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I am looking to design a transimpedance amplifier for use with an avalanche photodiode in a battery powered application and was hoping to get some assistance.
I am looking to detect pulsed light in the range of 800nm. I understand that a high speed op-amp would be required due to the nature of light, but am very unsure of what I need. I was looking at the OPA684 due to its slew rate of 820 V/us, but from what I've researched, a low bias current is very beneficial. Having a low bias current op amp reduces voltage offset and helps achieve the highest sensitivity. The OPA128 has a very low bias current but also a very average slew rate. I am also unsure of what the bandwidth range should be for such a device.
Also, how does one go about choosing the right value for the feedback resistance?
Any assistance or guidance on what parameters I should consider when choosing the right op-amp for such an application (APD transimpedance amplifier) would be greatly appreciated.
Thanks for your help.
Do you have a photodiode selected and do you have details on the expected pulse in terms of the photodiode output, namely, the pulse width (e.g. 100ns pulse) and pulse amplitude (e.g. 20uA)? The pulse characteristics are central to the part selection. It is also useful to know photodiode capacitance for part selection as this plays into the bandwidth and compensation.
The value of the feedback resistance is selected to get the desired transimpedance gain, which will depend on the amplitude of the input current pulse and the desired amplitude at the output of the transimpedance stage. Usually a second stage amplifier is used to amplify the voltage output of the first stage, since higher feedback resistance (transimpedance gain) results in in lower transimpedance stage bandwidth.
This app note may help you get started: Transimpedance
Considerations for High-Speed Operational Amplifiers - sboa122
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In reply to Kris F:
As for the photodiode, I've been looking at the Hamamatsu APD; Specifically the S2381 (http://sales.hamamatsu.com/assets/pdf/parts_S/s2381_etc_kapd1007e09.pdf). Since this is going to be used in a very low IR light application (Laser range finder).
I am looking to use a time to digital converter (Acam TDC-GP21) that has a fire pulse generator on board that I will use to pulse the laser. Since the pulse generator is programmable, I can use whatever is most convenient.
The output pulse amplitude is going to be dependent on the gain of the APD. I am looking to achieve a gain of 100. So that will make the APD sensitivity (for the S2381) 50A/W. So for reflected light of 500 nW the output curernt would be somewhere around 25uA.
The APD has a capacitance of 1.5 pF.
The app note that you reference was quite useful. My main concern with choosing the right part is deciding on what slew rate and cut-off I need.
In reply to Joel Yello:
Sorry for the delay. I did not see that you had replied.
If you haven't already identified a part, I would suggest beginning the part selection process by considering the required gain and bandwidth for the application. For example, if you want the output of the transimpedance amplifier to be 1Vpp corresponding to the 25uA pulse, you will need a transimpedance gain of 40 kohm. The higher the gain, however, the lower the achievable transimpedance bandwidth as shown in Figure 6 of the app note for 10pF.
In terms of the required bandwidth, I would go with the following approach for estimating the bandwidth needed. Say you have pulse width of 100ns and the rise and falling edge each occupy 10%, or 10ns, of the pulse width. In other words, in the first 10ns, the current is rising, for the next 80ns, the current remains high, and for the last 10ns, the current falls back down to the low level. From small-signal rise time/bandwidth analysis, the required bandwidth based just on rise time is 0.34/rise time = 0.34/10ns = 34MHz.
You can also think of it this way. Let's assume instead of a single 100ns wide pulse, you have a pulse train (square wave) of period 200ns (100ns high, 100ns low...). This would translate to a fundamental frequency of 5MHz (1/200ns). Since we're dealing with a square wave and not a sinusoid, the bandwidth will need to be wide enough to pass several odd harmonics of the fundamental frequency. Let's say we want to pass up to the 7th (35MHz) harmonic to be able to get a square wave output. That means the bandwidth should be around 35MHz, which is close to the same bandwidth calculated based on rise time.
