LMH2110 Datasheet, PDF (22/33 Pages) National Semiconductor (TI) – 8 GHz Logarithmic RMS Power Detector with 45 dB Dynamic Range

LMH2110
SNWS022C – JANUARY 2010 – REVISED MARCH 2013
V_DD
RF_{IN B1 A1 A2 OUT} ^RS
C_S
LMH2110
EN
C2
B2,C1
GND
+
ADC
-
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Figure 51. Low-Pass Output Filter for Residual Ripple Reduction
The output impedance of the LMH2110 is HIGH in shutdown. This is especially beneficial in pulsed mode
systems. It ensures a fast settling time when the device returns from shutdown into active mode and reduces
power consumption.
In pulse mode systems, the device is active only during a fraction of the time. During the remaining time the
device is in low-power shutdown. Pulsed mode system applications usually require that the output value is
available at all times. This can be realized by a capacitor connected between the output and GND that “stores”
the output voltage level. To apply this principle it should be ensured that discharging of the capacitor is
minimized in shutdown mode. The connected ADC input should thus have a high input impedance to prevent a
possible discharge path through the ADC. When an additional filter is applied at the output, the capacitor of the
RC-filter can be used to store the output value. An LMH2110 with a high impedance shutdown mode save power
in pulse mode systems. This is because the capacitor C_S doesn’t need to be fully re-charged each cycle.
Supply
The LMH2110 has an internal LDO to handle supply voltages between 2.7V to 5V. This enables a direct
connection to the battery in cell phone applications. The high PSRR of the LMH2110 ensures that the
performance is constant over its power supply range.
Specifying Detector Performance
The performance of the LMH2110 can be expressed by a variety of parameters. This section discusses the key
parameters.
Dynamic Range
The LMH2110 is designed to have a predictable and accurate response over a certain input power range. This is
called the dynamic range (DR) of a detector. For determining the dynamic range a couple of different criteria can
be used. The most commonly used ones are:
• Log conformance error, E_LC
• Variation over temperature error, E_VOT
• 1 dB step error, E_{1 dB}
• 10 dB step error, E_{10 dB}
• Variation due to modulation, E_MOD
The specified dynamic range is the range in which the specified error metric is within a predefined window. An
explanation of these errors is given in the following paragraphs.
Log Conformance error
The LMH2110 implements a logarithmic function. In order to describe how close the transfer is to an ideal
logarithmic function the log conformance error is used. To calculate the log conformance error the detector
transfer function is modeled as a linear-in-dB relationship between the input power and the output voltage.
The ideal linear-in-dB transfer is modeled by 2 parameters:
• Slope
• Intercept
22
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