AD8319 VSET PULSE
A Ch1 2.60V
Figure 33. Oscilloscope Screenshot Showing the
Response Time of the AGC Loop
Response time and the amount of signal integration are
controlled by CFLT. This functionality is analogous to the
feedback capacitor around an integrating amplifier. While it is
possible to use large capacitors for CFLT, in most applications
values under 1 nF provide sufficient filtering.
Calibration in controller mode is similar to the method used in
measurement mode. A simple two-point calibration can be
done by applying two known VSET voltages or DAC codes and
measuring the output power from the VGA. Slope and intercept
can then be calculated with the following equations:
Slope = (VSET1 − VSET2)/(POUT1 − POUT2)
Intercept = POUT1 − VSET1/Slope
VSETX = Slope × (POUTX − Intercept)
More information on the use of the ADL5330 in AGC applications
can be found in the ADL5330 data sheet.
For applications in which maximum video bandwidth and,
consequently, fast rise time are desired, it is essential that the
CLPF pin be left unconnected and free of any stray capacitance.
The nominal output video bandwidth of 50 MHz can be
reduced by connecting a ground-referenced capacitor (CFLT) to
the CLPF pin, as shown in Figure 34. This is generally done to
reduce output ripple (at twice the input frequency for a
symmetric input waveform such as sinusoidal signals).
Figure 34. Lowering the Postdemodulation Bandwidth
CFLT is selected using the following equation:
× 1.5 kΩ × Video Bandwidth )
The video bandwidth should typically be set to a frequency
equal to about one-tenth the minimum input frequency. This
ensures that the output ripple of the demodulated log output,
which is at twice the input frequency, is well filtered.
In many log amp applications, it may be necessary to lower the
corner frequency of the postdemodulation filtering to achieve
low output ripple while maintaining a rapid response time to
changes in signal level. An example of a 4-pole active filter is
shown in the AD8307 data sheet.
OPERATION BEYOND 8 GHZ
The AD8319 is specified for operation up to 8 GHz, but it provides
useful measurement accuracy over a reduced dynamic range of
up to 10 GHz. Figure 35 shows the performance of the AD8319
over temperature at 10 GHz when the device is configured as
shown in Figure 22. Dynamic range is reduced at this frequency,
but the AD8319 does provide 30 dB of measurement range within
±3 dB of linearity error.
–40 –35 –30 –25 –20 –15 –10 –5 0
Figure 35. VOUT and Log Conformance vs. Input Amplitude at 10.0 GHz,
Multiple Devices, RTADJ = Open, CLPF = 1000 pF
Implementing an impedance match for frequencies beyond 8 GHz
can improve the sensitivity of the AD8319 and measurement
Operation beyond 10 GHz is possible, but part to part variation,
most notably in the intercept, becomes significant.
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