FAQ: Calculating the Resistor Values to Alter the Slope of a RF Log Amp or RMS Detector

Document created by Jim Employee on Feb 16, 2010Last modified by AndyR on Jan 31, 2012
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Q:

I want to increase the slope of the AD8319 log amp. Is there a formula to cacluate the  resistor values needed?

 

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A.

The required resistor values can be calculated from the equation: 

 

R2 = ((SLOPEnew/SLOPEnom) - 1).( Rext||Rin)

 

where

 

SLOPEnew is the desired slope

 

SLOPEnom is the nominal slope of the device when Vout is directly to Vset

 

Rext is the resistor connecting Vset to ground

 

R2 is the resistor that connects Vout to Vset

 

The input impedance of VSET on the AD8319 is 40k ohms. Slope setting resistors should be kept below approximately 10k ohms to prevent this input impedance from significantly affecting the resulting slope.

 

The table below shows resistors required and the resulting voltage swing for different slopes. It is also important to set these resistor values so that Vout is not required to source significant current. In the examples below the load current is < 1 mA.

 

This equation applies to any log or rms responding RF detector that has a scaleable output.

 

 

 

Input Range (dBm)Desired SlopeActual SlopeR2 K ohmsRext K ohmsVoltage Swing
-60 - +5-22 mV/dB-21.968 mV/dB0-1.52 V - 0.37 V
-60 - +5-44 mV/dB-46.445 mV/dB4.754.753.22 V - 0.777 V
-60 - +5-66 mV/dB-68.199 mV/dB4.752.374.72 V - 1.14 V

 

 

Slope detail.JPG

8319_slope_graph.jpg

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