---
# System prepended metadata

title: 'Issue: \<Solved\> Change to High Speed PD and Circuit'
tags: [HW-Circiut, DeWei, issue]

---

# 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)

![](https://i.imgur.com/gMaSUHr.png)
* 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
  * ![](https://i.imgur.com/fo0jcfA.png)
* Package problem
  * ![](https://i.imgur.com/OEsyfcC.png)
  * Custom? MOQ

### Solution

* Replace BPW21R with [SFH250v](https://docs.broadcom.com/doc/AV02-3095EN)
  * No pacakage problem
    * ![](https://i.imgur.com/X7QYFsl.png)
    * 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`