OPA857: Output of OPA857 staying constant.

Hi Ankur,

I was able to look at your schematic and it looks like the device is configured correctly when setting the configuration pins. The negative output pin is expected to be around 1.85V and is designed to be fixed at that voltage. The concern is the positive output which ideally should change as the current signal changes. At that value of .45V, the output is saturated, which means there is some current being sourced from the photodiode. Is there a way to measure how much current the photodiode is sourcing? Could you maybe measure how much is being supplied. Just to simplify the design would it be possible to remove the THS4541 stage and short R121 so the device has a load close to the datasheet value. Removing this stage will ensure the photodiode is what is causing the device to saturate.

Best Regards,

Ignacio

We have disconnected THS4541 stage and shorted R121 with 0 Ohms resistance. Output rails remain same and we see no change in the output rails in any of the below scenario

1) Laser turned On and APD bias supply Turned ON

2) Laser turned OFF and APD bias supply Turned ON

3) Laser turned OFF and APD bias supply Turned OFF

Hi Ankur,

There is one test that would be worth running to see if it is the device itself. There is an option to run the device as an inverting amplifier which your schematic has the ability to do. Would it be possible to disconnect the APD and connect a 5kOhm at R268 and put a voltage of around 2.5V (left pad of R268) to see if the output does change? This will ultimately rule out the device does not have some kind of damage. 

Best Regards,

Ignacio

Hii,

I have tested as you have suggested. 

Currently in the board,

1) Op-AMp output is disconnected from THS stage.

2) Output P and N are not shorted

3) APD is disconnected. R268 is 5k and supplied 2.5V on left PAD. 

We see no change in the output rails. Even when output P and N were shorted , there was no change in the outputs.

Hi Ankur,

That should have definitely resulted in a change as the device is simply in an inverting gain circuit. Is there an iq value you can measure when powering on this device to see if it is higher or lower than the typical value this device should be pulling. If there is a way to measure iq in your system and its not in an expected range the device is likely damaged.

Best Regards,

Ignacio

hiii,

We have taken a new card and done the same test. There we see o/p change upon supplying 2.5V through series 4.99k resistor(R268). We have not disconnected THS4541 while running this test. Below are our observations

1) When R268 is not connected to 2.5V, Voltages at the outputs of op-amp and THS4541 are as follows

R117 = 1.84V

R118 = 1.84V

R109 = 0.36V

R106 = 1.78V

2) When R268 is connected to 2.5V, Voltages at the outputs of op-amp and THS4541 are as follows

R117 = 1.2V

R118 = 1.84V

R109 = 12 mV

R106 = 3.2V

 Are these readings okay to validate that op-amp and THS4541 are working fine? Pl suggest any change if required

Hi Ankur,

Yes, the values you are sharing now fall in line with what is expected out of the OPA857 in this specific configuration. It seems like the previous device was damaged. The output of the FDA is railing which is expected with the condition you have the TIA stage in. The goal on configuring the TIA stage as an inverting amplifier was to see if the device was damaged which you debugged that it was.

Best Regards,

Ignacio

Hii, Further to this discussion,

Once we had tested the the Op-Amp, We had installed the APD and threw some light On it but didn't observe any change in the outputs so we reverted back to op-amp test(R268=4.99k and left pad of R268= 2.5V) but now we are getting 1.84V on both rails of opamp irrespective of R268 connected with 2.5V or not. Does this mean that our APD circuit has damaged op-amp? 

Pl suggest changes in Op-Amp input circuit so that op-amp is not damaged? 

Hi Ankur,

If the device is behaving this way again, I would assume it is the APD causing the damage. I was able to ask about this issue and there is not a common circuit to somehow protect the device as all the current will be going through the TIA as it is intended to. Is there a way to maybe reduce the voltage biasing the APD to the lowest value and see if this will drop the input current being sent to the TIA? Could you see anything when running the circuit like any sort of transient before the output locked up to 1.84V?

Best Regards,

Ignacio

Hii

I have observed when there is no input to TIA, there is 1.8V present at the input of TIA. so with this test, I assume that we are injecting (2.5V-1.8V)/5Kohms = 0.14mA current into TIA. Is my calculation correct?

Hi Ankur,

If you are referring to the configuration with a 5kohm on R268 with 2.5V on the left pad, then yes, the current going through the device across the feedback pin is around 0.14mA like you're calculating. With the APD, did this latch up behavior at 1.8V change at all or does it seem like the device did get damaged? It would be worth adjusting the bias voltage of the APD to see if this can help limit the current it sends to the device if it is being damaged.

Best Regards,

Ignacio