From this example, and referring back to Figure 6 in the app note, both the OPA657 and OPA847 should be able to meet a 40kohm transimpedance gain requirement. The figure is for 10pF source capacitance. However, when you add the input capacitance and parasitic board capacitance to your APD capacitance of 1.5 pF, the effective source capacitance will probably be closer to 5-6pF, so the calculations for 5-6pF should yield similar results. The OPA847 will be able to achieve wider bandwidth than the OPA657, but you will have to consider the higher offsets of the non-FET input OPA847. The OPA657 is a FET input which will have much better offset but lower bandwidth. This tradeoff is discussed in the app note as well.
I've attached a circuit for a transimpedance amplifier with an OPA657 and another but similar APD. The laser pulse should be short and have a fast rise time so you can get good range resolution and keep thr duty cycle of the laser low. If you know the shape of your laser pulse (a short laser pulse will be approximately Gaussian),
You can generate a piecewise linear approximation of that pulse in TINA and then use TINA to do a Fourier analysis of it to determine the minimum transimpedance amplifier bandwidth that you need to get reasonable pulse fidelity.
I hope that you find this helpful.
Regards, Neil P. Albaugh ex-Burr-Brown
In reply to Neil Albaugh:
Hi, Thanks for the circuit. I may use this design.
I have received opa847 and opa657, however I am having trouble prototyping with them. For testing purposes, I am attempting to operate the 847 in a non-inverting configuration. My output waveform is completely distorted and does not resemble the input (sine 1 Vpp 1khz) at all. It's a SOIC-8 package. I am using a simple solderable DIP adaptor for bread-boarding.
I've tried 3 different op-amps, all with the same result. Could this be a result of capacitive board parasitics?
The OPA847 is a rather sensitive device, it would be best to solder the device directly on a board and avoid bread-boarding at all cost.
In non inverting configuration, the minimum gain stable is 12V/V, with a flat frequency response for 20V/V. You best chance to make the OPA847 works would be to use one of the unpopulated EVM DEM-OPA-SO-1B or DEM-OPA-SOT-1B. This will limit all the parasitics near the device.
Also if you are using a large gain, the 1Vpp input may be too large, so the amplifier could be clipping. For ±6V supply, do not expect the amplifier output swing to be above 10Vpp. This voltage combined with the minimum gain for the OPA847 would indicate that this could be the problem.
What gain are you using in your evaluation?
If you try the circuit that I attached in my last post, I think it will work better than what you are attempting. A non-inverting amplifier following an APD will have poor SNR compared to a good transimpedance amplifier.
You still have not said what your pulse width is, so I can't be too much more helpful.
Xavier, Thanks for the help, I'm going to try to prototype without a breadboard. Also, I am attempting to operate with a gain of 10.
Neil, I'm only using the non-inverting configuration for testing purposes to verify that my op- amp is working as it should (no APD attached).
However, in regards to the TIA, I want to use a pulse width of about 10ns. I imagine the period will be much greater since the duty cycle should be low. Also is it common practice to use an active voltage limiting circuit after the TIA? Since this will be for a range finder, the APD will have a dynamic current that depends on distance. So the output voltage could get very large depending on distance.
A pulse with a Full Width Half Maximum (FWHM) of 10ns will require a bandwidth of about 50MHz to preserve its waveform reasonably accurately but in your application perhaps 20MHz would be sufficient. The pulse can be limited by a second-stage amplifier that has a very fast overload recovery time or you might be able to simply use a comparator with very low offset voltage to sense the presence of your return pulse.
The threshold of the comparator needs to be set as close to zero as possible to obtain high sensitivity but far enough above the baseline noise level so that you do not get false triggering. This fixed-threshold detection (FTD) method is simple but it has one distinct disadvantage-- it produces "timing walk". That is, the timing changes with the amplitude of the return pulse. You might look into what is called a Constant-Fraction Discriminator (CFD).
A CFD is useful where you don't have prior knowledge of what the return pulse is likely to be, for example a single- shot measurement. In a free-running system, you could use a fast peak detector for an AGC control and a fixed-threshold detector to eliminate timing walk.
I hope this helps.
The comparator seems like the way to go. I found the ALD2321 made by ADVANCED LINEAR DEVICES that has a Vos of 0.2mV. I'll be doing lab testing and will report what I find.
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