Tektronix AM503B Current Probe Amp Noise Levels - Page 1
Forum section: electronics / testgear

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: September 14, 2016, 06:54:03 pm
content: I'm getting set up for current probe work using an 6302A probe and I just got an AM503B Probe Amp which seems to work fine but I'm concerned about the noise level.At the recommended volts per division of 10 mV/div, I get about 5 mV of noise on the display regardless of range (see picture showing the probe set on DC reading a 40 mA DC current) is this normal or do I have a problem?

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2016, 06:49:57 am
content: You did not give the scale factor but the specifications say that the noise should be less than 250uA RMS to 100 MHz. Your display of 5.6mV PP is less than this at 1mA/div.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: September 15, 2016, 11:26:21 am
content: The front panel was set to 20 mA/div and the scope was set to 10 mV/div which is what is printed on the front panel. If my rms voltage was say 4 mV then that would translate to 20 x 0.4 mA which is 8 mA so it looks like it's very noisy. Do you concur with my math?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 15, 2016, 12:25:39 pm
content: Yes, it looks like about the right level of noise. I am getting about 4.5mV p-p for full scope BW and 2mV p-p for 20MHz BW (AM503 + A6302) for all ranges but the lowest 1 and 2mA/div. At 1mA/div the noise is 12mV p-p (8mV p-p for 20MHz BW).CheersAlex

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: September 15, 2016, 02:38:30 pm
content: Thanks guys. Alex, please confirm you're saying that my noise levels are roughly the same as you see on your equipment, i.e. you think my AM503B is OK.

author: MarkL
author_rank: Unknown
timestamp: September 15, 2016, 07:19:54 pm
content: Quote from: Gandalf_Sr on September 15, 2016, 11:26:21 amThe math is right. With your settings I also get a similar level of noise (3.6mV pk-pk, 377uV RMS --> 7.2mA pk-pk, 754uA RMS). This is using an AM503/A6302.An easier way to read the scope trace is to set the probe units to amps and the scaling to match the AM503. As long as the scope is set to 10mV/div to start, I just look at the displayed "amps/div" on the scope until it matches the AM503 dial. Less math to think about.Perhaps your noise level is slightly higher from whatever you're using to create to 40mA through the probe? I measured mine with no input.The full spec that David quotes is below (pg 4-2 AM503B manual, pub #070-8766-05). I get 149uA RMS. Although the spec doesn't say, I'm assuming that's measured on the most sensitive range. Note that the noise spec is typical and not guaranteed.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: September 15, 2016, 08:50:52 pm
content: Isn't V rms = V pk-pk x 0.707? Or has my brain gone wonKy? That would make your 3.6 mV pk-pk = 2.62 mV rms

author: Someone
author_rank: Super Contributor
timestamp: September 15, 2016, 11:10:07 pm
content: Quote from: Gandalf_Sr on September 15, 2016, 08:50:52 pmOnly for a sine wave, Analog suggest a 6.6x multiplier for noise:http://www.analog.com/media/en/technical-documentation/application-notes/AN-615.pdfBut you can use other statistical limits, bandwidth of the measurement and number of points in the statistical sample start to weigh in if you want more accurate estimates.

author: rx8pilot
author_rank: Super Contributor
timestamp: September 15, 2016, 11:29:50 pm
content: Here is one of my AM503B's with 6302 probe. I did not have time to measure both of the ones I have.Top trace is 200Mhz BW limited and shows 3.55mV P-P and 370uV RMSThe math trace below is a low-pass at 50Mhz - the effective probe BW. That shows the P-P noise at 2.2mV and RMS of 243uV.These noise levels are not a problem since you can amplify the signal above that noise floor even at very low currents. I have been rather happy with these so far, even with small currents. For very small currents, I use a LeCroy DA1855 differential amplifier and a precision sense resistor.

author: MarkL
author_rank: Unknown
timestamp: September 16, 2016, 02:28:41 am
content: Quote from: rx8pilot on September 15, 2016, 11:29:50 pmI'll add to this that averaging can perform miracles in removing noise from these amplifiers, as long as your signal is repetitive and you can trigger on it. And if the signal is too far into the noise for a reliable trigger, you can usually find an associated voltage node to get a clean trigger via a different channel.With enough averaging you can also set the scope to <10mV/div for even higher gain. And once averaging is on, some scopes provide a magnify math function, so you can really get carried away with the sensitivity. Screen shot below of getting somewhat carried away with a 55uA RMS 1kHz sine wave, AM503 set to 1mA. The purple trace is Math = 2 * Ch1 --> 50uA/div. The second screenshot is with the math channel off and averaging off.High Res mode can be similarly helpful, but with the usual caveats on displayed BW.EDIT: Ch1 on the scope is 1mV/div with probe factor of 10V/A (0.100:1), units=Amps, if you were wondering.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: September 16, 2016, 09:44:17 am
content: Awesome info guys, I love it when I learn something.Thanks to all

author: MarkL
author_rank: Unknown
timestamp: September 16, 2016, 03:33:49 pm
content: One additional note on the averaging thing...It's important that the scope's input (signal+noise) stay within the dynamic range of the ADC. If you get beyond the limits of the ADC it will clip, and you will start to get distortion in the displayed average.A small amount of clipping can usually be tolerated, meaning a few bad acquisition points will not make a big difference in the displayed result. Some scopes will indicate clipping and others not, so just be aware of this possibility if the waveform doesn't look like what you'd expect. It would tend to look flatter at the min's and max's, or have reduced amplitude.In the previous screenshot I posted with no averaging (am503_no_averaging.png), it would appear that I have massively overdriven the input to the scope when set to 1mV/div. And I should have gotten a distorted result. However, the MSOX3000 "cheats" on 1mV/div and 2mV/div, and they are just display magnifications of the 4mV/div range. Had it been a true analog 1mV/div front end, the ADC would have clipped. If you look closely at the yellow trace, you can see the large ADC steps in the shading resulting from the magnification.Posted below is the same signal on the 4mV/div range. This is what is actually being processed by the averaging algorithm. Nothing is clipping.

author: David Hess
author_rank: Super Contributor
timestamp: September 17, 2016, 06:34:43 am
content: Quote from: Someone on September 15, 2016, 11:10:07 pmThere is anolder AD application notewhich includes a table showing the relationship between RMS and peak-to-peak noise and probability.If you are using a DSO, then the practical ratio of peak-to-peak and RMS is easy enough to determine with the built in automatic measurements so no guesswork is needed. The ratio varies somewhat between instruments and operating modes.