LMH2120_15 Datasheet, PDF (20/34 Pages) Texas Instruments – LMH2120 6 GHz Linear RMS Power Detector with 40 dB Dynamic Range

LMH2120
SNWS021C – JULY 2010 – REVISED FEBRUARY 2013
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³ VOUT = k vRF2dt
(8)
in which k is the conversion gain. Note that as a result of the feedback loop a square root is also implemented,
yielding the RMS function.
Given this architecture for the RF detector, the high performance of the LMH2120 can be understood. In theory
the accuracy of the linear transfer is set by:
• The linear feedback network, which basically needs to process a DC signal only.
• A high loop gain for the feedback loop, which is ensured by the amplifier gain A.
The RMS functionality is inherent to the feedback loop and the use of a multiplier. Thus, a very accurate LIN-
RMS RF power detector is obtained.
To ensure a low dependency on the supply voltage, the internal detector circuitry is supplied via a low drop-out
(LDO) regulator. This enables the usage of a wide range of supply voltage (2.7V to 5V) in combination with a low
sensitivity of the output signal for the external supply voltage.
RF Input
RF systems typically use a characteristic impedance of 50Ω; the LMH2120 is no exception to this. The RF input
pin of the LMH2120 has an input impedance of 50Ω. It enables an easy, direct connection to a directional
coupler without the need for additional components (Figure 54). For an accurate power measurement the input
power range of the LMH2120 needs to be aligned with the output power range of the power amplifier. This can
be done by selecting a directional coupler with the appropriate coupling factor.
Since the LMH2120 has a constant input impedance, a resistive divider can also be used instead of a directional
coupler (Figure 58).
RF
PA
ANTENNA
R₁
V_DD
RF_IN
A1
B1
A2
LMH2120
OUT
EN C2
B2, C1
GND
ADC
Figure 58. Application with Resistive Divider
Resistor R₁ implements an attenuator, together with the detector input impedance, to match the output range of
the PA with the input range of the LMH2120. The attenuation (A_dB) realized by R₁ and the effective input
impedance (R_IN) of the LMH2120 equals:
ª ^R1 º
A_dB
=
20LOG
«1
¬
+
R_IN
»
¼
(9)
Solving this expression for R₁ yields:
20
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