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AD8319 Datasheet(PDF) 2 Page - Analog Devices |
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AD8319 Datasheet(HTML) 2 Page - Analog Devices |
2 / 4 page CN-0082 Circuit Note Rev. 0 | Page 2 of 4 amplifier output) until the level at the RF input corresponds to the applied setpoint voltage. GAIN settles to a value that results in the correct balance between the input signal level at the detector and the setpoint voltage. The basic connections for operating the ADL5331 in an AGC loop with the AD8319 are shown in Figure 1. The AD8319 is a 1 MHz to 10 GHz precision demodulating logarithmic amplifier. It offers a detection range of 45 dB with ±0.5 dB temperature stability. The VOUT pin of the AD8319 controls the GAIN (gain control) pin of the ADL5331. When the AD8319 is in controller mode, as it is in this application, VOUT on the AD8319 can drive the ADL5331 GAIN pin over its full linear range of 0 V to 1.4 V. Under very low power RF in conditions, outside the linear control range of the loop, VOUT on the AD8319 may be driven to its maximum value very close to VPOS. To avoid overdrive recovery issues with the ADL5331 GAIN input, a voltage divider can be placed between VOUT on the AD8319 and GAIN on the ADL5331. This may have a slight effect on the overall speed of the loop, for instance, when the input power to the ADL5331 is stepped. A coupler/attenuation of 23 dB is used to match the desired output power range from the VGA to the linear operating range of the AD8319. In this case, the desired output power range of the VGA is −15 dBm to +15 dBm. With the given attenuator/coupler, the range of power to the AD8319 RF input is −8 dBm to −38 dBm, within the specified range of −3 dBm to −43dBm for a ±1 dB error. The detector’s error amplifier uses CLFP, a ground-referenced capacitor pin, to integrate the error signal (in the form of a current). A capacitor must be connected to CLFP to set the loop bandwidth and to ensure loop stability. Figure 2, Figure 3, and Figure 4 show the transfer function of the ADL5331 output power vs. the AD5621 DAC code for a 100 MHz sine wave with an input power of 0 dBm, −10 dBm, and −20 dBm. Note that the power control of the AD8319 has a negative sense. Decreasing the DAC code, which corresponds to demanding a higher signal from the ADL5331, tends to increase GAIN. In order for the AGC loop to remain in equilibrium, the AD8319 must track the envelope of the ADL5331 output signal and provide the necessary voltage levels to the ADL5331’s gain control input. Figure 5 shows an oscilloscope screenshot of the AGC loop in Figure 1. A 100 MHz sine wave with 50% AM modulation is applied to the ADL5331. The output signal from the ADL5331 is a constant envelope sine wave with amplitude corresponding to a setpoint voltage at the AD8319 of 1.5 V. Also shown is the gain control response of the AD8319 to the changing input envelope. –40 –30 –20 –10 0 10 20 1024 1524 2024 2524 3024 3524 AD5621 DAC CODE –5.0 –2.5 0 2.5 5.0 POWER OUT STRAIGHT LINE ERROR Figure 2. ADL5331 Power Out vs. AD5621 DAC Code with RF Input Signal = 0 dBm –40 –30 –20 –10 0 10 20 1280 1780 2280 2780 3280 3780 AD5621 DAC CODE –5.0 –2.5 0 2.5 5.0 POWER OUT STRAIGHT LINE ERROR Figure 3. ADL5331 Power Out vs. AD5621 DAC Code with RF Input Signal = −10 dBm –40 –30 –20 –10 0 10 20 1280 1780 2280 2780 3280 3780 AD5621 DAC CODE –5.0 –2.5 0 2.5 5.0 POWER OUT STRAIGHT LINE ERROR Figure 4. ADL5331 Power Out vs. AD5621 DAC Code with RF Input Signal = −20 dBm |
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