LMH2120 Datasheet, PDF (19/34 Pages) National Semiconductor (TI) – Linear RMS power detector particularly suited for accurate

LMH2120
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SNWS021C – JULY 2010 – REVISED FEBRUARY 2013
LMH2120 RF POWER DETECTOR
For optimal performance, the LMH2120 needs to be configured correctly in the application. The detector will be
discussed by means of its block diagram (Figure 57). Details of the electrical interfacing are separately discussed
for each pin below.
A1
V_DD
LDO
Internal Supply
B1 ^RFIN
C2
EN
V/I
V/I
i₁
i_OUT
A
i₂
V/I
V/I
OUT A2
V_OUT
GND
C1, B2
Figure 57. Block Diagram
For measuring the RMS (power) level of a signal, the time average of the squared signal needs to be measured
as described in section ACCURATE POWER MEASUREMENT. This is implemented in the LMH2120 by means
of a multiplier and a low-pass filter in a negative-feedback loop. A simplified block diagram of the LMH2120 is
depicted in Figure 57. The core of the loop is a multiplier. The two inputs of the multiplier are fed by (i₁, i₂):
i₁ = i_LF + i_RF
(4)
i₁ = i_LF - i_RF
(5)
in which i_LF is a current depending on the DC output voltage of the RF detector and i_RF is a current depending on
the RF input signal. The output of the multiplier (i_OUT) is the product of these two current and equals:
i_LF2 - i_RF2
i_OUT =
I₀
(6)
in which I₀ is a normalizing current. By a low-pass filter at the output of the multiplier the DC term of this current
is isolated and integrated. The input of the amplifier A acts as the nulling point of the negative feedback loop,
yielding:
³ ³ iLF2dt = iRF2dt
(7)
which implies that the average power content of the current related to the output voltage of the LMH2120 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 linear transfer for this RF detector, the feedback network implements a linear function as
well resulting in an overall transfer function for the LMH2120 of:
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