OPA847: Voltage Noise Peaking in Transimpedance Amplifier (TIA) - why does the addition of a feedback capacitor cause voltage noise peaking?

Hello Samir,

Thank you so much for your in-depth response, it is incredibly helpful.

The input capacitance being much larger then expected is not something I had considered. I am relatively new to circuit design so it is possible that there is extra capacitance at the input (although I have tried to follow many of the design techniques for TIA's suggested online). With this in mind I have changed the input capacitance in my model to try and fit the theoretical noise curves to experimental data:

Here is the noise curve for R_f = 10kohm and C_f = 2pF. I have increased the input capacitance to 1200pF as this produced the best fit with the experimental data. I have only plotted the voltage noise contribution to total noise as that is the dominating term at higher frequencies (although the total noise does consist of current, shot and Johnson noise components). As you said, due to the higher input capacitance the noise gain increases much earlier and we see a much better fit to experiment.

However, if I keep the input capacitance the same and consider the case of C_f = 0pF I observe the following:

Again due to the higher input capacitance the voltage noise starts increasing at a lower frequency, however this time it is not consistent with experimental values as our experiment suggests with no feedback capacitor the roll up in noise begins at a much higher frequency.

So it appears that by increasing the input capacitance significantly the theoretical noise better fits experiment only in the case where a feedback capacitor is present. In my understanding I have considered the feedback and input stages separately, however the data suggests that a feedback capacitor is causing drastic changes to the input stage. Is this something you are aware of or have come across before?

For reference please find attached the schematic and board layout of this circuit, if you have any questions about my design choices I would be happy to expand further.

Regards

Joshua

Hi Samir,

Just another quick (and I think rather interesting) update. I replaced the OPA847 with the OPA657 and observed the following:

Clearly now we see a much better agreement with theory (I believe the elevated white floor in the blue data is the floor of the spectrum analyser). My first thought is that the 847 is a bipolar input whilst the 657 is a FET input device. Would you know why a bipolar input device would cause such issues?

Thanks again

Josh

Hello Joshua,

Are you using a physical capacitor or are you using a photodiode when measuring the noise? Your layout looks good and I cannot see how there could be an extra 200pF of capacitance.

Does the Iq of the OPA847 change when going between no Cf and Cf = 2pF? I checked the phase margin of the amplifier and with Cf = 0 (or very low) your phase margin is under 20 degrees which suggests instability. I wonder if that is changing the characteristic of the amplifier. Can you compare Cf = 2pF and Cf = 4pF and see what happens.

When comparing BJT vs FET inputs the 1st thing to consider is the current noise. You can ignore current noise in a FET input amp. I checked the current noise of the OPA847...3.5pA/rtHz * 10kOhm = 35nV/rtHz. That is where you see your noise floor. Also, I do know that the current noise of the OPA847 starts to increase above 10 MHz. I wonder if the low phase margin in the case of Cf = 0 is making the amplifier act up and change something in its profile.

Do you have the ability to inject a signal into the non-inverting pin? If you did we would be able to check the actual noise gain of the amplifier.

-Samir

Hello Samir,

In this test I am using a physical capacitor, previous measurements I have used a photodiode whilst measuring noise and the behaviour observed with the OPA847 is consistent in both cases. It might be worth changing the feedback capacitor to a higher value to see what happens, my current choice of 2pF allows a reasonable phase margin whilst preserving the required bandwidth, however, for our understanding it might be worth sacrificing bandwidth by increasing Cf further.

Although the current noise is lower in the FET input device I do not believe we are dominated by current noise in the OPA847 at high frequency, it appears to me that when Cf is increased from 0pF to 2pF it is voltage noise which is increasing as the slopes match (both go as ~f).

I do not have the ability to inject a signal into the non-inverting pin without breaking tracks in the circuit. Is it possible to inject a low voltage test signal into the inverting pin or does this raise other problems?

Thank you for all your help

Josh

Hello Josh,

  Unfortunately it wouldn't do much good to inject into the inverting side. Injecting into the noninverting side would give us a noise gain measurement.

-Samir

Hi Samir,

I will look at the possibility of taking a noise gain measurement on my current circuit.

Another question I have regarding the OPA847, in figure 3 of the spec sheet they give an example of a TIA with the non-inverting terminal grounded through a 12kohm resistor to "reduce the contribution of input bias current errors (for total output offset voltage) to the offset current times the feedback resistor". In my current circuit the non-inverting terminal is routed straight to ground, do you think it is a possibility that input bias current errors are somehow causing an increase in noise at the MHz region? It could explain why I do not see this issue in the FET input op-amp as bias currents are negligible.

Josh

Hello Josh,

  The matching resistor on the noninverting terminal is only to match the offsets on either side and will not in itself affect the bias current. However, if the amplifier is unstable that could cause the Ib to vary. Can you please check the output offset voltage of the OPA847 with Cf = 2pF and open.

-Samir

Hi Samir,

Here is the time trace for Cf = 0pF and Cf - 2.2pF:

It appears that, as you were suggesting Ib is changing with a changing Cf. It should be noted that with Cf = 0pF the output offset is very stable (as it is for Cf = 2.2pF).

Thanks

Josh

Josh, Can you please try 4-8pF and see if there is further change?

Also do you by any chance have a loop antenna and a spectrum analyzer so you can make noninvasive measurements to see if there are any high frequency oscillations?

-Samir

Hi Samir,

I do have access to a high frequency (up to GHz) spectrum analyzer, although I do not have the data recorded I have seen quite bright spectral lines at ~750MHz. If necessary I can record this data and send it to you.

Thanks

Josh

Joshua, Do you see the spectral peaks for both feedback caps? Don't bother capturing the data. I am just trying to make sense of what may be happening here. I am a little behind on work right now so don't have the time to verify all this myself in the lab.

-Samir

Hi Samir,

I have recorded high frequency (up to 1GHz) data for the case where Cf = 0pF and have increased the feedback capacitance as you suggested to Cf = 5.6pF, this is shown below:

So by increasing the feedback capacitor it seems to dampen the 5MHz peaking but increase the white floor significantly. We also this time see the bright spectral lines between 100MHz and 1GHz. It appears that adding a feedback capacitor causes the lines to brighten and side-bands become more prominent.

Josh

Hi Josh, there is something wrong with the 5.6pF curve. The higher noise even at low frequencies (1kHz-100kHz) is not expected. Did you maintain the same feedback resistor or did you increase the resistor? If the resistor is the same size then I think the device is possibly oscillating which is increasing the amplifier noise. Was the power consumption with 5.6pF Cf the same as with 2.2 pF?