lmh2120umx National Semiconductor Corporation, lmh2120umx Datasheet - Page 20

• Manufacturer Part Number: lmh2120umx
• Description: Lmh2120 6 Ghz Linear Rms Power Detector With 40 Db Dynamic Range
• Manufacturer: National Semiconductor Corporation

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plifier. This can be done by selecting a directional coupler with
the appropriate coupling factor.
Since the LMH2120 has a constant input impedance, a re-
sistive divider can also be used instead of a directional cou-
pler
Resistor R
tector input impedance, to match the output range of the PA
with the input range of the LMH2120. The attenuation (A
realized by R
LMH2120 equals:
Solving this expression for R
Suppose the desired attenuation is 30 dB with a given
LMH2120 input impedance of 50Ω, the resistor R
be 1531Ω. A practical value is 1.5 kΩ. Although this is a
cheaper solution than the application with directional coupler,
it has a disadvantage. After calculating the resistor value it is
possible that the realized attenuation is less than expected.
This is because of the parasitic capacitance of resistor R
which results in a lower actual realized attenuation. Whether
the attenuation will be reduced depends on the frequency of
the RF signal and the parasitic capacitance of resistor R
Since the parasitic capacitance varies from resistor to resis-
tor, exact determination of the realized attenuation can be
difficult. A way to reduce the parasitic capacitance of resistor
R
resistors.
Enable
To save power, the LMH2120 can be brought into a low-power
shutdown mode by means of the enable pin (EN). The device
is active for EN = HIGH (V
shutdown mode for EN = LOW (V
output of the LMH2120 is switched to high-impedance. This
high impedance prevents the discharge of the optional low-
pass filter which is good for power efficiency. Using the shut-
down function, care must be taken not to exceed the absolute
maximum ratings. Since the device has an internal operating
voltage of 2.5V, the voltage level on the enable should not be
1
is to realize it as a series connection of several separate
(Figure
FIGURE 5. Application with Resistive Divider
1
5).
implements an attenuator, together with the de-
1
and the effective input impedance (R
EN
1
yields:
> 1.1V), and in the low-power
EN
< 0.6V). In this state the
1
needs to
IN
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) of the
(10)
(9)
dB
1
1
)
.
20
higher than 3V to prevent excess current flowing into the en-
able pin. Also enable voltage levels lower than 400 mV below
GND should be prevented. In both cases the ESD devices
start to conduct when the enable voltage range is exceeded
and excess current will be drawn. A correct operation is not
guaranteed then. The absolute maximum ratings are also ex-
ceeded when EN is switched to HIGH (from shutdown to
active mode) while the supply voltage is switched off. This
situation should be prevented at all times. A possible solution
to protect the device is to add a resistor of 1 kΩ in series with
the enable input to limit the current.
Output
The output of the LMH2120 provides a DC voltage that is a
measure for the applied RF power to the input pin. The output
voltage has a linear-in-V response for an applied RF signal.
RF power detectors can have some residual ripple on the
output due to the modulation of the applied RF signal. The
residual ripple on the LMH2120’s output is small; therefore,
additional filtering is usually not needed. This is because its
internal averaging mechanism reduces the ripple significant-
ly. For some modulation types having very high peak-to-
average ratios or low-frequency components in the amplitude
modulation, additional filtering might be useful.
Filtering can be applied by an external low-pass filter. It should
be realized that filtering reduces not only the ripple, but also
increases the response time. In other words, it takes longer
before the output reaches its final value. A trade-off should be
made between allowed ripple and allowed response time. The
filtering technique is depicted in
filter is realized by resistor R
bandwidth of this filter can be calculated by:
The output impedance of the LMH2120 is HIGH in shutdown.
This is especially beneficial in pulsed mode systems. It en-
sures a fast settling time when the device returns from shut-
down 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 usu-
ally 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
FIGURE 6. Low-Pass Output Filter for Residual Ripple
f
−3 dB
= 1 / (2
Reduction
S
π
Figure
and capacitor C
R
S
C
S
)
6. The low-pass output
S
. The -3 dB
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