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Entry  Mon Nov 7 19:26:33 2016, yinzi, DailyProgress, TempCtrl, Testing driver circuit 6x
    Reply  Tue Nov 8 11:22:48 2016, awade, DailyProgress, TempCtrl, Testing driver circuit RC_CircuitLoadedUnLoaded.pngRC_CircuitLoadedUnLoaded_AC_Analysis.png
Message ID: 1762     Entry time: Tue Nov 8 11:22:48 2016     In reply to: 1761
Author: awade 
Type: DailyProgress 
Category: TempCtrl 
Subject: Testing driver circuit 

That second input sine wave doesn't look very sine wavey.  That should be a clue to something there. Do you remember if you had the osiloscope input set to 50 Ω or 1 MΩ? A passive LP filter is just a frequency dependant voltage divider, loading the output will alter its corner freqeuncy and  change the pass band attenuation.  f_c = 1/2pi(R || R_load x C).  

Attached is comparison of no load (eqv. to 1MΩ) and 50 Ohm.  Also here is a (non-periminant) link to interactive plot +schematic : http://www.partsim.com/simulator/#63263

Life might be easier for you if I get the SR785 network analyzer to the electronics lab for you to use.


Edit: Fixed link -awade Tue Nov 8 14:32:28 2016


After troubleshooting some supply power problems (apparently the top and bottom half of the breadboard power strips aren't connected???), I took some measurements on the following circuit that Andrew suggested, which had the desired properties in simulation. However, AD743's were not really available, so I used LT1012's instead, but it shouldn't make a difference for these measurements.

The output of the AD620 stage (see elog 1749) was as expected (with a .5Vp, .1Hz input, a 5Vp output), and this was used as the input to the buffer circuit:

The output of the buffer circuit was just 0V, so I took some intermediate measurements. The following is at the positive input to the first amplifier (aka after the 100k resistor):

And at the output of the first stage (aka Vout_1st):

From a hand-calculation, the noninverting input should be about 0.85Vin at 0.1Hz, so there's definitely something wrong there.

I tried just connecting Vin straight to a voltage follower, and that actually pulled down Vin:

I'd expect the input resistance to the op amp to be almost infinite, but here it almost looks like it's loading down the input signal, except for the fact that the shape is weird (kind of like clipping). 

So, I'm not really sure what's going on. I'll try digging through the datasheet, testing the circuit in isolation (drive it straight from the function generator), and running a simulation that includes the first half of the circuit to see if that provides any insight.


Edit: Fixed hand calculation (used f instead of w earlier), but the gist is the same.


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