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Hi all,
I'm running at 20 MHz for an SPI SCLK, and the bus I'm using is a 1.8 V bus. I have two 1.8 V slaves, and 8 slaves to go on the bus that all operate at a minimum of 2.0 V for a high logic level signal. I'm using a level converter from Maxim to fix that.
The signals on my PCB are...
Hmmm... sorry, nothing should change by adding a voltage amplifier after the initial photodiode amp because the problem with that initial stage is input referred voltage noise. That noise would then just get amplified by the second stage like it would have gotten amplified by a larger feedback...
The randomly moving thing is directly proportional to the input referred voltage noise amplified by the gain formula above. That formula shows how the input voltage noise will be amplified by the ratio of the feedback resistance and the shunt resistance of the photodiode. The shunt resistance...
I would recommend you step back to the 1 M resistor, or even 500k, and use two stages, one transimpedance, one voltage gain. Add a second resistor to the transimpedance stage on the non inverting terminal of the same value as your feedback resistor. You can do this before changing your...
The AD797 does look like a better amplifier, but you got me with the "shoots off scale" comment. If the amplifier is pegging out at 5V (~5 uA of current) with a small temperature change, then I must be missing something. If we were relying on the noise gain transfer function of the amplifier...
FvM:
Yeah, I always looked at Johnson noise as an additive noise source, but I always use AC signals so I'm not sure any more about this producing too much effect on the output. I always pictured little current sources in parallel with the resistance, but I could be wrong. Typically it's in...
Thermal agitation of charge carriers in a circuit causes a small current to flow. Is not perpetual, it's Johnson noise and has been known for about 100 years. You could null the offset, but it will just change with temperature.
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I think, fundamentally, the problem with your low shunt resistance is current noise. If you look at the Johnson current noise equation, it increases as resistance goes down. This is going to get amplified by your TI amp and show up as noise/offset that changes with temperature. I read about...
I think silicon pdiodes are typically for visible, and InAs are typically for IR, so would also be interested to know what source you are using. Also curious if it is an AC or DC measurement you are trying to make. There's an app note from Hamamatsu that describes two effects, a charge build...
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