LMH2100 Datasheet, PDF (27/32 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

function, care must be taken not to exceed the absolute max-
imum ratings. Forcing a voltage to the enable input that is 400
mV higher than V_DD or 400 mV lower than GND will damage
the device and further operations is not guaranteed. The ab-
solute maximum ratings can also be exceeded when the
enable EN is switched to HIGH (from shutdown to active
mode) while the supply voltage is low (off). This should be
prevented at all times. A possible solution to protect the part
is to add a resistor of 100 kΩ in series with the enable input.
3.4 Output and Reference
This section describes the possible filtering techniques that
can be applied to reduce ripple in the detector output voltage.
In addition two different topologies to connect the LMH2100
to an ADC are elaborated.
3.4.1 Filtering
The output voltage of the LMH2100 is a measure for the ap-
plied RF signal on the RF input pin. Usually, the applied RF
signal contains AM modulation that causes low frequency rip-
ple in the detector output voltage. CDMA signals for instance
contain a large amount of amplitude variations. Filtering of the
output signal can be used to eliminate this ripple. The filtering
can either be realized by a low pass output filter or a low pass
feedback filter. Those two techniques are depicted in
Figure 11.
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FIGURE 11. Low Pass Output Filter and Low Pass Feedback Filter
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Depending on the system requirements one of the these fil-
tering techniques can be selected. The low pass output filter
has the advantage that it preserves the output voltage when
the LMH2100 is brought into shutdown. This is elaborated in
section 3.4.3. In the feedback filter, resistor R_P discharges
capacitor C_P in shutdown and therefore changes the output
voltage of the device.
A disadvantage of the low pass output filter is that the series
resistor R_S limits the output drive capability. This may cause
inaccuracies in the voltage read by an ADC when the ADC
input impedance is not significantly larger than R_S. In that
case, the current flowing through the ADC input induces an
error voltage across filter resistor R_S. The low pass feedback
filter doesn’t have this disadvantage.
Note that adding an external resistor between OUT and REF
reduces the transfer gain (LOG-slope and LOG-intercept) of
the device. The internal feedback resistor sets the gain of the
transimpedance amplifier.
The filtering of the low pass output filter is realized by resistor
R_S and capacitor C_S. The −3 dB bandwidth of this filter can
then be calculated by: f_{−3 dB} = 1 / 2πR_SC_S. The bandwidth of
the low pass feedback filter is determined by external resistor
R_P in parallel with the internal resistor R_TRANS, and external
capacitor C_P in parallel with internal capacitor C_TRANS (see
Figure 13). The −3 dB bandwidth of the feedback filter can be
calculated by f_{−3 dB} = 1 / 2π (R_P//R_TRANS) (C_P+C_TRANS). The
bandwidth set by the internal resistor and capacitor (when no
external components are connected between OUT and REF)
equals f_{−3 dB} = 1 / 2π R_TRANS C_TRANS = 450 kHz.
3.4.2 Interface to the ADC
The LMH2100 can be connected to the ADC with a single
ended or a differential topology. The single ended topology
connects the output of the LMH2100 to the input of the ADC
and the reference pin is not connected. In a differential topol-
ogy, both the output and the reference pins of the LMH2100
are connected to the ADC. The topologies are depicted in
Figure 12.
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FIGURE 12. Single Ended and Differential Application
27
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