Variable gain amplifiers question..
@homebrewed :
Hi Mark
On your signal conditioning board, you had a choice of three gains, 1X, 10X, 100X, done by tapping into the outputs between opamp stages, though I am not exactly sure how the P2 device does it, and the output is clamped not to exceed 3.3V+0.65, nor go below -0.65V.
Do we know how much gain we need, or that we still want to switch gains in the amplifier chain?
If the dynamic range of the ADC is enough (about 105dB), and around 2V is expected for the biggest energy we expect, then scaling the X-axis can be done in software. This can be anything from setting the X-axis range, or set to portions of interest, to having log scaling, octave or decade. I see (in videos) this is readily done in MCA displays of XRF videos on data already collected.
I can imagine one might want to "zoom in", perhaps collecting counts from a range of lower energy returns, while ignoring pulses from higher peaks known to be there, just letting them clip on the rail, and not get plotted anyway.
In my design, there is the first stage TIA, followed by op-amp stages for post-TIA gain, one of which is a place for an optional 50Hz/60Hz interference filter stage. There is also the final ADC driver which has gain=1. This is where I decide whether there is a software switched variable gain addition.
It's the interface?
In my circuit, the power for the analogue signals section and ADC is isolated entirely, using a cheap low power high frequency switcher with transformer (MAX253), followed by a 2.5V ultra low noise LDO regulator (ADM7160AUJZ2.5R7). There is some passive filtering and noise control as well.
To compete with a photomultiplier tube method, gain of the entire arrangement is enormous, quite hard to keep stable, and tricky over noise, bias, and offsets. Any arrangement to switch gains, or select tapped signal routes, in my case, would need opto-isolated digital control, because the alternative would bypass the attempt to prevent connection of the computer digital 0V to the clean 0VA.
Hence the question. Are we intending having the ability to alter at least some of the gain range by hardware?
Perhaps yes, while prototyping, until we know how much gain we need?
I had thought, given you have a whole signal conditioning board, and some experience with it, you might have had a first-order guess.
It's OK if we just don't know yet. I was just poking around looking at opto-isolated stuff commonly used for things like RS485 and whether or not a ready-made software switchable variable gain amplifier might be good.
