Building a Measurement Amplifier

RE: Building a Measurement Amplifier

shabaz — Mon, 31 May 2021 23:44:16 GMT

Here's the latest rev 3 candidate (close to rev 2 but I needed a new number since I'd already attached rev 2 files previously).

The differences between rev 3 and rev 1 are:

* Both input and output are DC or AC coupled selectable separately (input is selectable via a switch, and output is selectable via a different output BNC connector)

* Higher bandwidth (maybe 1 MHz?)

* In AC mode, the response is down to 1 Hz hopefully

* Gain selectable (100/200, via a switch)

* Three-setting switch-selectable low-pass filter (100Hz, 1kHz, 10kHz)

* Zeroing capability via a push-button, to rapidly settle the output capacitor in AC output mode

* Potentiometer for output offset null adjustment

* Prototyping area for extending/fixing the design

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Board layout:

[View:/resized-image/__size/1217x829/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-8a9aeb2b-3af1-4ac5-ac64-3a033657e385/8836.contentimage_5F00_208330.png:1217:829]

This is theoretical, it might not work! I'll order the board in a few days or so after thinking about it some more.

RE: Building a Measurement Amplifier

shabaz — Thu, 11 Mar 2021 23:17:35 GMT

It took a long time to get back to this : ( but here's the new revision 2 candidate. I've added the DC capability (SW2 selects that) along with an offset null that may be needed for that. AC mode response is down to 1 Hz. Also gain is now switched 100/200. Since the bandwidth may be too high for some measurements (e.g. multimeters may not respond beyond tens of kHz) there is switchable 100Hz/1kHz/10kHz filtering capability. When the filter is disabled, the bandwidth should be greater than 100 kHz (might reach 1 MHz) but that can be reduced by adjusting C16. To make life easier, there is an AC-only output (it means the null adjustment doesn't need to be made) which can be on the back panel, since there is no space on the front panel with so many switches. The front panel BNC output is for DC, or for AC but would need trimming with the offset control. I've removed the zeroise button, but now I'm thinking I should have left it in. Anyway I won't work on the PCB layout for a few days, so that any feedback can be accomodated.

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RE: Building a Measurement Amplifier

jw0752 — Fri, 09 Oct 2020 01:44:27 GMT

Hi Shabaz,

You have inspired me and I have followed through and built a small piece of test equipment using your 200 gain amp and the low battery circuit. For many years of my life I was a service tech of audio equipment. Many of my repairs were done using a small cheap Radio Shack signal injector and a Radio Shack signal tracer amplifier. While I still have the RS signal injector the Amp disintegrated somewhere along the line. While I do not do very much audio service anymore nostalgia and your Amplifier circuit got me enthused to make a Signal Tracer. Along with the Amplifier design that you supplied and the Low Battery circuit I also added a 555 circuit to be the source signal. Here are a few photos of the build.

[View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-8a9aeb2b-3af1-4ac5-ac64-3a033657e385/3225.contentimage_5F00_192346.jpg:620:465][View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-8a9aeb2b-3af1-4ac5-ac64-3a033657e385/1172.contentimage_5F00_192347.jpg:620:465]

[View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-8a9aeb2b-3af1-4ac5-ac64-3a033657e385/0676.contentimage_5F00_192348.jpg:620:465][View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-8a9aeb2b-3af1-4ac5-ac64-3a033657e385/4718.contentimage_5F00_192349.jpg:620:465]

[View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-8a9aeb2b-3af1-4ac5-ac64-3a033657e385/2664.contentimage_5F00_192350.jpg:620:465][View:/resized-image/__size/620x465/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-8a9aeb2b-3af1-4ac5-ac64-3a033657e385/6765.contentimage_5F00_192351.jpg:620:465]

I added a volume control between the two amplifier stages. The primary output of the amp goes to a small speaker but I have also provided pins so that the oscilloscope can be tied into the output. Finally a set of very thin bench top short test leads was made up with as many options for termination as I could come up with. Since my audio equipment service is so infrequent I do not know when I will be able to actually use the unit but it is fun to have one available.

Thanks for the fun day.

John

RE: Building a Measurement Amplifier

jw0752 — Tue, 06 Oct 2020 22:55:46 GMT

Hi Shabaz,

Your postings always inspire me and I am currently working on building this one. I decided to begin by investigating the Low Battery Circuit that you have posted in the text. After building it I found that it did not work as advertised so I checked to make sure that I had not made any mistakes and then I looked at the logic. In the end I determined that the LED on the output pin #1 needed to go to V- instead of to V+. Though I do not show the LED as reversed in the corrected schematic obviously it must be reversed to have the cathode to ground and the anode to the 2K2 resistor. Here is my revision to your schematic and notes that I took.

[View:/resized-image/__size/620x451/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-8a9aeb2b-3af1-4ac5-ac64-3a033657e385/2133.contentimage_5F00_192341.jpg:620:451]

To my surprise and delight the circuit has a very nice feature in that the battery low indicator LED starts to blink at 16 volts. As the voltage drops towards 6 volts the frequency of the flashing slows until at 6 volts the LED stays on. Once the voltage has dropped to 4 volts the power is not sufficient for proper operation and the light goes out. The circuit draws about 5 mA which is very acceptable for a piece of test equipment that is not likely to be on for extended periods of time. I think it is an awesome design. Here is a short video of the circuit in action.

Thanks

John

RE: Building a Measurement Amplifier

jw0752 — Wed, 23 Sep 2020 18:03:18 GMT

Hi shabaz,

I have been studying the schematic and I was wondering why you chose the 12 uF size capacitor for the input. It seems to me that the input cap value is usually an order of magnitude or more smaller than this though I don't know the why for that either. Just trying to get some insights.

Thanks John

RE: Building a Measurement Amplifier

fmilburn — Tue, 22 Sep 2020 02:59:44 GMT

Very nice. This is something I would be interested in building as well.

RE: Building a Measurement Amplifier

jw0752 — Mon, 21 Sep 2020 20:05:25 GMT

Hi Shabaz,

Does it make any sense to put in switchable shunts on the input and output caps so that it could also serve in situations where DC amplification is needed? I needed something similar the other day when I was working with a pressure sensor. In my case a signal amplifier would not do as it had to be a DC amplification. I see that Jan mentioned something similar.

John

RE: Building a Measurement Amplifier

Jan Cumps — Mon, 21 Sep 2020 19:54:05 GMT

A little signal amplifier is a great lab tool. Thanks for the design.

The microcurrent gizmo that I use to measure low currents, has a similar approach, a two-stage x100 amplifier.

That one doesn't use input and output caps, to make it work for DC measurements.

Selectable shunt resistors at the input for different current ranges.

I also sometimes use it as a generic x100 amp.

RE: Building a Measurement Amplifier

genebren — Mon, 21 Sep 2020 19:19:35 GMT

Great project! I can see a lot of useful functionality for this project.

RE: Building a Measurement Amplifier

jw0752 — Mon, 21 Sep 2020 18:39:57 GMT

Hi Shabaz,

This looks like a very useful bench tool. Thank you for sharing the design with us. Now I have a new project to work on for a few days.

John