This is for a transimpedance amplifier. I have a modulated signal (~1MHz) with 75% duty cycle. (i.e. 750nsec on, 250off).
We currently have designed OPA355 and it works well in terms of noise etc. except it is not fast enough. If I increase the gain above 200K, the rise time of the opamp violates my 250nsec rise time hard limit. (We have two stages and total of 1.6Meg gain target as we work with a 50nA signal. We make sure things are low noise and we try to gain as much as possible in the first stage to get better SNR)
So, I like to get another recommendation from TI and other masters in the forum.
My hard requirements are:
- Good gain (400-500K target) in the first stage.
- 250nsec rise time
- Single supply (3.3v preferred but I will take 5V as well)
You seem to be limited by the gain bandwidth product of the OPA355. A higher gain bandwidth product amplifier such as the THS4304 should resolve the gain limitation you are seeing.
One important parameter is missing-- the capacitance of your source (photodiode?).
Regards, Neil P. Albaugh ex-Burr-Brown
Single supply is good however I am not so sure this would be suitable as a transimpedance amplifier.
At our bias (i.e. 3.3V) it is about 25pf.
An OPA355 looks OK to me-- see attached TINA circuit. A single-supply transimpedance amplifier works OK-- and you can use the common-mode bias voltage to reverse bias your photodiode. As you can see in the output Total Noise plot, you should low-pass filter the output so that frequencies above those that contain signal information are attenuated since they only contribute noise.
Since the THS4304 is bipolar as opposed to CMOS input for the OPA355, the circuit will have less voltage noise but added current noise. My belief is that these will cancel one another making both solution comparable from a SNR standpoint.
To support 250ns rise time, you will need 1.4MHz bandiwdth. The OPA355 should allow you to support 650kohm transimpedance gain. To compensate the amplifier correctly, the feedback capacitance should be 0.25pF.
Targeting the same bandwidth with the THS4304, the maximum transimpedance gain that can be achieved is 2.6Mohm with a 60fF compensation capacitor. The issue here is that since this is a bipolar input, the output offset voltage will exceed the available voltage swing. This can be offsetted by using T-network in the feedback path. I attached a schematic to achieve 500k transimpedance gain with such a configuration.
Run a noise analysis on your THS4304 circuit and compare it to my OPA355 circuit's noise.
The issue is in the THS4304 current noise model that is 2.4 time the noise of the THS4304 datasheet. See attached extracted noise. The OPA355 is also off on the current noise (700fA/rtHz extracted instead of the nominal 50fA/rtHz) but since the gain and the bandwidth are lower, the total noise is also lower. Also since the current noise density is also much lower it will have less impact in the total noise calculation. 7870.OPA355 Noise Analysis.TSC
6320.THS4304 Noise Analysis.TSC
Comparing input referred current noise density, the THS4304 is simulating ~4.5pA/rtHz input referred noise but once recalculated with the actual device current noise the input referred current noise density should be 2.3pA/rtHz. I attached the OPA355 and the THS4304 simulations for noise density.
What counts is the total noise in the signal bandwidth refered to the signal voltage at the TZA output.
Run an SNR analysis in the Noise section of TINA. With 100nA input, the THS4304 circuit gives a signal voltage of about 50mV and the OPA355 gives 30mV. The total noise is very different and that results in about 12dB worse SNR for the THS4304 circuit.
Here is the THS4304:
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