OPA1S2384EVM for amplifying photodiode

Hi Jordan,

Please take a look at TIPD176 (1MHz, Single-Supply, Photodiode Amplifier Reference Design). This document should discuss most design consideration for a photodiode application.

I also recommend taking a look at our TI Precision Lab training videos that discuss amplifier design considerations.

Thank you,

Tim Claycomb

Hi Timothy,

I have looked through the documentation you suggested and found plenty of useful information, so thank-you for that. However, I am having difficulties getting things to work with the high frequencies I will be using.

The photodiode signal has a beat frequency of 40MHz. I am not sure of the exact signal strength yet, but I imagine I will be generating pA level current. As a starting point, I would like to simulate 0-200nA current into the OPA1S2384.

I have used the TINA simulation from this forum as a starting point, and I have attached the amended file. I do not necessarily need the track and hold functionality so I have changed the sinusoidal switching supply to be a constant 5V. I've included a 50mV bias at the non-inverting input to prevent the amplifier output from saturating at the negative power supply rail. My main problem is the second amplification stage - I need to further amplify my signal as it is only approx 1mV after the first stage. I have no strict requirements as to what the voltage swing is, I just need a measurable signal >1mV which is linear in the range 0-200nA.

Could anyone offer any more advice/help please?

Thanks,

Jordan

6866.PhotodiodeTIA.TSC

Hi Jordan,

You will need to add at least a 100mV bias to the non-inverting input to keep the output of the first stage in a linear operating region (the output swing using a 5V single supply is 100mV to 4.9V) if you are expecting an input current of 0A as a minimum. If you want to simulate how the circuit will act with a 0-200nA input current you can perform a DC transfer characteristic in TINA-TI, this allows you to sweep an input signal. Go to Analysis => DC Analysis => DC Transfer Characteristic and input the values you want to sweep and what input signal you want to sweep.

If you need a second gain stage you can place the second amplifier in the OPA1S2384 in a non-inverting configuration to amplify the signal. Or use another amplifier on the output of the OPA1S2384 in a non-inverting configuration.

Thank you,

Tim Claycomb

Hi Tim,

Thanks again for your quick reply. I was actually already doing the DC Transfer characteristic and could get things to work for the first stage, just not the second. I have placed the second amp in the non-inverting configuration, but my calculations don't seem to tally up with the simulations.

The first stage has given me a DC transfer of 251-251.5mV for 0-200nA. This is the largest gain I can achieve at 40MHz.

For the next amplification stage, I tried to get the output to swing from 0.26-0.25mV as a test. This gives a gain of 20 (4.75-0.251/0.2515-0.251), which corresponds to an offset of -99.74 (Vout=Vin*gain + offset). This gives a value of 50ohms for the offset resistor for a 1kohm feedback resistor value (offset = -Vos*(Rf/Ros)). This also gives a value of 1mOhm for the resistor labelled Rg in my simulation. When I run the DC Transfer on this, I get a uV sweep (shifted + attenuation!) around 131.07 mV at the output. Can you see what I'm doing wrong?

Thanks,

Jordan

1258.PhotodiodeTIA.TSC

Hi Jordan,

It might be easiest to follow the design procedure shown in TIPD176 using an amplifier of your choice. I recommend following the guidelines in section 3.1 of the document to choose an amplifier.

Almost all of our amplifiers can be placed on an evaluation board like the Universal Operation Amplifier Evaluation Module or in a proto board using DIP adapter boards.

Thank you,

Tim Claycomb

Hi Tim,

Thanks for your reply. I've managed to get things working with the OPA1S238xEVM board when using a signal generator at the input. For this I used a 10MHz signal gen with 4mV pk-pk sine wave and I get the corresponding outputs according to my TIA model (Rf=5k and Cf=1p for each amplifier).

However, when I connect my photodiode (Thorlabs DET025A) I do not get any output. For this test, I am using a HeNe laser with a chopper at 500Hz to produce the periodic signal. If I measure the output directly (without amplifier) I can clearly see the signal on a scope. Would you have any idea what could be happening? I've removed resistors R12, R10 and put a 0ohm in R6. I've also removed R13 and R5 and grounded the non-inverting input. Any suggestions would be helpful.

Thanks,

Jordan

Photodiode TIA.TSC

Hi Jordan,

What is the expected output current of your photodiode during the test? If it is at 200nA like you mentioned in your previous post, the output of the amplifier will only be approximately 1mV which you may not be able to read on an oscilloscope. Using a 10x scope probe the typical volts/division you can get is 10mV/div on the scope. So having a 1mV output will only be a maximum of a 1/10th of a division on the oscilloscope screen.

For testing purposes only, can you increase resistor R1, to say 50k ohms, to increase the gain of the circuit so that the output of the amplifier is approximately 10mV with a 200nA input? This will help determine if the circuit is functioning properly.

Thank you,

Tim Claycomb

Hi Tim,

For this test i have about 5uA of current coming from the photodiode which is strange how I don't see any signal.

Could it be something to do with the way I have modelled the photodiode? Maybe it's capacitance is not actually 1.7pF? Or maybe the input impedance is too high? I calculated this value according to the dark current produced at 5V bias (from the manual). Although I am operating it in with 12V reverse bias from a battery included in the enclosure.

