LMH2100 Datasheet, PDF (29/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

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LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
Feature Description (continued)
The error metrics in the datasheet that describe the accuracy of the detector for a change in the input power are
E_{1 dB} (for a 1-dB change in the input power) and E_{10 dB} (for a 10-dB step, or ten consecutive steps of 1 dB). Since
it can be assumed that the temperature does not change during the power step the differential error equals the
difference of the drift error at the two involved power levels:
E_1dB(P_IN,T)= E_DRIFT(P_IN+1dB,T) - E_DRIFT(P_IN,T)
E_10dB(P_IN,T)= E_DRIFT(P_IN+10dB,T) - E_DRIFT(P_IN,T)
(11)
It should be noted that the step error increases significantly when one (or both) power levels in the above
expression are outside the detector dynamic range. For E_{10 dB} this occurs when P_IN is less than 10 dB below the
maximum input power of the dynamic range, P_MAX.
7.4 Device Functional Modes
7.4.1 Shutdown
To save power, the LMH2100 can be brought into a low-power shutdown mode. The device is active for EN =
HIGH (V_EN>1.1 V) and in the low-power shutdown mode for EN = LOW (V_EN < 0.6 V). In this state the output of
the LMH2100 is switched to a high impedance mode. Using the shutdown function, care must be taken not to
exceed the absolute maximum 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 ensured. The absolute 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.
7.4.1.1 Output Behavior in Shutdown
In order to save power, the LMH2100 can be used in pulsed mode, such that it is active to perform the power
measurement only during a fraction of the time. During the remaining time the device is in low-power shutdown.
Applications using this approach usually require that the output value is available at all times, also when the
LMH2100 is in shutdown. The settling time in active mode, however, should not become excessively large. This
can be realized by the combination of the LMH2100 and a low pass output filter (see Figure 81).
In active mode, the filter capacitor C_S is charged to the output voltage of the LMH2100, which in this mode has a
low output impedance to enable fast settling. During shutdown-mode, the capacitor should preserve this voltage.
Discharge of C_S through any current path should therefore be avoided in shutdown. The output impedance of the
LMH2100 becomes high in shutdown, such that the discharge current cannot flow from the capacitor top plate,
through R_S, and the LMH2100 devices's OUT pin to GND. This is realized by the internal shutdown mechanism
of the output amplifier and by the switch depicted in Figure 85. Additionally, it should be ensured that the ADC
input impedance is high as well, to prevent a possible discharge path through the ADC.
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