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).
|-----100pF----|
| ___ 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... |