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Finally – I conquered the Wien Bridge bread board prototype that included a VGND of +2.5V (detailed in this post).  Now on to the 2nd stage – reading the E.C voltage.

# The Goal

Add the Second stage of the EC circuit to the breadboard prototype.  Measure the output with an oscilloscope and compare results with the LTSpice IV model. # The Schematic

Here is an image of the LTSpice IV schematic for the EC Circuit with the second stage within the pink area. ## Wien Bridge Oscillator

Here is the output from the Wien Bridge Oscillator stage: YIPPEE!  The frequency is right where it should be at 1.6KHz.

## Shrink Vpp

Moving right along…here is the output after the voltage divider (to get the input into the non-inverting pin close to a Vpp of 200mA): YIPPEE! The scope’s readings appear close enough…now onto reading the ECv.

I’m going to try both a 200Ω and a 1400Ω resistor for R0 (see image above – R0 represents the input of an E.C. probe.  It acts like a variable resistor within a voltage divider similar to a thermistor sensor).  Here’s what I get from the LTSpice IV model: The green line is when R = 200Ω.  The black line is when R = 1KΩ.

When R=200Ω, the Vpp ~= 1.4V.  When R=1KΩ, the Vpp != .46V

I added the op amp and resistors to the breadboard then checked out the results on output of the 2nd op amp.  Here I used a 200Ω R: Another YIPPEE!! The Vpp is “close enough” to the model, off by .1V (1.5V on scope versus 1.4 in LTSpice IV model). And the breadboard prototype of the EC circuit through stage 2 seemed to work for a 1K resistor.

# What’s Next

On to the last stage – sending the ECv through a rectifier.  Hopefully I have more op amps and diodes around…