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AD8338 output noise

Question asked by wang_jiayou on Jun 5, 2016
Latest reply on Jun 20, 2016 by JinoL

Hi All

     I some questions about AD8338. I use the following circuit to design AD8338 AGC function, single-end input and single-end output, the input signal is 58Khz, but I have some problems.

1. Is the circuit has any error?

2. During the test, I found that when  Vgain > 800mv, the AD8338 output is all noise, 

     when Vgain < 800mv, the AD8338 output is normal.

    I think this is caused by noise, so I added a capacitor across nodes(OUTP and FBKP).

 

3. From datasheet page 13, Adding feedback capacitors, CFBK, across nodes (OUTP, FBKP and OUTM, FBKM) reduces bandwidth of the output amplifiers of the AD8338 and the signal path of the VGA. Reducing the bandwidth of the AD8338 minimizes output noise and simplifies the design of the antialiasing filter when using the VGA to drive an ADC.
I connected a 220pF capacitor across nodes(OUTP and FBKP) to reduce noise, but VAGC changed to 2.3V, AD8338 cannot work, why?

4. How to reduce noise?

Can some specialists give me some advice?Thank you!

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