The Photodiode Wizard online design tool simplifies the process of selecting optimal op amps and calculating component values to meet the performance requirements given by the user.
Photodiodes generate a small amount of current, proportional to the level of light the sensor is exposed to. A standard method for amplifying photodiode current (and converting that current to a voltage) is through use of a transimpedance (current-to-voltage) amplifier circuit configuration.
Photodiode transimpedance amplifier
See "High Impedance Sensors" for a more detailed discussion of photodiode transimpedance amplifiers. (Excerpted from Practical Design Techniques for Sensor Signal Conditioning)
CN-0312: Dual-Channel Colorimeter with Programmable Gain Transimpedance Amplifiers and Synchronous Detectors
CN-0312 is a reference design which includes photodiode signal conditioning circuitry that achieves 16-bit resolution color measurements of liquids. The signal path includes a photodiode, a transimpedance amplifier (with two selectable gains), and sychronous detection. Detailed information about CN-0312 can be found in the circuit note.
The transimpedance amplifier circuitry in CN-0312 is implemented using switches, which enables programmable gain. Two gain values, 1MΩ and 33kΩ, are used. Below is an excerpt from the circuit note:
Using Photodiode Wizard to design transimpedance amplifier circuit
Let's pick one of the two TIA designs illustrated above to design using Photodiode Wizard:
- Requirements per CN-0312 (TIA design is also detailed in accompanying Analog Dialogue article)
Step 1: Entering photodiode specifications on "Photodiode" tab
Use the photodiode datasheet to determine the photodiode capacitance (Cd) and shunt resistance (Rsh). You will also need to know the peak photodiode current (Ip). In the case of CN-0312, it is stated in the circuit note that the expected peak current is 2.5uA. Below shows the relevant table in the photodiode datasheet, with specs circled in red:
Finding photodiode specifications in datasheet - example using Hamamatsu S1336-44BK
Entering photodiode specifications in Photodiode Wizard - Cd, Rsh, and Ip in red
Tip: Need help with Photodiode Wizard? Look for help icons throughout the tool, and post questions on Engineer Zone.
Step 2: Entering circuit specifications on "Circuit Design" tab
Enter the design requirements - 2.5V peak output voltage, and 50kHz Bandwidth:
Entering circuit specifications in Photodiode Wizard - Vp and BW in red
Step 3: Selecting an amplifier
Photodiode Wizard will initially select an amplifier based on the design criteria entered. The amplifier selection may change as the you tweak the design specifications.
If you want to force Photodiode Wizard to use a particular amplifier, you can do so by clicking "Change" in the "Op Amp" section:
A pop-up window will appear, with a list of all of the op amps in the Photodiode Wizard library. You can filter the list to show only amplifiers that are suitable for your design requirements:
Since the photodiode TIA in CN-0312 uses the AD8615, let's select that op amp:
Step 4: Adjusting the peaking
Photodiode Wizard allows you to further tweak your design through use of the "peaking slider." Moving the peaking will change the value of Cf, which will in turn effect the step response, frequency response, noise gain, and SNR. Low peaking results is a more damped circuit, with a slower repsponse time, but better noise performace. Higher peaking results is a quicker response, but with a higher noise tradeoff.
To most closely match the circuit in CN-0312, we can move the slider completely to the left. This doesn't quite get us to the 4.7pF value used in CN-0312, but it's close:
Comparing the Results
The previously mentioned Analog Dialogue article goes through a detailed analysis of the performance of the TIA design we've just created using Photodiode Wizard.
I highly recommend reading the entire article, if you want to understand the equations and calculations. But in the interest of brevity, the total noise (due to Rf, op amp current noise, and op amp voltage noise) is summarized in equation 13 (page 3 of article):
Compare that to the total noise reported on the SNR view in Photodiode Wizard:
I hope this was a helpful and simple illustration of how to use Photodiode Wizard to design a photodiode TIA circuit. Photodiode Wizard has been designed to enable the designer to very quickly explore how design requirements affect various aspects on the design performance.
- Part 2: Using SPICE simulations to verify Photodiode Wizard designs
- Part 3: Exploring CN-0312 signal chain - Using synchronous modulation to improve ENOB/noise performance
- Photodiode Wizard - http://www.analog.com/designtools/en/photodiode/
- CN-0312 Circuit Note and Reference Design - http://www.analog.com/en/circuits-from-the-lab/CN0312/vc.html
- "Programmable-Gain Transimpedance Amplifiers Maximize Dynamic Range in Spectroscopy Systems" - http://www.analog.com/library/analogdialogue/archives/47-05/pgtia.pdf
- Datasheet for Hamamatsu S1336-44BK photodiode - http://www.hamamatsu.com/resources/pdf/ssd/s1336_series_kspd1022e07.pdf
- "High Impedance Sensors" - http://www.analog.com/high_impedance_sensors5