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• Project update

  Jesse Farrell • 04/15/2022 at 23:13 • 0 comments

  So the widget works at 10MHz… but I think I can do better. I should probably call this project a day and move onto a new one, but here we are. I’m going to try adding one additional amplifier stage to introduce some frequency compensation to the system. This allows me to spread the overall CSA gain across more op-amps (better BW), and also allows me to introduce some HF gain to hopefully push the BW a bit higher.

  I also moved some stuff around on the new board (shown below), I'll be doing some simulation of the design before I order the next revision.

• Validation (the hunt for precision gear)

  Jesse Farrell • 03/12/2022 at 17:43 • 0 comments

  Quick comment on my test setup… I’m using a SDG 1032x function generator and a SDA 1104x-E for my testing here. Since the function generator has a 50-ohm output, I put a 50-ohm shunt into my CSA widget… the hope being to reduce the reflections. Note that there was no compensation at the output, just the CSA output probed with a 10x probe. 

  *on with the validation*
  

  Originally, I populated the board with 1% resistors matched with a fluke 115. The amplifiers gain and linearity was a bit questionable to say the least. So, using some of my school’s equipment, I rummaged through about 100 0603’s to find 1k and 2k resistors with ~0.05% tolerance (not using 4 terminal measurement here… the Keysight DMM didn’t have it). Here are some screenshots of the input and output waveform for the widget. Notice that the vertical volts/div were kept at 10x apart so if the amplifier gain is correct, both waveforms should appear to be the same amplitude on the scope. 

  *Here is 100kHz with and without BWL on the input signal*
  

  
  *Here is 1MHz with and without BWL on the input signal*
  

  
  *Here is 10MHz with and without BWL on the input signal*
  

  
  *Here is 20MHz with and without BWL on the input signal, some reflections on the input signal :P*
  

  Luckily my Siglent gear plays nicely together so I was able to use an automated bode plot function to get the following graph. Definitely a lower BW then I wanted, but 10MHz isn’t too bad. I’ll have to do some more research to figure out ways of improving this.

• Schematic Overview

  Jesse Farrell • 03/12/2022 at 01:05 • 0 comments

  The main design process can be found in the HAS located in the project files, it goes over why I chose the components used here. A big driving requirement for this design is configurability. I’m looking to use the same CSA design for several different current ranges. The entire schematic is shown below, then I’ll go over each noteworthy block.

  This CSA is based on an instrumentation amplifier using 3x OP810’s. These op-amps were mainly chosen for their higher gain product bandwidth, wider operating range, and low input offset voltage. I might add a potentiometer somewhere in this design to allow the user to fine tune the circuits gain.

  This CSA is fed by the differential signal from a shunt resistor. To allow for several different packages I stacked a 2512/1206/0805 footprint on top of one another. 

  The reference ground is controlled by the voltage follower shown below. Using a potentiometer, the user can tweak the output offset. 

  Lastly there is a small battery monitor circuit that will notify the user when the 9V battery reaches ~5V. 
  

• Simulation

  Jesse Farrell • 03/12/2022 at 00:54 • 0 comments

  After researching different CSA implementations, I decided to go with an instrumentation amplifier for my design, primarily for its increased CMRR compared to other amplifiers. Once I chose the desired OPAMP (see HAS for details), I simulated the design in LTSPICE, see below. Note that a voltage follower is used to generate a reference ground for the circuit. 

  Using higher resistances seems to add some non-linearity to the end of the response. (Similar to higher Q factor on filters). I could add some active filtering to counteract these affects, but it would require about 1pF, which at that point I’m suspicious my layout and power planes are already adding that much… either way it seems simpler to use the lower value resistors. 

• Read Me

  Jesse Farrell • 03/06/2022 at 02:19 • 0 comments

  This page will act as a dumping ground for validation results and some progress updates. For the polished documentation see the HAS that will be included in the project files.

  I’m a little delayed for the documentation of this project, but you can expect regular posts for the next month or so.

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