LMH2120_15 Datasheet, PDF (21/34 Pages) Texas Instruments – LMH2120 6 GHz Linear RMS Power Detector with 40 dB Dynamic Range

LMH2120
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SNWS021C – JULY 2010 – REVISED FEBRUARY 2013
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R₁ = ^«¬10 ²⁰ - 1^»¼ R_IN
(10)
Suppose the desired attenuation is 30 dB with a given LMH2120 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 has a disadvantage. After calculating the resistor value it is possible that the realized attenuation is
less than 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 LMH2120 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 LMH2120 is switched to high-impedance. This high impedance prevents the
discharge of the optional low-pass filter which is good for 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 excess current flowing into the
enable pin. 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 excess current will be drawn. A
correct operation is not ensured then. The absolute maximum ratings are also exceeded when 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 LMH2120 provides a DC voltage that is a measure for the applied RF power to the input pin.
The output voltage has a linear-in-V 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 LMH2120’s output is small; therefore, additional filtering is usually not needed. This is
because its internal averaging mechanism reduces the ripple significantly. For some modulation types having
very high peak-to-average ratios or low-frequency components in the amplitude modulation, 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 59. 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)
V_DD
RF_{IN B1 A1 A2 OUT}
LMH2120
EN
C2
B2,C1
GND
R_S
C_S
+
ADC
-
Figure 59. Low-Pass Output Filter for Residual Ripple Reduction
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