LMH2110 Datasheet, PDF (20/33 Pages) National Semiconductor (TI) – 8 GHz Logarithmic RMS Power Detector with 45 dB Dynamic Range

LMH2110
SNWS022C – JANUARY 2010 – REVISED MARCH 2013
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which implies that the average power content of the current related to the output voltage of the LMH2110 is
made equal to the average power content of the current related to the RF input signal.
For a negative-feedback system, the transfer function is given by the inverse function of the feedback block.
Therefore, to have a logarithmic transfer for this RF detector, the feedback network implements an exponential
function resulting in an overall transfer function for the LMH2110 of:
V_out
=
V₀
log
¨©§
1
V_x
³^VRF2dt ¸¹·
(8)
in which V₀ and V_X are normalizing voltages. Note that as a result of the feedback loop also a square-root is
implemented yielding the RMS function.
Given this architecture for the RF detector, the high-performance of the LMH2110 can be understood. In theory
the accuracy of the logarithmic transfer is set by:
• The exponential feedback network, which basically needs to process a DC signal only.
• A high loop gain for the feedback loop, which is specified by the amplifier gain A.
The RMS functionality is inherent to the feedback loop and the use of a multiplier. So, a very accurate LOG-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 LMH2110 is no exception to this. The RF input
pin of the LMH2110 has an input impedance of 50Ω. It enables an easy, direct connection to a directional
coupler without the need for additional components (Figure 46). For an accurate power measurement the input
power range of the LMH2110 needs to be aligned with the output power range of the power amplifier. This can
be done by selecting a directional coupler with the correct coupling factor.
Since the LMH2110 has a constant input impedance, a resistive divider can also be used in stead of a directional
coupler (Figure 50).
RF
PA
ANTENNA
R₁
V_DD
A1
RF_IN
B1
OUT
A2
LMH2110
EN C2
B2, C1
GND
ADC
Figure 50. Application with Resistive Divider
Resistor R₁ implements an attenuator together with the detector input impedance to match the output range of
the PA to the input range of the LMH2110. The attenuation (A_dB) realized by R₁ and the effective input
impedance of the LMH2110 equals:
20
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