Eliminating the Effect of First Stage Offset

Question

Hello, 
 I am planning to use the AD641 in a cascaded configuration. I have a question regarding the section &ldquo;Eliminating the Effect of First Stage Offset&rdquo; on page 13 of the datasheet. 
 In this section, it is mentioned that using a low-resistance choke for AC coupling requires the inductor to present high impedance within the operating frequency range. I would like to ask: what constitutes &ldquo;high impedance&rdquo; quantitatively in this context? Is there a known rule of thumb or empirical guideline to estimate this value? 
 For reference, my signal frequency is expected to be in the range of 1 MHz to 200MHz. 
 Thank you.

enash · Accepted Answer

I would say that you want the impedance of the inductor to be, say, 1kohm at your minimum frequency, 1 MHz in your case. So using ZL = 2 x Pi x Freq x L, we would get an inductor size of 159uH.

I should note that there are newer superior devices to AD641. If you are using the limiter output and need operation down to 1 MHz, then AD641 is a good choice. But if you only need the RSSI log output, a device such as AD8310 would be a better choice that is easier to operate.

SLA Status	Assignee	Support Status
SLA Closed. Met after 18 hours		Open

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Eliminating the Effect of First Stage Offset

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