Photodiode Sensor & TIA Calculator
Calculate photodiode photocurrent, optical power, responsivity, inverting transimpedance output voltage, feedback resistor size, dark-current offset, shot-noise reference, RF-CF pole and ideal optical power per ADC code.
This is a sensor signal-chain calculator. It is not a full SPICE model, photodiode device simulator, op-amp stability solver, optical link budget, laser-safety tool or automatic op-amp part selector.
Engineering tool
Photodiode Sensor & TIA Calculator
Analyze photodiode optical power, responsivity, photocurrent, inverting TIA output voltage, feedback resistance, dark current, shot-noise reference, RF-CF pole and ADC optical-code reference.
Calculation mode
Parameter panel
Result console
- TIA Output Voltage
- 1.499V
- Photocurrent
- 10µA
- Optical Voltage Gain (V/W)
- 50,000V/W
- Optical Voltage Gain (V/mW)
- 50V/mW
- Optical Power
- 20µW
Photodiode Sensor & TIA Formula Audit
| Photocurrent Sign Convention | Iphoto is entered and displayed as a positive magnitude for incident light. |
|---|---|
| TIA Polarity Convention | Default inverting photodiode TIA uses VOUT = VREF - ITOTAL RF. |
| Reference Voltage Definition | VREF is the non-inverting reference or virtual-ground voltage; it is not assumed to be 0 V. |
| Optical Power → Photocurrent | Iphoto = Rλ Popt. |
| Photocurrent → Optical Power | Popt = Iphoto / Rλ, requiring Rλ > 0. |
| Dark Current Convention | Idark is a positive magnitude and is assumed to flow in the same output-pulling direction as photocurrent. |
| Optical-to-Voltage Gain | |dV/dP| = Rλ RF in V/W. |
| Feedback Resistor Formula | RF = (VREF - VOUT,target) / ITOTAL,max for the adopted inverting direction. |
| Output Saturation Policy | Ideal predicted output is shown even when outside the usable output range; no silent clamp is applied. |
| Shot Noise Formula | in = sqrt(2 q I), and irms = sqrt(2 q I B) when bandwidth is provided. |
| RF-CF Pole Definition | fRC = 1/(2π RF CF), reported only as a simple feedback pole reference. |
| Exact Bandwidth Boundary | Photodiode capacitance, op-amp GBW, noise gain and parasitics are not modeled as full TIA stability. |
| Responsivity Definition | Rλ is photodiode current per incident optical watt at the wavelength of interest. |
| Quantum Efficiency Relation | Rλ = η q λ/(h c), or approximately η·λ(nm)/1239.84 A/W. |
| ADC Optical Resolution | P/code = VLSB/(Rλ RF), an ideal quantization reference rather than a detection limit. |
| Noise Boundary | Shot noise is not total TIA noise; op-amp, resistor, leakage and optical noise remain outside this model. |
| Spectral Boundary | Broadband optical sources require effective responsivity or spectral integration. |
Formula reference
Photodiode Sensor and TIA Formulas
The calculator uses a positive photocurrent magnitude and the fixed inverting photodiode TIA convention VOUT = VREF - ITOTAL × RF.
Iphoto = Rλ PoptPopt = Iphoto / RλVOUT = VREF - (Iphoto + Idark) RFITOTAL = (VREF - VOUT) / RF|dV/dP| = Rλ RFRF = (VREF - VOUT,target) / ITOTAL,max|ΔVdark| = Idark RFin = sqrt(2 q I)irms = sqrt(2 q I B)fRC = 1 / (2π RF CF)Rλ = η q λ / (h c)P/code = VLSB / (Rλ RF)Variable definitions
- Popt
- incident optical power
- Rλ
- photodiode responsivity at the wavelength of interest
- Iphoto
- photocurrent magnitude
- Idark
- dark-current magnitude in the same current direction
- ITOTAL
- Iphoto + Idark
- RF
- transimpedance feedback resistance
- CF
- optional feedback capacitance
- VREF
- TIA reference or virtual-ground voltage
- η
- quantum efficiency fraction
Photodiode Sensor & TIA Formula Audit
| Photocurrent Sign Convention | Iphoto is a positive magnitude for incident optical power. |
|---|---|
| TIA Polarity Convention | The adopted topology is an inverting photodiode TIA: VOUT = VREF - ITOTAL RF. |
| Reference Voltage | VREF is the non-inverting reference or virtual ground; it is not assumed to be 0 V. |
| Optical Power to Current | Iphoto = Rλ Popt. |
| Current to Optical Power | Popt = Iphoto / Rλ, requiring Rλ > 0. |
| Dark Current | Idark is entered as a positive magnitude and added to photocurrent for total current. |
| Optical-to-Voltage Gain | |dV/dP| = Rλ RF. |
| Feedback Resistor Design | RF = (VREF - VOUT,target) / ITOTAL,max. |
| Saturation Policy | Ideal output is still shown when outside the usable range; warnings report likely clipping. |
| Shot Noise | in = sqrt(2qI); irms = sqrt(2qIB) when bandwidth is provided. |
| RF-CF Pole | fRC = 1/(2πRFCF), a simple feedback pole reference. |
| Exact Bandwidth Boundary | This page does not calculate exact closed-loop TIA bandwidth or phase margin. |
| Responsivity | Rλ is A/W at the wavelength of interest. |
| Quantum Efficiency | Rλ = ηqλ/(hc); η = Rλhc/(qλ). |
| ADC Optical Reference | P/code = VLSB/(RλRF), an ideal ADC quantization reference. |
| Spectral Boundary | Broadband sources require spectral responsivity integration or an effective Rλ approximation. |
Worked Examples
Optical power to current
Known: Rλ=0.5 A/W, P=1 mW
Iphoto=0.5 mA.
