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Entry  Wed Mar 3 00:22:05 2010, Dmass, Electronics, General, OP AMP WOES 
    Reply  Wed Mar 3 08:39:41 2010, rana, Electronics, General, OP AMP WOES 
Message ID: 628     Entry time: Wed Mar 3 00:22:05 2010     Reply to this: 629
Author: Dmass 
Type: Electronics 
Category: General 
Subject: OP AMP WOES 

I tried to make a simple current to voltage convert for my Green PDs, and measure the noise. Hilarity did not ensue.

I made the following connections for an LT1792 (I also tried a LT1012).



            | ___ 100k_____|__   

            |    |\           |   

Gnd-50Ohm-BNC----|-  \ _______|___BNC____ to SR785

Gnd--------------|+  /


I stuffed this into my existing hydra board, and pulled out all the other components. My Noise looked WAY TOO HIGH

I was getting well over a uV/rtHz up to 800 Hz. This does not add up.

  • Johnson Noise should be 40 nV/rtHz
  • Voltage noise of the 1792 is plotted to be 4nV/rtHz with a 30 Hz corner
  • Current Noise of the 1792 is said to be 10 fA/rtHz (which gives 1nV/rtHz)
  • This does not add up to microvolts!

I also tried:

  • Changing the Capacitor values to 22 pF and 220 pF, neither seemed to fix anything.
  • Pulling both the R and the C from the feedback, replacing it with a 3k resistor, and still about 1uV....
  • Changed in the LT1792 for a LT1012 (which had similar values for I and V noise, so this doesn't add up either).
  • Checked the terminals of the op amps for power, I was sending +15V to 7 and -15V to 4.
  • Yes I double checked that the 785 was not in a stupid gain state, and that the noise floor of the measurement was way below this.

I am curious about:

  • My voltage supply: I stole an unused new focus +/- 15V supply from the shelf above the GYRO, I don't know if this could be causing problems to that extent.
  • I tried two versions of a 3M board, with the same disappointing results.
  • I also tried a TENMA 72-6615 "Laboratory DC Power supply" It is regulated to 0.01% according to its data sheet. Maybe it is the connection scheme I am using?
  • The transfer function, I will take this tomorrow and check to see that it is A/B=1

There really has to be something stupid I am doing...

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