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10 years, 2 months ago.
PWM out noise
Hi,
I am using P21 for 3.3V pwmout, with period 100us, duty 50us. Since I am feeding this signal to opamp (opa211) that controls current source, I need to reduce voltage of mbed 3.3V signal to 0.100V (100mV).
To lower voltage I am using voltage divider with 32k and 1k as seen in picture. The voltage after 32k resisotr is already pretty noisy (check picture). How can I reduce that noise?
Thanks for any suggestion
3 Answers
10 years, 2 months ago.
Calculating an RC constant for 32k (your top resistor) and a 1nF cap across the 1k resistor, gives 73.6 us pr Tau. 0.7 Tau gives 50% charge of the cap, which will be: 0.7 * 73.6 us = 51.52 us.
This will be close enough for the application I think.
Lerche
10 years, 2 months ago.
OK, I used short gnd probe on osciloscope to measure this. I found out that if I move gnd of power supply closer to osciloscope probe gnd, the noise reduce significantly... Is there even a chance to work with so small voltages (100mV) on breadboard - and make quality measurements? Are there more quality breadboards that solve this noise problem?