Micro-watt signal
Known: Rλ=0.6 A/W, P=10 µW
Iphoto=6 µA.
Current to optical power
Known: Iphoto=50 µA, Rλ=0.5 A/W
Popt=100 µW.
Zero responsivity boundary
Known: Rλ=0 in inverse mode
No finite optical-power solution is reported.
TIA forward voltage
Known: VREF=2.5 V, Iphoto=10 µA, RF=100 kΩ
VOUT=1.5 V.
Zero light without dark current
Known: I=0
VOUT remains at VREF.
Optical power to TIA output
Known: P=20 µW, Rλ=0.5 A/W, RF=100 kΩ, VREF=2.5 V
Iphoto=10 µA and VOUT=1.5 V.
TIA inverse
Known: VOUT=1.5 V with the same RF and VREF
ITOTAL=10 µA and Popt=20 µW.
Dark current offset
Known: Idark=10 nA, RF=1 MΩ
Zero-light output shift magnitude is 10 mV.
Dark current at zero light
Known: P=0 with positive dark current
Output moves below VREF by IdarkRF.
Feedback resistor solve
Known: VREF=2.5 V, Imax=20 µA, target VOUT=0.5 V
RF=100 kΩ.
Output clipping warning
Known: Ideal output below usable minimum
The ideal value is shown and saturation warning is displayed.
No silent clamp
Known: Calculated VOUT=-2.5 V
The negative ideal value remains visible for review.
Optical voltage sensitivity
Known: Rλ=0.5 A/W, RF=100 kΩ
|dV/dP|=50,000 V/W = 50 V/mW.
RF-CF pole
Known: RF=100 kΩ, CF=10 pF
fRC≈159.155 kHz.
No CF boundary
Known: CF=0
No finite RF-CF pole is reported.
ADC LSB
Known: 12-bit ADC, 0-3.3 V
VLSB≈805.664 µV/code.
Optical power per code
Known: Rλ=0.5 A/W, RF=100 kΩ
P/code≈16.113 nW/code.
Shot-noise reference
Known: I=1 µA, B=1 kHz
irms=sqrt(2qIB), output noise ≈irmsRF.
Zero-current shot noise
Known: I=0
Ideal photocurrent shot-noise contribution is 0 in this model.
QE at 1000 nm
Known: η=100%
Rλ≈0.80655 A/W.
QE at 500 nm
Known: η=100%
Rλ≈0.4033 A/W.
Responsivity to QE
Known: Known Rλ and wavelength
η=Rλhc/(qλ).
QE warning
Known: Derived η>100%
The calculator warns about the simple one-electron-per-photon model boundary.
Round trip
Known: P→I→VOUT→I→P
The original optical power is recovered within numerical tolerance.
Unit consistency
Known: mW/µW/nW and mA/µA/nA
Internal calculations remain in W and A.
