LMH2100 Datasheet, PDF (42/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
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Layout Guidelines (continued)
10.1.5 Reference (REF)
The Reference pin can be used to compensate for temperature drift of the internal reference voltage as
described in Interface to the ADC. The REF pin is directly connected to the inverting input of the transimpedance
amplifier. Thus, RF signals and other spurious signals couple directly through to the output. Introduction of RF
signals can be prevented by connecting a small capacitor between the REF pin and ground. The capacitor
should be placed close to the REF pin as depicted in Figure 91.
10.1.6 Output (OUT)
The OUT pin is sensitive to crosstalk from the RF input, especially at high power levels. The ESD diode between
OUT and VDD may rectify the crosstalk, but may add an unwanted inaccurate DC component to the output
voltage.
The board layout should minimize crosstalk between the detectors input RFIN and the detectors output. Using an
additional capacitor connected between the output and the positive supply voltage (VDD pin) or GND can
prevent this. For optimal performance this capacitor should be placed as close as possible to the OUT pin of the
LMH2100.
10.2 Layout Example
DECOUPLING
CAPACITOR
GND
GND
TRANSMISSION LINE
VDD
OUT
RFIN
REF
CROSSTALK FILTER
CAPACITOR
GND
GND
EN
GND
Figure 91. Recommended LMH2100 Board Layout
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