Hi,
I used OPA1622 for LPF, follow PGA4311,a part of schematic as below(4pcs OPA1622 role as OR+/-, OL+/-). The problem is: If OPA1622 is Enabled, when a cellphone approaching, there is beep in the earphone; If OPA1622 is Shutdown or taked off from the circuit, It is ok .would you help please? Thanks.
Hello Gump,
The issue you are seeing is likely caused from a high frequency signal (signal from the cell phone) being fed from the output of the OPA1622 through the feedback capacitor (C297) and into the input of the amplifier. This makes the signal appear as an EMI signal at the input of the OPA1622.
To reduce or eliminate the signal being fed from to the input you can...
1. Remove the feedback capacitor. This will remove the high frequency path from the output to the input. Since you want a low pass filter, this likely is not an option.
2. Add a resistor in series with the feedback capacitor. See below.
3. Add an RC low pass filter with a cut off frequency much less than the signal being transmitted from the cell phone (100MHz should work) to the input pins of the OPA1622. See below. This will attenuate high frequency signals at the inputs of the OPA1622. If this option is used, it is important to verify the OPA1622 is stable with the RC filter on the input pins.
I have attached a TINA-TI simulation file for you to simulate the effects of each solution I have proposed. To simulate the effects run a Transient analysis and monitor the amplitude of the signal between the inputs of the amplifier.
Please let me know if you have any questions.OPA1622 Cellphone EMI .TSC
-Tim Claycomb
