# Issue: \<Solved\> Change to High Speed PD and Circuit

## Bacnground: 40ns pulse

* 25MHz square wave
* [BW\*Tr=0.35](https://tinyurl.com/mr2e9ucm)
* Note: Be care of using this formula
* [Slew Rate and Tr](https://tinyurl.com/3zhaukvr)
* V/us -> V/ns -> V/ns \* 0.8 -> Tr

## OPA192 should be criticle

* [Datasheet](https://www.ti.com/lit/gpn/opa192) said:
* BW: 10MHz
* Slew Rate: 20 V/us
* Replacement: [OPA810](https://www.ti.com/lit/gpn/opa810)
* Price close to OPA192: 100NTD/PCS
* Pros
* Slew Rate: 20V/us -> 200V/us
* BW: 20MHz -> 140MHz
* Cons
* Offset -> 5uV -> 500uV

## We should change the policy for photodiode

### Previous: [Photovoltaic mode](https://www.ti.com/lit/pdf/tidu535) with [BPW21R](https://www.vishay.com/docs/81519/bpw21r.pdf)

* Photovoltaic mode also can be understood as *Solar Chager*
* R/F time ~ 6 us: too bad
* Our target: 40ns pulse would have problem
* Low intensity of light also would have problem
* 
* Package problem
* 
* Custom? MOQ

### Solution

* Replace BPW21R with [SFH250v](https://docs.broadcom.com/doc/AV02-3095EN)
* No pacakage problem
* 
* Can be mount onto PCB directly
* R/F time ~ 10 ns
* Working on the photoconductive mode
* [Understanding Photovoltaic and Photoconductive Modes of Photodiode Operation](https://tinyurl.com/mr3n822d)
* New problem: We must add **-5** voltage
* ~~Possiable solution: Voltage reference + OPA~~
* Solution:
* ~~[TechReport: Non-inverting PD AMP](https://hackmd.io/@pd-driver-ntuhep/HyBnq4Xvj)~~
* Because inversing 1st PD I/V stage + inversing 2nd amp stage = positive

###### tags: `issue` `DeWei` `HW-Circiut`