20:14 am
content: Quote from: ejeffrey on July 20, 2011, 02:23:44 pmI have seen a Tek scope (forgot the model number) that can be switched also to 75 ohm for video application though its BNC jack is a 50 ohm type. Problem arises because there is mechanical difference between 50 and 75 ohm BNC plug and jack.Don't know if there will be significant signal degradation when using a 75 ohm BNC cable with the scope's 50 ohm BNC jackBNC maleInsulator nestles up throughout against outer contact, and an air gap between center contact and insulator for 50 ohm BNC maleReduced insulator in mating end, and an air gap between center and outer contacts for 75ohm BNC maleBNC femaleInsulator surrounds female center contact wholly for 50 ohm BNC femalePart of female center contact doesn’t be surrounded by insulator and an air gap between center contact and outer contact for 75 ohm BNC femalehttp://www.wellshow.com/technical-support/rf_connector-bnc_connector/

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 21, 2011, 09:21:21 am
content: Nice info about the 75 Ohm BNC , but it looks to be an common TV aerial plug with metallic body.I presume that easily will get up to 800 MHz, even the plastic ones does it.

author: Ronnie
author_rank: Regular Contributor
timestamp: July 21, 2011, 10:34:48 am
content: I was also surprised there is difference between 50 and 75 ohm BNC jack and plugNow Tektronix should consider this when offering a scope with 75 ohm input termination capability.BNC cables are applicable up to around 4 GHz.http://www.l-com.com/content/Coaxial-Cabling-Tutorial.html

author: Unknown
author_rank: Unknown
timestamp: July 21, 2011, 07:14:16 pm
content: Quote from: Ronnie on July 21, 2011, 10:34:48 amMake that 50 ohm BNC connectors. 75 ohm connectors have less bandwidth.Standard 75 ohm BNC connectors are 100% compatible with 50 ohm ones from a mechanical point of view. I think there used to be a difference in center pin size, but the current models are compatible. There of course will be an impedance mismatch at the connection, but since 75 ohm BNC is typically used for low frequency stuff like video, this is rarely a major issue.

author: scrat
author_rank: Frequent Contributor
timestamp: July 21, 2011, 07:38:05 pm
content: Quote from: jahonen on July 21, 2011, 08:03:19 amExactly the same as our Chauvin Arnoux. Works very well, we use it for motor control and power inductance measurement. For these applications it results to be much less noisy than the tektronix probe with external amplifier, which by the way costs a lot and is really mechanically bad engineered (at least not robust enough for an electric drives and power converters lab!).

author: Unknown
author_rank: Unknown
timestamp: July 21, 2011, 10:49:12 pm
content: Quote from: scrat on July 21, 2011, 07:38:05 pmTry comparing them at 50MHz, that might explain the price difference. A system with more bandwidth will also be more noisy, everything else equal. The Tek design is the most advanced DC/AC design with a Hall Sensor in a ferrite core without any air gap. The alternatives are less sensitive (= more gain = more noise). 100kHz bandwidth is easy, just get an off-the-shelf Hall sensor. Not so with DC-50MHz or DC-100MHz.Of course for a DMM this is complete overkill.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: July 21, 2011, 11:36:56 pm
content: Quote from: alm on July 21, 2011, 10:49:12 pmIt looks that you excluding from the equation the noise of the clamp it self by operating up to 50MHz !!The noise in the clamp it self, translates to unstable output in mV .In my Chauvin Arnoux PAC12 at 10mV= 1 A ... it did caused trouble to me with the last digits, they was continually changing,and I baptized this as Play-up.The truth is that it is called asoutput noise.From the specifications ...DC range 10mV= 1 A0 - 1KHz = less or equal to8mV!!1KHz - 5KHz = less or equal to12mV !!AC range 10mV= 1 A0.1Hz - 5KHz = less or equal to2mV!!At 1mV = 1Athere is much less noise.0 - 1KHz = less or equal to1mV!!1KHz - 5KHz = less or equal to1.5mV !!AC range 10mV= 1 A0.1Hz - 5KHz = less or equal to500uV!!Originally I thought that my clamp was bad or damaged,because at DC 10mV= 1 A , I was getting mad because of this play-up,and I was very confused and did not know who to blame.Now I know that the dumb clamp does it with the blessings of the manufacturer.I had contacted the representative of Chauvin Arnoux in Greece, and enjoyed a terrible service.They do not have anything as parts in stock,they order from France and they charge you extra and the shipping from France and the local shipping ( 5 EUR)in the total shipping cost of 80 EUR, + 30 EUR so to get an spare connection cable for the clamp.Not to say that they ask 300EUR for calibration.. LOLHey Chauvin Arnoux drop dead I do not care.

author: ipman
author_rank: Regular Contributor
timestamp: July 22, 2011, 06:48:56 am
content: I have a Fluke i410. It's a Hall-type probe, outputs 1mV/A, and did not notice any change with any DMM I've ever come by, but not tried with osciloscopes tough.This probe is a lot "bigger" as it measures up to 400A, but the minimum recomended current is 1A, altough it can detect from 0.5A upwards with some extra error.