What is the minimum frequency input supported by AD8330?

Question

Hi, 
 We are using the AD8330 in one the board, when i am checking the Gain Amplification with different input frequencies, i found that lower frequencies Output is not coming. 
 Here when i apply the input frequency around 300KHz, output is amplified. 
 When i apply the 20KHz input, Some times Amplified Output, but, most of the time, No Output ta all. 
 WHen probe the Device Input pins, proper input is applied, and VDBS voltage is constant and no change from when High Frequency test done. 
 Here only, Input frequency is reduced. 
 Is ther any problem?. Has device is verfied from DC to 150MHz, as mentioned?. 
 What is the minimum frequency can apply for working?. 
 I had attched the AD8830 schematics. 
 Thanks 
 Reddy

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prb · Answer

Hi Reddy, 
 It may be that the 100nF capacitors are limiting the low frequency performance of you signal chain. Have you factored C29 and C26 into you calculations? The AD8330 does work at arbitrarily low frequencies, but such performance is usually limited by the size of capacitors one is willing to use at the input.

prb · Answer

Hi Reddy, 
 I believe your problem is not from the input capcitors, but from the AD8330 offset null circuit. At very low signal frequncies, the offset compensation circuit confuses your signal with and offset and tries to null it out. Here is an excerpt from the AD8330 datasheet. 
 https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/324/7d55364764713b10ebaf77a1a6b0cd76.html https://ez.analog.com/cfs-file/__key/communityserver-discussions-components-files/324/1586.7d55364764713b10ebaf77a1a6b0cd76.html 
 Based on this description, there will be a HPF from the offset null of about 200 kHz in your circuit. 
 There is a way around this on page 21 of the datasheet: 
 http://www.analog.com/static/imported-files/data_sheets/AD8330.pdf 
 Which outlines how to either disable or lower the corner of the offset null and still eliminate offset. 
 Regards, 
 Paul