LMH2120_15 Datasheet, PDF (22/34 Pages) Texas Instruments – LMH2120 6 GHz Linear RMS Power Detector with 40 dB Dynamic Range

LMH2120
SNWS021C – JULY 2010 – REVISED FEBRUARY 2013
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The output impedance of the LMH2120 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, discharging of the capacitor should be 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 LMH2120 with a high-impedance shutdown mode saves power in pulse mode
systems. This is because the capacitor C_S doesn’t need to be fully recharged each cycle.
Supply
The LMH2120 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 LMH2120 ensures that the
performance is constant over its power supply range.
SPECIFYING DETECTOR PERFORMANCE
The performance of the LMH2120 can be expressed by a variety of parameters. This section discusses the key
parameters.
Dynamic Range
The LMH2120 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:
• Linear conformance error, E_LC
• Variation over temperature error, E_VOT
• 1 dB step error, E_{1 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.
Linear Conformance error
The LMH2120 implements a linear detection function. In order to describe how close the transfer is to an ideal
linear function the linear conformance error is used. To calculate the linear conformance error the detector
transfer function is modeled as a linear-in-V relationship between the input power and the output voltage.
The ideal linear-in-V transfer is modeled by 2 parameters:
• Slope, K_SLOPE
• Intercept, P_INT
and is described by:
V_OUT = K_SLOPE (P_IN – P_INT)
(12)
where V_OUT is the output voltage in dBV, K_SLOPE is the slope of the function in dB/dB, P_IN the input power level in
dBm and P_INT is the power level in dBm at which the function intersects V_OUT = 0 dBV = 1V (See Figure 60).
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