Getting a large change from small change

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Discussion Overview

The discussion revolves around the challenge of achieving a significant voltage change from small variations in capacitance and resistance in electronic circuits. Participants explore potential circuit configurations and methods to enhance signal-to-noise ratio (SNR) when dealing with minor changes in component values, particularly in the context of capacitive and resistive measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks to amplify small changes in capacitance (from 40pF to 40.5pF) to achieve a larger voltage change (0.5V to 0.8V) for better visibility in measurements.
  • Another participant questions the reasoning behind changing the hardware setup, implying that the current configuration may already be sufficient.
  • A suggestion is made to consider a bridge setup, such as a Wheatstone bridge, adapted for capacitors and AC excitation, although one participant expresses uncertainty about the workings of bridges.
  • There is a mention of the classical Wheatstone bridge as a balanced system for resistance measurements, indicating its relevance to the discussion.
  • One participant references a resource on AC bridges and application notes from manufacturers of lock-in amplifiers, suggesting these could provide useful insights.
  • A participant expresses skepticism about the effectiveness of a bridge in their current setup, noting that they already have a configuration involving capacitors and resistors, but they are still struggling with achieving the desired resolution in voltage change.
  • There is a query about the possibility of transforming a linear relationship into an exponential one to enhance the sensitivity of the measurements.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to achieve the desired signal change. Multiple competing views and suggestions are presented, with some participants expressing uncertainty about the effectiveness of proposed methods.

Contextual Notes

Participants acknowledge limitations in their current setups and the need for further exploration of circuit configurations. There is also an indication that the relationship between capacitance changes and voltage output may not be straightforward, suggesting potential complexities in achieving the desired results.

edmondng
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Need some help or suggestion here.
I'm trying to collect some data resulting from very small changes. What i want is for example a change from 40pF to 40.5pF to result in a bigger voltage change say 0.5V to 0.8V, enough for me to see some difference. In other words, a very big SNR.

The way i have my setup now is using a cap reference and comparing it with the variable, and going through a differential amplifier.

I was thinking in the sense of circuit manipulation where for example if you have 2 caps in series, where 1 is very very large like 1000pF and the variable cap is only 40pF, Then the resulting capacitance from 40pF to 40.5pF will result in only a very small change.

Same is true for resistance in parallel. Put a 52 Ohm in parallel with 1100 Ohm, but now change the 52 to 53 ohm and the resulting signal is still small. So is there some way for me to manipulate such that i can get a bigger signal change with my small input change.

Ideas, suggestion would be great. Thanks
 
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What is the reasoning for changing the hardware?
 
Maybe you can use some sort of a bridge setup to get a bigger response to changes:

http://en.wikipedia.org/wiki/Wheatstone_bridge

I think you can do something similar to the Wheatstone resistive bridge, but use capacitors and AC excitation instead, but to be honest, I never really understood how bridges worked anyway, so I don't think I'll be much more help beyond pointing you at the subject matter. Good luck!
 
Hey, great minds think alike!
 
Oliver & Cage, Electronic Measurements and Instrumentation, McGraw-Hill, 1975, the AC bridge "bible;" also, you'll find tricks under application notes from various manufacturers of lock-in amplifiers.
 
berkeman said:
Hey, great minds think alike!

Some just a little faster. Maybe we need to display posting time seconds.
 
hmm i don't know how much of a bridge would help cause i already have something like that.
Cap ref in series with resistor, parallel with cap series resistor. Measure the voltage differential at resistor. If all values are same, difference is zero.

What I'm more concerned is the change. In other words changing the values from 40pF to 40.5 pF can give me enough resolution to see that change. I could change the ref cap to give me different slopes of change but it all still comes out about the same in terms of max change. Is there maybe some way to convert the linear relationship to some exponential relationship or other equation that can be related to it?
 

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