Hi
I am debugging on a microphone-preamplifier and discovered some 50Hz hum when the microphone is connected. When the microphone is disconnected the RMS value is halved. When I short-circuit the inputs the noise disappear and I am down to less than 1mV at the output which correspond to the noise reported in the datasheet by the opamp manufacture and noise in resistors etc.
The system is powered by batteries (3.3V).
the gain in the active filters are 270 times and have a 3dB cutoff FREQ at 1250Hz.
The amplifier is build on a 4layer print, area 1.8 square cm. GND in hidden layer, Vdd in outer layers.
The Noise are measured to be around 5.5mV RMS at the output from the amplifier or (5.5/270) = 20μV RMS at the input, when shielded by alu-paper.
When un-shielded the noise are measured to be around 8.5mV RMS at the output from the amplifier or (8.5/270) = 31μV RMS if I move closer to my desk with Monitor/PC etc the RMS value Double 16mV RMS.
When the Mic are removed and the OpenLoop RMS noise is 3mV at the output or 11μ at the input.
The attached frequency spectrums covers 1000Hz. The Wall-supply in Denmark is 50Hz.
Is it normal for a Electret Condensor Microphone to capture surrounding noise from fluorescent lamps/monitors etc. and how much should I expect ?
Are there some optimization to the circuit, I have tried to change the Capasitor C7 on the schematic to 12.2μF but it did not seem to enhance the performance. ?
Thanks.
I am debugging on a microphone-preamplifier and discovered some 50Hz hum when the microphone is connected. When the microphone is disconnected the RMS value is halved. When I short-circuit the inputs the noise disappear and I am down to less than 1mV at the output which correspond to the noise reported in the datasheet by the opamp manufacture and noise in resistors etc.
The system is powered by batteries (3.3V).
the gain in the active filters are 270 times and have a 3dB cutoff FREQ at 1250Hz.
The amplifier is build on a 4layer print, area 1.8 square cm. GND in hidden layer, Vdd in outer layers.
The Noise are measured to be around 5.5mV RMS at the output from the amplifier or (5.5/270) = 20μV RMS at the input, when shielded by alu-paper.
When un-shielded the noise are measured to be around 8.5mV RMS at the output from the amplifier or (8.5/270) = 31μV RMS if I move closer to my desk with Monitor/PC etc the RMS value Double 16mV RMS.
When the Mic are removed and the OpenLoop RMS noise is 3mV at the output or 11μ at the input.
The attached frequency spectrums covers 1000Hz. The Wall-supply in Denmark is 50Hz.
Is it normal for a Electret Condensor Microphone to capture surrounding noise from fluorescent lamps/monitors etc. and how much should I expect ?
Are there some optimization to the circuit, I have tried to change the Capasitor C7 on the schematic to 12.2μF but it did not seem to enhance the performance. ?
Thanks.
Attachments
-
10.7 KB Views: 163
-
12.3 KB Views: 87
-
7 KB Views: 85
-
12.9 KB Views: 84
-
11 KB Views: 66
-
12.2 KB Views: 73
Last edited: