Any tips on measuring analog signals down to hundreds uV to few mV range? - Page 1
Forum section: electronics / projects

author: ryantran
author_rank: Newbie
timestamp: December 21, 2022, 05:15:08 pm
content: Hi All,This is a generic question, not really project-specific question. I wonder if anyone here has experiences in measuring signals with a scope in a range of ~0.5mV to ~10mV ish. In this range, every design aspects are crucial inclduing: grounding scheme, PCB layout, how to probe etc.My current project includes a signal generator that produces an input (a Gaussian waveform) that are in the range mentioned above, a PCB (some black box) and output from the black box. The sig generator is connected to the PCB via a BNC + MMCX cable. I use a ground spring on the probe and the "Average" feature of scope to measure input and output otherwise the signal would super noisy. But the Average feature also gives you an illusion of how "clean" the signal is.Then I notice that when I move or wiggle the MMCX cable, the signal's noise got affected significantly and I could see the waveform dancing, hence the output's behavior is also affected due to the disturbance. That makes me question how I know my test bench is reliable.Anyone has any tips or any experiences to share on measuring low level analog signals like this? Really appreciate your feedback.

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2022, 05:33:05 pm
content: For an oscilloscope, such low level measurements mean either directly connecting the signal to the oscilloscope BNC input, or at even lower levels the use of a low voltage differential probe or input like a Tektronix 7A22 which operates down to 10 microvolts per division and up to 1 MHz.More often I build an amplifier into the circuit to generate a higher level output for testing.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 21, 2022, 07:16:49 pm
content: Those diff plug-ins have built-in adjustable filtering, lest you be viewing a bunch of noise. You may need low-microphonic cable, though I don't know how available that is. Or, keep everything still!

author: bdunham7
author_rank: Super Contributor
timestamp: December 21, 2022, 07:21:02 pm
content: Quote from: ryantran on December 21, 2022, 05:15:08 pmTry using a boxcar averaging filter (typically ERES) instead of the capture averaging. This will lower your BW which is what you need to do to lower your noise level.

author: ryantran
author_rank: Newbie
timestamp: December 21, 2022, 10:28:50 pm
content: Quote from: bdunham7 on December 21, 2022, 07:21:02 pmWhat do you mean boxcar averaging filter (or ERES)?

author: bdunham7
author_rank: Super Contributor
timestamp: December 21, 2022, 10:39:12 pm
content: Quote from: ryantran on December 21, 2022, 10:28:50 pmI should have asked what scope you are using the 'Average' feature on. Many DSO's have a function, typically labelled 'ERES' (extended resolution or something like that) where it uses a per-trace moving average function on the samples. You get the same number of samples but with what is effectively a LPF.

author: srb1954
author_rank: Super Contributor
timestamp: December 21, 2022, 10:40:31 pm
content: Quote from: David Hess on December 21, 2022, 05:33:05 pmThe Tek 7A22 is excellent for tracking down low level noise signals but you need a Tek 7000 series scope mainframe to house it.The same functionality as the 7A22 can be obtained from a Tek AM502 differential amplifier which can act as a preamplifier for any normal scope. The AM502 requires Tek TM500 series mainframe but this is cheaper and a lot simpler than a 7000 scope mainframe.

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2022, 11:28:30 pm
content: Quote from: srb1954 on December 21, 2022, 10:40:31 pmIt is too bad that 7A22s are so much more common than AM502s, although I finally managed to get an AM502.Oregon Analog Tools made a 7000 mainframe chassis for holding 7000 series plug-ins so they could be used with any oscilloscope:https://w140.com/tekwiki/wiki/Oregon_Analog_Tools_ASM7

author: niconiconi
author_rank: Frequent Contributor
timestamp: December 21, 2022, 11:34:48 pm
content: Quote from: David Hess on December 21, 2022, 11:28:30 pmI always wanted one, but I can find no technical information, user manual, schematics, or used instruments. Now it effectively doesn't exist. I hope one day relevant information can resurface so to allow one rebuilding it.

author: niconiconi
author_rank: Frequent Contributor
timestamp: December 21, 2022, 11:40:41 pm
content: Quote from: niconiconi on December 21, 2022, 11:34:48 pmOh, and not to mention the mechanical challenges, such as unobtainable edge connectors...

author: Damianos
author_rank: Frequent Contributor
timestamp: December 22, 2022, 10:01:58 am
content: Usually,when I have to deal with low level signals, the "box under test" contains "enough amplification" so the output can be probed "normally". For the input I use an attenuator and termination as possible directly at the input of the box. This way the generator is adjusted for a 100 times higher level, simplifying the measurement and the attenuator reduces the interference.

author: David Hess
author_rank: Super Contributor
timestamp: December 22, 2022, 10:55:27 pm
content: Quote from: niconiconi on December 21, 2022, 11:34:48 pmIf you want that capability, most or all of the 7000 mainframes have a vertical output which can be used to allow a separate instrument to take advantage of a 7000 series plug-in.

author: TimFox
author_rank: Super Contributor
timestamp: December 22, 2022, 11:27:29 pm
content: Another possibility for looking at low-level analog signals is the lock-in amplifier, if a suitable reference signal exists.

author: David Hess
author_rank: Super Contributor
timestamp: December 23, 2022, 01:44:20 pm
content: Quote from: TimFox on December 22, 2022, 11:27:29 pmIf you have a reference signal, then it can be applied to the oscilloscope as an external trigger and then averaging used to lower the noise level and extract the signal.

author: mawyatt
author_rank: Super Contributor
timestamp: December 23, 2022, 02:32:23 pm
content: Quote from: David Hess on December 22, 2022, 10:55:27 pmWe often used this Tek 7000 Vertical Output feature and plugged into a Spectrum Analyzer, this gave us the various Tek scope plug-ins & probes and such for use with the SA. Very useful feature indeed!!Wish the DSOs had a similar Vertical Output feature, they could tap and buffer signal just before the channel ADC.Best,

author: David Hess
author_rank: Super Contributor
timestamp: December 24, 2022, 02:19:35 am
content: Quote from: mawyatt on December 23, 2022, 02:32:23 pmThe 7000 mainframe vertical output was actually sourced from the trigger signal path, so the trigger source determined which signal was forwarded to the vertical output.The larger advantage however was that the 7000 mainframes also provided the b-gate signal which could be used in combination with the vertical output to allow measurements of a specific portion of the displayed waveform, indicated by the intensified zone. The b-gate output could for instance be applied to the ARM input of a counter. This combination was present inside timer/counter oscilloscopes like the 2247A and 2236 allowing them to make gated measurements.I would be easy enough for DSO manufacturers to add a vertical output in the same way Tektronix did, but I do not see it happening.Most often I have connected the 7000 vertical output to one of my DSOs so that I can use a 7A13 or 7A22 differential amplifier with my DSO.