Q.
I want to use just the AD608's log/limiting amplifier and want to disable the mixer and amplifier. How should I tie off the mixer and amplifier pins?
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A.
1. AC-couple RFHI and RFLO to ground using two 100 pF capacitors.
2. Connect MXOP to VMID with the nominal 330 ohm resistor (R1) value that is recommended in the datasheet.
3. C5, C1 and R4 should be installed as per Table II on page 8 of the datasheet (recommended values vary with input frequency)
4. Connect VMID to C5 as shown below and add the 100 nF decopling capacitor as shown.
5. The input signal should be ac-coupled into pin 5 (IFHI) using a capacitor.
6. Set the input impedance using resistor R3. Since the impedance from INHI to INLO is high (10K), a 50 ohm resistor can be used for R3 to provide a broadband 50 ohm input impedance.
7. The input coupling capacitor mentioned above and R3 create an input high-pass filter. Make sure that (1/2*pi*R*C) is well below the desired input frequency.