Engineering Notes
| Photodiode | Within its linear region, photodiode current is approximately proportional to incident optical power. |
|---|---|
| Responsivity | Responsivity is specified in A/W and should be taken from the datasheet at the target wavelength. |
| Quantum Efficiency | Quantum efficiency and responsivity are related but not interchangeable. |
| Wavelength | A single Rλ value represents monochromatic light or an effective approximation for broadband light. |
| Transimpedance Amplifier | A TIA converts photodiode current into voltage using a feedback impedance. |
| Feedback Resistor | Larger RF increases voltage sensitivity but reduces output headroom and often available bandwidth. |
| Reference Voltage | Single-supply photodiode TIAs often set VREF above ground to allow downward output swing. |
| Dark Current | Dark current creates zero-light output offset and adds to shot-noise current. |
| Shot Noise | Shot noise is only one noise term; it does not include op-amp or resistor noise. |
| Junction Capacitance | Photodiode capacitance is a major bandwidth and stability parameter. |
| Feedback Capacitor | CF can help stability and noise-gain shaping, but the simple RF-CF pole is not exact bandwidth. |
| Op-Amp GBW | Real TIA performance depends on op-amp gain-bandwidth product, input capacitance and noise gain. |
| ADC Resolution | Optical power per code is a quantization reference, not a true detection limit. |
| Saturation | Use the actual usable output swing, not ideal supply rails, when choosing RF. |
| Critical Designs | Validate precision photodiode systems with datasheets, noise analysis, stability review, PCB leakage control and measurement. |
Common Mistakes
- Confusing A/W photodiode responsivity with V/W optical voltage sensitivity.
- Forgetting W, mW, µW and nW conversions.
- Mixing A, µA and nA current units.
- Using the wrong TIA polarity sign.
- Forgetting VREF or assuming it must be 0 V.
- Changing dark-current direction between forward and inverse calculations.
- Forgetting to subtract dark current in inverse optical-power calculations.
- Assuming a larger RF is always better.
- Calling 1/(2πRFCF) exact TIA bandwidth.
- Ignoring photodiode junction capacitance.
- Ignoring op-amp GBW and phase margin.
- Treating shot noise as total TIA noise.
- Treating ADC optical power per code as a detection limit.
- Assuming responsivity is wavelength independent.
- Confusing responsivity and quantum efficiency.
- Silently clamping negative inverse optical-power results.
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FAQ
How do I calculate photodiode current from optical power?
Use Iphoto = Rλ × Popt, where Rλ is photodiode responsivity in A/W at the wavelength of interest and Popt is incident optical power in watts.
How do I calculate optical power from photodiode current?
Use Popt = Iphoto / Rλ. The inverse calculation requires responsivity greater than zero and a consistent current sign convention.
What is photodiode responsivity?
Responsivity is photodiode current per incident optical watt, expressed in A/W. It depends strongly on wavelength and operating conditions.
What does A/W mean for a photodiode?
A/W means amperes of photodiode current per watt of incident optical power at a specified wavelength.
How does wavelength affect photodiode responsivity?
Responsivity changes with wavelength because photon energy and quantum efficiency change. Use the datasheet value at the wavelength being measured.
What is a transimpedance amplifier?
A transimpedance amplifier converts input current into output voltage. A photodiode TIA commonly uses an op-amp with a feedback resistor.
How do I calculate TIA output voltage?
This calculator adopts the inverting convention VOUT = VREF - ITOTAL × RF, where ITOTAL includes photocurrent and optional dark current.
How do I choose a TIA feedback resistor?
Choose RF from the maximum expected current and usable output swing: RF = (VREF - VOUT,target) / ITOTAL,max for the adopted inverting direction.
Why is TIA output inverted?
In the common inverting photodiode TIA topology, positive photodiode current through RF pulls the output below the reference voltage.
What is the purpose of the TIA reference voltage?
VREF establishes the virtual-ground reference. In single-supply systems it is often placed above ground so the output has room to move downward.
How does photodiode dark current affect the output?
Dark current creates a zero-light offset. With the adopted convention, a positive dark-current magnitude shifts the output by -Idark × RF.
What is photodiode shot noise?
Shot noise is the current noise associated with average photodiode current. The ideal current noise density is sqrt(2qI).
Is shot noise the total TIA noise?
No. Real TIA noise also includes op-amp voltage noise, op-amp current noise, feedback resistor thermal noise, leakage, capacitance interaction and ambient-light variation.
How does feedback capacitance affect a TIA?
Feedback capacitance creates a simple RF-CF pole and is often used for stability and noise-gain shaping, but the pole alone is not exact TIA bandwidth.
How does photodiode capacitance affect TIA bandwidth?
Photodiode junction capacitance interacts with op-amp input capacitance, RF, CF, GBW and phase margin. It is a major real-world stability parameter.
What is the difference between responsivity and quantum efficiency?
Responsivity is A/W electrical response. Quantum efficiency is the fraction of photons converted to charge carriers. They are related by Rλ = ηqλ/(hc).
How do I calculate optical power resolution with an ADC?
An ideal quantization reference is P/code = VLSB/(RλRF). This is not the same as the true optical detection limit because noise is not fully modeled.
Why can a TIA saturate at high optical power?
Higher optical power creates more current, and RF converts that current into a larger voltage shift. If the ideal output exceeds the usable op-amp swing, clipping is likely.