Suggestions for simple voltage amplifier?

[StarFishPrime]

Hi
I'm looking to build a simple amplifier to boost the mV signal from an infra-sound sensor (effectively a 20 inch microphone consisting of a stretched mylar sheet with a rare-earth magnet in the middle with a pick-up coil positioned a couple of mm away). This to be fed to a 0-5V adc connect to a Raspberry Pi. I have a simple 1 transistor audio amplifier circuit but can anyone suggest what may be a more suitable design of amplifier.

Thanks

Ian

[rob77]

for infra-sound (low frequency) - any opamp will do the cheap ass jelly-bean LM358 would be my first choice

[macboy]

A simple op-amp will do. You will need a lot of gain, but with such low frequencies, you won't need any fancy op-amp [gain is traded for bandwidth in op-amps, look up gain-bandwidth-product or GBP]. You will need to think about whether you want AC coupling or DC coupling. With AC, you need to consider the low frequency roll-off, especially given your signals are low frequency themselves. With DC, you need to worry about amplifying any DC offset in addition to the signal, which can overdrive your op-amp and/or ADC.

Make sure that you implement an anti-aliasing filter so that frequencies above Nyquist (1/2 sampling frequency) don't cause you grief.

[katzohki]

100MHz GBWP should do.

[rob77]

100MHz GBWP should do.

100MHz ? for what ? the OP is willing to connect it to arduino's ADC.... 1MHz is more than enough.. (the LM358 i suggested)

a little thinking about it:
infrasound means less than 16Hz (tones so low humans can't hear them) even if you choose a gain of 1000 (mV to V amplification) you are still in the tens of kilohertz gain bandwidth product ballpark. - seriously, any opamp will do (o.k. except the ultra high precision but ultra low speed opamps)

so  just go ahead and spend 15 cents on a LM358 you get 2 opamps in a 8 pin package with a 1MHz GBP.

[jlmoon]

What source of Infra sound are you hoping to explore?

[katzohki]

100MHz GBWP should do.

100MHz ? for what ? the OP is willing to connect it to arduino's ADC.... 1MHz is more than enough.. (the LM358 i suggested)

a little thinking about it:
infrasound means less than 16Hz (tones so low humans can't hear them) even if you choose a gain of 1000 (mV to V amplification) you are still in the tens of kilohertz gain bandwidth product ballpark. - seriously, any opamp will do (o.k. except the ultra high precision but ultra low speed opamps)

so  just go ahead and spend 15 cents on a LM358 you get 2 opamps in a 8 pin package with a 1MHz GBP.

I missed the infrasound part I guess. My comment was a bit offhand. Well any opamp should do then.

[StarFishPrime]

Thanks for your advice. I'll brush up on my op-amp theory.

Cheers

Ian

[Zero999]

What's the power supply voltage?

If you have another, higher voltage power supply (8V or more) then the LM358 will do but it's no good if all you have is 5V, in which case you need a rail-to-rail op-amp: the MCP602 will do.

In order to interface with the ADC, the op-amp's output will need to be biased to 2.5V. If the input is DC coupled and it's not convenient to float the microphone at 2.5V, you'll need a negative power supply, otherwise a single supply is fine.

[babysitter]

I guess starfish prime (You are still around? I thought your miliseconds of fame are over) is just sad that he was not as audible as his brothers and sisters which detonated at lower altitudes. Get on with it!