A
Anonymous
Guest
I have been worrying about amplifier noise because I am thinking about using an AD8031 which has a higher noise figure than a 5534. The 8031 is 15 nV per square root Hz while the 5534 is 4 nV. There are two ways the 8031 could give more noise. One is the input voltage noise which is larger by a factor of 15/4 = 3.75. The other is that the 8031 has a higher bandwidth. The gain bandwidth product is 80 MHz vs 10 MHz for the 5534. If the amplifier is running wide open without any feedback capacitor the bandwidth for the 8031 is 8 times as wide as the 5534. So the noise should increase by a factor of the square root of 8. All together the 8031 noise should be higher by 3.75 times the square root of 8 = 10.6.
To test this I removed the coil to minimize the external noise and looked at the output of the integrator on the analog PI board. When I switch amplifier chips I only see an increase of about 2.5 for the 8031. I can think of two reasons that this might be less than the 10.6 mentioned above.
One is that the amplifier is not the only noise source. There is also thermal noise from the amplifier input resistor. And even with the coil removed there might be some external noise getting into the circuit, and there may be other internal sources of noise. These parts of the noise do not change when switching chips so the percentage change could be less than I calculated.
Also the amplifier bandwidth might not really be 8 times as wide. Parasitic capacitance may be limiting the bandwidth for the 8031 to something less than 8 times the 5534.
But even 2.5 times as much noise is more than I would like. So I am still thinking about amplifiers. An amplifier that runs off a single 5V supply will be easier to interface to the microprocessor, and I would like something that settles a little bit faster than the 5534. But if it runs off a single supply the input voltage must be able to go a little below the negative supply, about one diode drop below.
Robert
To test this I removed the coil to minimize the external noise and looked at the output of the integrator on the analog PI board. When I switch amplifier chips I only see an increase of about 2.5 for the 8031. I can think of two reasons that this might be less than the 10.6 mentioned above.
One is that the amplifier is not the only noise source. There is also thermal noise from the amplifier input resistor. And even with the coil removed there might be some external noise getting into the circuit, and there may be other internal sources of noise. These parts of the noise do not change when switching chips so the percentage change could be less than I calculated.
Also the amplifier bandwidth might not really be 8 times as wide. Parasitic capacitance may be limiting the bandwidth for the 8031 to something less than 8 times the 5534.
But even 2.5 times as much noise is more than I would like. So I am still thinking about amplifiers. An amplifier that runs off a single 5V supply will be easier to interface to the microprocessor, and I would like something that settles a little bit faster than the 5534. But if it runs off a single supply the input voltage must be able to go a little below the negative supply, about one diode drop below.
Robert