Thanks,

Jordan

Hi Jordan,

Although I am not at all familiar with the I took a quick look at the datasheet and according the Specifications in Chapter 6 the max diode capacitance is 1.73pF and the bias voltage (Vr) is 12V.

Is it possible that the Thorlabs DET025A is outputting a current in the opposite direction than what you are expecting? This would cause the amplifier to rail to the negative supply and appear that there is no signal on the output of the amplifier.

Please note that technical responses will be delayed until after January 3, 2017 due to US Holidays.

Thank you,

Tim Claycomb

Hi Jordan,

Yes, you are correct, using a dual supply you should be able to see the output signal go negative.

In section 4.7 of the DET025A datasheet it discusses placing a load resistance on the output of the DET025A to view the voltage on an oscilloscope. If you remove the Thorlabs DET025A from the circuit and place a 100kohm resistor load resistor on the output, what is the output voltage?

Please note that technical responses will be delayed until after January 3, 2017 due to US Holidays.

Thank you,

Tim Claycomb

Hi Jordan,

One thing I forgot to mention in my previous post is that the OPA1S2384 has a maximum supply voltage of +5.5V (or +/-2.75V). If you the device has been supplied with +/-5V it is likely damaged because the absolute maximum supply voltage of 6V (+/-3V) was exceeded.

Thank you,

Tim Claycomb

Hi Tim,

I am not in the lab at the moment but I will try this in the morning. I assume you're wondering if that will produce a voltage drop of 500mV across the resistor? Should the current not still flow through the feedback resistor when connected directly without a resistor across the output?

In the 'photodiode tutorial' section of the DET025A webpage it gives the circuit diagram of what is in the enclosure. It contains an RC circuit to remove any high frequency noise that may be present when using a power supply for the bias instead of a battery. There is a 1kohm resistor and a 1uF capacitor in this. Would that be causing issues?

Thanks,
Jordan

Hello Jordan,

Was the OPA1S2384 supplied with +/-5V? If so, the device is likely damaged because the absolute maximum supply voltage is 6V.

Thank you,

Tim Claycomb

Hi Tim,

Apologies for the confusion in my previous post, I didn't supply the board with +/-5V, rather +/-2.5V as stated in the manual. I have since managed to get things working - I increased R1 to a much higher resistance and was able to see the expected signal. I have 2 further questions:

1) As mentioned in your previous post, I want further stages to increase the gain bandwidth (as I cannot get the required gain at 40MHz with a single stage). Based on calculations and simulations (attached), I need 6 stages to get ~115dB of gain at 40MHz. I have attached a diagram of the modifications I have made to the board, but I seem to get the same signal at OUTA and OUTB even though I also have the same resistance across amplifier B. Although pins 2 and 6 are connected the opposite way around to bypass the switch in the simulations, I am not sure why I see the same signal after each amplifier? The manual suggests the CMOS switch closes when a logic high signal is applied to J4, but when I replaced R9 with a 0ohm resistor my signal disappeared. Do I need to remove the C12 capacitor and R3 resistor for this to work since I don't want the 'sample-and-hold functionality?

2) I have purchased 2 more evaluation boards and have replaced the necessary components. I have attached their diagrams (Boards 2 & 3) also. Do these look correct? I will be attaching the output of board 1 (J3) to the non-inverting input of board 2 (J2), and then the output of board 2 (J3) to the non-inverting input of board 3 (J2).

Many thanks,

Jordan

Board 1.pdf

Boards 2 & 3.pdf

Photodiode TIA MultiStage.TSC

Hi Jordan,

1. You are likely getting the same signal at OUTA and OUTB because amplifier B is placed in a gain of 1. To get a larger output voltage at OUTB amplifier B will need to put in to a non-inverting configuration.

2. To create a multistage gain circuit I recommend purchasing some DIP adapter boards and ordering some samples of op amps to put on the DIP adapter boards. You can then use the DIP adapter boards to add multiple amplifiers in a non-inverting configuration to the output of the OPA1S2384EVM.

If multiple OPA1S2384EVMs are used to create a multistage gain both amplifiers in the OPA1S2384 should be placed in a non-inverting configuration to amplify the signal. Remember to keep in mind the input common mode and output swing limitations of the amplifiers.

Thank you,

Tim Claycomb

Tim,

I have set up the circuit in the same way as the simulation attached in my previous post which shows that the second amplifier is set up in the non-inverting configuration, with a feedback resistor of 7.5k. You can also see this in the circuit diagrams I included. Do you agree with the setup?

I see the problem now. I am testing the circuit with only 500Hz since the piece of equipment which will give me the 40MHz signal has not yet arrived. From the AC transfer plot of the simulation in my previous post I see the gain at each amplification step is the same at frequencies below 1MHz. Above this, I get a higher gain after each step.

Is there a way to remove the phase shift after all the amplification stages? The processing of my signal after amplification will be phase sensitive so this may cause problems. From the simualtions, I am getting approx -300deg of phase shift at the output which is steeply decreasing.

Thanks,

Jordan