FAQ: Operation of AD8375 and AD8376 Digitally Controlled VGAs at Low Frequency

Document created by enash Employee on May 18, 2010Last modified by AndyR on Jan 31, 2012
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The frequency response plots in the AD8376 and AD8376 DGA data sheets show roll-off at low frequency. Is there any way around this?





The AD8375 uses an open-collector output structure that requires dc bias through an external bias network. Typically, choke inductors are used to provide bias to the open-collector outputs. Choke inductors work well at signal frequencies where the impedance of the choke is substantially larger than the target ac load impedance. At lower frequencies, the impedance of these inductors decreases and causes the gain to decrease.



In broadband applications, it may not be possible to find large enough choke inductors that offer enough reactance at the lowest frequency of interest while offering a high enough self resonant frequency (SRF) to support the maximum bandwidth available from the device. The circuit below can be used when frequency response below 10 MHz is desired. This circuit replaces the bias chokes with bias resistors. The bias resistor has the disadvantage of a greater IR drop, and requires a supply rail that is several volts above the local 5 V supply used to power the device. Additionally, it is necessary to account for the ac loading effect of the bias resistors when designing the output interface. Whereas the gain of the AD8375 is load dependent, RL, in parallel with R1 + R2, should equal the optimum 150 Ω target load impedance to provide the expected ac performance depicted in the data sheet. Additionally, to ensure good output balance and even-order distortion performance, it is essential that R1 = R2.


This circuit is discussed in more detail on page 14 of the Rev 0 data sheet.



AD8375-6 Low Frequency Operation.JPG





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