LMV225 Datasheet, PDF (26/39 Pages) National Semiconductor (TI) – RF Power Detector for CDMA and WCDMA

LMV225, LMV226, LMV228
SNWS013L – AUGUST 2003 – REVISED MARCH 2013
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Output Ripple with Additional Filtering
The calculated result above is for an unfiltered configuration. When a low pass filter is used by shunting a
capacitor of e.g. C_OUT = 1.5 nF at the output of the LMV225/LMV228 to ground, this ripple is further attenuated.
The cut-off frequency follows from:
1
f_C = _{2 S COUT RO}
(8)
With the output resistance of the LMV225/LMV228 R_O = 19.8 kΩ typical and C_OUT = 1.5 nF, the cut-off frequency
equals f_C = 5.36 kHz. A 100 kHz AM signal then gets attenuated by 5.36/100 or 25.4 dB. The remaining ripple
will be less than 20 mV. With a slope of 40 mV/dB this translates into an error of less than ±0.5 dB. Since the
LMV226 has a low output impedance buffer, a capacitor to reduce the ripple will not be effective.
Output Ripple Measurement
Figure 71 shows the ripple reduction that can be achieved by adding additional capacitance at the output of the
LMV225/LMV228. The RF signal of 900 MHz is AM modulated with a 100 kHz sinewave and a modulation index
of 0.3. The RF input power is swept while the modulation index remains unchanged. Without the output capacitor
the ripple is about 200 mV_PP. Connecting a capacitor of 1.5 nF at the output to ground, results in a ripple of 12
mV_PP. The attenuation with a 1.5 nF capacitor is then 20 • log (200/12) = 24.4 dB. This is very close to the
calculated number of the previous paragraph.
1000
NO ADDITIONAL CAPACITOR
100
10
C_OUT = 1.5nF
1
-50 -40 -30 -20 -10 0 10
RF INPUT POWER (dBm)
Figure 71. Output Ripple vs. RF Input Power
PRINCIPLE OF OPERATION
The logarithmic response of the LMV225/LMV226/LMV228 is implemented by a logarithmic amplifier as shown in
Figure 72. The logarithmic amplifier consists of a number of cascaded linear gain cells. With these gain cells, a
piecewise approximation of the logarithmic function is constructed.
+
+
+
+
Y
A/0
A/0
A/0
A/0
X₀
X₁
X₂
X₃
X₄
Figure 72. Logarithmic Amplifier
26
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