LMH2110 Datasheet, PDF (21/33 Pages) National Semiconductor (TI) – 8 GHz Logarithmic RMS Power Detector with 45 dB Dynamic Range

LMH2110
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SNWS022C – JANUARY 2010 – REVISED MARCH 2013
ª ^R1 º
A_dB
=
20LOG
«1
¬
+
R_IN
»
¼
(9)
Solving this expression for R₁ yields:
ª ^AdB º
R₁ = ^«¬10 ²⁰ - 1^»¼ R_IN
(10)
Suppose the desired attenuation is 30 dB with a given LMH2110 input impedance of 50Ω, the resistor R₁ needs
to be 1531Ω. 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 will be reduced depends on the frequency of the RF
signal and the parasitic capacitance of resistor R₁. Since 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.
Enable
To save power, the LMH2110 can be brought into a low-power shutdown mode by means of the enable pin (EN).
The device is active for EN = HIGH (V_EN>1.1V) and in the low-power shutdown mode for EN = LOW (V_EN <
0.6V). In this state the output of the LMH2110 is switched to a high impedance mode. This high impedance mode
prevents the discharge of the optional low-pass filter which is good for the power efficiency. Using the shutdown
function, care must be taken not to exceed the absolute maximum ratings. Since the device has an internal
operating voltage of 2.5V, the voltage level on the enable should not be higher than 3V to prevent damage to the
device. Also enable voltage levels lower than 400 mV below GND should be prevented. In both cases the ESD
devices start to conduct when the enable voltage range is exceeded and excessive current will be drawn. A
correct operation is not ensured then. The absolute maximum ratings are also exceeded when the enable (EN) is
switched to HIGH (from shutdown to active mode) while the supply voltage is switched off. This situation should
be prevented at all times. A possible solution to protect the device is to add a resistor of 1 kΩ in series with the
enable input to limit the current.
Output
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 LMH2110’s output 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. It should be realized that 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 should be made between allowed ripple and allowed response time. The filtering technique is
depicted in Figure 51. 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)
(11)
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