LMH2110_15 Datasheet, PDF (28/36 Pages) Texas Instruments – LMH2110 8-GHz Logarithmic RMS Power Detector with 45-dB Dynamic Range

LMH2110
SNWS022D – JANUARY 2010 – REVISED JUNE 2015
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Solving Equation 16 for R₁ yields:
ª ^AdB º
R₁ = ^«¬10 ²⁰ - 1^»¼ R_IN
(17)
Suppose the desired attenuation is 30 dB with a given LMH2110 input impedance of 50 Ω, the resistor R₁ needs
to be 1531 Ω. A practical value is 1.5 kΩ. Although this is a cheaper solution than the application with directional
coupler, it also comes with a disadvantage. After calculating the resistor value it is possible that the realized
attenuation is less then expected. This is because of the parasitic capacitance of resistor R₁ which results in a
lower actual realized attenuation. Whether the attenuation is reduced depends on the frequency of the RF signal
and the parasitic capacitance of resistor R₁. Because the parasitic capacitance varies from resistor to resistor,
exact determination of the realized attenuation can be difficult. A way to reduce the parasitic capacitance of
resistor R₁ is to realize it as a series connection of several separate resistors.
8.2.3 Application With Low-Pass Output Filter for Residual Ripple Reduction
The output of the LMH2110 provides a DC voltage that is a measure for the applied RF power to the input pin.
The output voltage has a linear-in-dB response for an applied RF signal.
RF power detectors can have some residual ripple on the output due to the modulation of the applied RF signal.
The residual ripple on the output of the LMH2110 device is small though and, therefore, additional filtering is
usually not needed. This is because its internal averaging mechanism reduces the ripple significantly. For some
modulation types however, having very high peak-to-average ratios, additional filtering might be useful.
Filtering can be applied by an external low-pass filter. Filtering reduces not only the ripple, but also increases the
response time. In other words, it takes longer before the output reaches its final value. A trade-off must be made
between allowed ripple and allowed response time. The filtering technique is depicted in Figure 55. The filtering
of the low pass output filter is realized by resistor R_S and capacitor C_S. The –3-dB bandwidth of this filter can be
calculated by:
f_{−3 dB} = 1 / (2πR_SC_S)
(18)
V_DD
RF_{IN B1 A1 A2 OUT} ^RS
C_S
LMH2110
EN
C2
B2,C1
GND
+
ADC
-
Figure 55. Low-Pass Output Filter for Residual Ripple Reduction
The output impedance of the LMH2110 is HIGH in shutdown. This is especially beneficial in pulsed mode
systems. It ensures a fast settling time when the device returns from shutdown into active mode and reduces
power consumption.
In pulse mode systems, the device is active only during a fraction of the time. During the remaining time the
device is in low-power shutdown. Pulsed mode system applications usually require that the output value is
available at all times. This can be realized by a capacitor connected between the output and GND that “stores”
the output voltage level. To apply this principle, capacitor discharging must be minimized in shutdown mode. The
connected ADC input must therefore have a high input impedance to prevent a possible discharge path through
the ADC. When an additional filter is applied at the output, the capacitor of the RC-filter can be used to store the
output value. An LMH2110 with a high impedance shutdown mode saves power in pulse mode systems. This is
because the capacitor C_S does not need to be fully re-charged each cycle.
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