## 1. Test via osc.
* Test Configure of circuit
* 

||5V|3.3V|
|---|---|---|
|Config. of Power|||
|Result|||

### 1.1 Why photo-current sudden increase about 3x~4x against previous test

* Following [this design](https://www.ti.com/lit/ds/symlink/opa928.pd) optimize layout
* Ground guard
* 
* SMD resistor to reduce loss of length
* *nA* grade photo-current

### 1.2 Output LPF is not stable while using > 70pF capacitor

* [Ref](https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1334790/opa992-tia-can-differential-input-impedance-of-op-amp-impact-vo)
* 
* Solution:
* This time -> Remove capacitor
* Would get more noise
* Next time ->
* Replace OPA992 with OPA991
* [Stable](https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1338599/opa991-simulation-for-tia-circuit-is-unstable-phase-margin/5098863#5098863)
* Can drive higher capacitor
* Also add buffer before input of ADC
* 
* Mentioned from ADC's [Datashet](https://www.ti.com/lit/ds/symlink/ads7950-q1.pdf)
* Keeping OPA992 is fine, just would have higher noise floor

## 2. Can be improve

* Feedback capacitor can be removed
* Rfb > 10Meg, at this time Cfb > 1 pF, so this system is stable
* [Reference](https://www.ti.com/lit/pdf/tidu535)
* Model
* 

## Summery
* 3x photocurrent improved
* This beam test:
* ~~3.3V psu(LDO)~~ -> 5v psu(LDO)
* Next board:
* Keep 3.3V psu (LDO)
* Reduce resistor ~~100Meg~~ -> 51, 22, 10, ... (Meg)
* For green fibers testing
* 1st time scale & ensure dynamic range
* Capacitor can be removed
* Add buffer OP-AMP before Switch and ADC
* Change symbol for the 1st pin of ICs