Hello guys!
I always feel a beginner when it comes to analog things, so I need help with this sensitive topic.
So, I need to design an LNA for a piezoelectric hydrophone to later digitize the signal with a single-ended ADC (the ADC of my STM32 for now). Hydrophone works both in TX and RX modes (it's for a sonar).
Antenna properties: operating at ~240 kHz, nothing else is known about it now.
Specs for the amplifier are: input voltage is expected to be in the order of 10's of uVolts, so I need ~10^5 voltage gain. Double-pole bandpass frequency response centered at 240 kHz with Q~10 is probably OK as it will be later processed with DSP. Also I want to digitally(from an MCU) control its gain in, at least, 20 dB range or so.
Also this thing is going to be installed not far from noisy things like a lot of DC-DC converters for powerful LEDs.
I intend to make it with high frequency opamps. For the gain-control part I will probably use some digital potentiometer between gain stages(or is there a better choise?). Also i can design the active filter part myself.
When it comes to the first stage of the amplifier, I am not sure what to do. I am considering two options:
1. I am not experienced with noise-sensitive things, but I guess that the common mode rejection ratio mentioned everywhere is an important part in this device to suppress common noise at inputs. I could make the first stage with a transformer(to serve as unbalanced->balanced converter) followed by a low noise high speed opamp, like one of these:
AD8646,
AD8651,
AD8655 or
OP37. But I am not sure about CMRR of a transformer, especially if it will be hand-wound on some ferrite ring. How high can it be?
2. Another option is to eliminate the transformer and take a nice instrumentation amplifier like
INA128 or
INA2128. One more cool instr. amp. I found is
AD8231, more noisy but hey, it has digitally-programmable gain.
One more piece of advice I have seen in the Internet is to go fully differential from input to ADC (Once I had a look at a professional Imagenex sonar board, and they did exactly that with fully-differential opamps
). But I am not sure how much sense it makes in my case if I am using a single-ended ADC.
Also one more general thing I would like to clarify about opamp circuit design is this: if I only care about high frequency part of spectrum, should I care much about opamp specs like offset voltage and input current? As I understand it makes huge difference only if I were to amplify DC signals.
Thanks to anyone who has made it through this wall of text!