# Design: Tunning I/V Convert(IVCvT) with OPA355

## 1. Major circuit

## 2. PD param

* [SFH250v](https://www.farnell.com/datasheets/3029078.pdf)
* Capacitance: 11pF @1MHz
* Photocurrent: 4.4uA @1uW/5VR
* TR/RF max: 16ns @5VR
* **Has fiber house and lock**
* This means we don't racking our's brains to desing house and lock
* 3D Printer?
* Does it realiable? lock is tight? well housing PD?
* Yield
* Plastic injection?
* How to draw?
* More time and more cost
* cons: SFH250V is expensive
## 3. OPA param

|Param\\OPA| [OPA192](https://www.ti.com/lit/gpn/opa192) | [OPA810](https://www.ti.com/lit/gpn/opa810) |[LTC6268-10](https://www.analog.com/media/en/technical-documentation/data-sheets/626810f.pdf) | [OPA356](https://www.ti.com/lit/gpn/opa356) |
| ------- | --------- | ---------- |----------- |----------|
| Ci-Comm | 6.4p | 1.6p | 0.45p | 1.5p |
| Ci-Diff | 2.5p | 0.5p | 0.1p | 1.5p |

## 4. Calc Gain-Bandwidth

### 4.1 Cin


* SHF250v
* OPA192
* Cin = Cj+Cd+Cm2 = 11pF + 6.4pF + 1.6pF = 19pF
* OPA810
* Cin = Cj+Cd+Cm2 = 11pF + 2.5pF + 0.5pF = 14pF
* LTC6268-10
* Cin = Cj+Cd+Cm2 = 11pF + 0.45pF + 0.1pF = 11.55pF
* OPA356
* Cin = Cj+Cd+Cm2 = 11pF + 1.5pF + 1.5pF = 14pF

### 4.2 Bandwidth

* **Bandwidth of system(circuit) is limited by *Cin* and *C1* and *R1***

$BW = \dfrac{1}{(2\pi)(R1)(Cin+C1)}$

### 4.3 To increase bandwidth

* Reducing R1?
* Since Vo = (Ipd)(R1)
* Vo will be reduced
* If Vo<<<, then background/noise would cover your signal
* For the low light application, we can't suggest this solution
* Reducing Cin?
* Change OPA
* Add negative bias for PD
* This means we will get about 3 times BW
* 
* Dual-rail power supply will increase complexity of circuit
* Low-Noise LDO for negitive, which is rare and expensive
* Reducing C1?
* I found minimum of MLCC is 0.1pF in the market

## Reference
* [1 MHz, Single-Supply, Photodiode Amplifier Reference
Design](https://www.ti.com/lit/ug/tidu535/tidu535.pdf)
* [LPF Calculator](http://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php)
* [LPF Plotter](https://www.trance-cat.com/electrical-circuit-calculators/en/rc-low-pass-filter-calculator.php)

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