Low voltage differential scope probe - Page 1
Forum section: electronics / testgear

author: Jester
author_rank: Frequent Contributor
timestamp: September 14, 2021, 10:10:19 pm
content: I'm looking at a problem that involves reasonably low level analog signals 1-10mV from an ultrasonic transducer that are being captured by a picoscope and then filtered and analyzed in software. The picoscope is operating near its limits amplitude wise. The connection between the transducer and picoscope is simply RG cable with no impedance matching (200k input Z on picoscope). I'm considering picking up a differential scope probe to see if this will reduce some of the noise, and I'm also considering some sort of pre amplifier perhaps with a gain of 10. Thoughts and comments?

author: nctnico
author_rank: Super Contributor
timestamp: September 14, 2021, 10:30:48 pm
content: It is not difficult to built a 10x (or more) pre-amplifier using an instrumentation amplifier chip on a piece of stripboard.

author: 2N3055
author_rank: Super Contributor
timestamp: September 14, 2021, 11:05:36 pm
content: Bandwidth?

author: ADT123
author_rank: Regular Contributor
timestamp: September 15, 2021, 06:00:13 am
content: Which PicoScope are you using? I would want to know a bit more about the application before making a firm recommendation (eg signal bandwidth) but have a look at the PicoScope 4444 - it has differential inputs, 14 bit resolution and depending on which product you are using now may have a more sensitive input range.https://www.picotech.com/oscilloscope/4444/picoscope-4444-overview?lvBefore trying anything else though make sure you have explored the resolution enhance and low pass filter options of the PicoScope you have already.

author: Jester
author_rank: Frequent Contributor
timestamp: September 16, 2021, 03:23:03 pm
content: Quote from: 2N3055 on September 14, 2021, 11:05:36 pmsampling rate is 42MHz

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2021, 03:28:39 pm
content: What analog bandwidth is required for your ultrasound signal?

author: Jester
author_rank: Frequent Contributor
timestamp: September 16, 2021, 03:41:02 pm
content: Quote from: ADT123 on September 15, 2021, 06:00:13 amPicoscope is 3404D, sampling at a minimum of 42MHz, the 20MHz bandwidth of the 4444 would be too low.

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2021, 03:44:56 pm
content: You seem to be confusing bandwidth and sample rate. By Nyquist, a 40 Msample/sec sampling rate is required to digitize a 20 MHz bandwidth without aliasing.

author: Jester
author_rank: Frequent Contributor
timestamp: September 16, 2021, 03:51:21 pm
content: Quote from: TimFox on September 16, 2021, 03:28:39 pmNo confusion with Nyquist. The 42MHz rate is for the 20MHz scenario.The signal of interest varies, as low as 20MHz and as high as 50MHz, however we collect enough data for ~2000 repetitions of that signal and then process the data.

author: bdunham7
author_rank: Super Contributor
timestamp: September 16, 2021, 04:01:49 pm
content: Quote from: Jester on September 16, 2021, 03:51:21 pm??QuoteObviously a 50MHz bandwidth makes a low noise preamplifier a wee bit harder to make. What are the output impedance characteristics of the transducer?

author: TimFox
author_rank: Super Contributor
timestamp: September 16, 2021, 04:08:37 pm
content: You should be more careful about punctuation: your commas implied another meaning.

author: ADT123
author_rank: Regular Contributor
timestamp: September 16, 2021, 04:15:27 pm
content: If you need 50MHz then as you say the PicoScope 4444 has too low a bandwidth. Consider the PicoScope 5000 series over the 3000 you have now. Depending on how many channels you need to use at once you can jump from 8 bit resolution to 14 or 15 bits (x64 or x128 more resolution) and x2 more sensitive input range. That is not going to combine to give you 256 improvement - your averaging of 2000 cycles is in effect increasing the resolution / reducing the noise but it will make a big improvement.If thats of interest ask your local distributor for a loan unit (if they dont play ball send me a PM).

author: Jester
author_rank: Frequent Contributor
timestamp: September 16, 2021, 05:30:47 pm
content: Quote from: bdunham7 on September 16, 2021, 04:01:49 pmI don't have a Zout plot for the transducer. The output impedance is specified as 60 Ohms and capacitance of 680pFThe 5443D, may be a better fit than what we are using now.

author: nctnico
author_rank: Super Contributor
timestamp: September 16, 2021, 07:59:51 pm
content: 60 Ohms is quite low for an ultrasonic transducer - assuming it is a piezo-electric one-. But it does give a lot of choices for getting a pre-amplifier. Maybe there is something premade. I would guess something already exists as a first signal conditioning stage for such transducers. If anything else fails then a differential to single ended amplifier chip is the way to go.