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Interactive Design Tools: Logarithmic Amplifiers : Raising and Lowering the AD8304 Intercept and Slope
A tool for calculating component values for changing the slope and intercept of the AD8304 logarithmic converter.
Instructions | Troubleshooting | Related Information

Instructions

  • This tool calculates components values for raising or lowering the AD8304 intercept and slope for single-positive-supply operation. It operates in two principle modes, either for raising or for lowering the intercept, and this mode is selected via the pulldown menu located above the schematic at upper right. The schematic shows the required mode-dependent topology.

  • Step 1: Begin by choosing the topology (Raise or Lower) and then entering the desired Slope, Intercept and impedance scaling resistor (RB) in the fields provided. Confirm the new values by using the "Enter" key after each field's data has been typed. Fields that have had data changed, but have not been accepted using the "Enter" key will have a black border around them.
  • Step 2: Adjust RB to achieve an approximately 5 KOhm input impedance to match the impedance of RLOG (i.e., "RA, RB, RC/Z in parallel").

  • It is possible to edit the RA, etc. resistor values to back-calculate the effect on slope and intercept, however, due to the logarithmic nature of the AD8304 there is some loss of precision and the component values will tend to "drift" from what is entered.
  • The input currrent, output voltage relationship is plotted in the graph at upper left.  MAX VOUT is the maximum output voltage possible for the selected gain = VLOG-MAX * Gain, where VLOG-MAX is 1.6V. MIN VOUT is the corresponding minimum gain set by VLOG-MIN, which is 0.1V. Additional limits may be imposed by the buffer opamp, which has a maximum of VP-0.1
  • It is possible to zoom in on an area of interest by click-dragging around the region. Once the chart is zoomed in, it is possible to zoom in further, or it is possible to clicke the "Zoom out" icon in the lower right of the chart to restore the original scale.


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Troubleshooting

For further troubleshooting information, please visit our Interactive Tools troubleshooting page.


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Related Information

AD8304 Product Page

Logarithmic Amplifier Selection Table


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