Extending the Dynamic Range of the ADL5513 Logarithmic Detector Using the AD8368 Variable Gain Amplifier
Figure 1. ADL5513 Logarithmic Detector and AD8368 VGA Providing 95 dB Dynamic Range at 120 MHz (Simplified Schematic) Note: VPOS = +5 V
Due to the amplification of out-of-band noise by the AD8368, a band-pass filter is used between the AD8368 and ADL5513 to increase the low-signal sensitivity. The VGA amplifies low power signals and attenuates high power signals to fit them in the detectable range of the ADL5513. If an amplifier with higher gain and lower noise figure is used, better than 90 dB sensitivity can be achieved for use in a receive-signal-strength-indicator (RSSI) application.
Figure 2 shows the data results of the extended dynamic range at 120 MHz and the corresponding error in VOUT.
Figure 2. Output and Conformance for the AD8368/ADL5513 Extended Dynamic Range Circuit at 120 MHz
Due to the high frequencies and wide dynamic ranges involved, the circuit must be constructed on a multilayer PC board that has at least one layer for the ground plane. Each IC power pin must be decoupled with a 0.1 μF low inductance ceramic capacitor as shown in Figure 1. Some power pins have an additional smaller value decoupling capacitor (low ESL) for extremely high frequencies. The “0 Ω” resistors on the schematic represent optional small resistor values (less than 10 Ω) or ferrite beads that may be added for more decoupling. The data sheets for the ADL5513 and the AD8368 show the optimal layouts of the respective evaluation boards.
On the underside of both the ADL5513 and the AD8368 chip scale package there is an exposed paddle. This paddle is internally connected to the ground of the chip. Solder the paddle to the low impedance ground plane on the PCB to ensure specified electrical performance and to provide thermal relief. It is also recommended that the ground planes on all layers under the paddle be stitched together with vias to reduce thermal impedance.