LMH2100 Datasheet, PDF (28/32 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

The differential topology has the advantage that it is compen-
sated for temperature drift of the internal reference voltage.
This can be explained by looking at the transimpedance am-
plifier of the LMH2100 (Figure 13).
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FIGURE 13. Output Stage of the LMH2100
It can be seen that the output of the amplifier is set by the
detection current I_DET multiplied by the resistor R_TRANS plus
the reference voltage V_REF:
V_OUT = I_DET R_TRANS + V_REF
I_DET represents the detector current that is proportional to the
RF input power. The equation shows that temperature varia-
tions in V_REF are also present in the output V_OUT. In case of a
single ended topology the output is the only pin that is con-
nected to the ADC. The ADC voltage for single ended is thus:
Single ended: V_ADC = I_DET R_TRANS + V_REF
A differential topology also connects the reference pin, which
is the value of reference voltage V_REF. The ADC reads
V_OUT - V_REF:
Differential: V_ADC = V_OUT - V_REF = I_DET R_TRANS
The resulting equation doesn’t contain the reference voltage
V_REF anymore. Temperature variations in this reference volt-
age are therefore not measured by the ADC.
3.4.3 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 measure-
ment 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 set-
tling time in active mode, however, should not become ex-
cessively large. This can be realized by the combination of
the LMH2100 and a low pass output filter (see Figure 11, left
side), as discussed below.
In active mode, the filter capacitor C_S is charged to the output
voltage of the LMH2100 — which in this mode has a low out-
put 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 be-
comes high in shutdown, such that the discharge current
cannot flow from the capacitor top plate, through R_S, and the
LMH2100's OUT pin to GND. This is realized by the internal
shutdown mechanism of the output amplifier and by the
switch depicted in Figure 13. Additionally, it should be en-
sured that the ADC input impedance is high as well, to prevent
a possible discharge path through the ADC.
4.0 BOARD LAYOUT RECOMMENDATIONS
As with any other RF device, careful attention must me paid
to the board layout. If the board layout isn’t properly designed,
unwanted signals can easily be detected or interference will
be picked up. This section gives guidelines for proper board
layout for the LMH2100.
Electrical signals (voltages/currents) need a finite time to trav-
el through a trace or transmission line. RF voltage levels at
the generator side and at the detector side can therefore be
different. This is not only true for the RF strip line, but for all
traces on the PCB. Signals at different locations or traces on
the PCB will be in a different phase of the RF frequency cycle.
Phase differences in, e.g. the voltage across neighboring
lines, may result in crosstalk between lines, due to parasitic
capacitive or inductive coupling. This crosstalk is further en-
hanced by the fact that all traces on the PCB are susceptible
to resonance. The resonance frequency depends on the trace
geometry. Traces are particularly sensitive to interference
when the length of the trace corresponds to a quarter of the
wavelength of the interfering signal or a multiple thereof.
4.1 Supply Lines
Since the PSRR of the LMH2100 is finite, variations of the
supply can result in some variation at the output. This can be
caused among others by RF injection from other parts of the
circuitry or the on/off switching of the PA.
4.1.1 Positive Supply (V_DD)
In order to minimize the injection of RF interference into the
LMH2100 through the supply lines, the phase difference be-
tween the PCB traces connecting to V_DD and GND should be
minimized. A suitable way to achieve this is to short both con-
nections for RF. This can be done by placing a small decou-
pling capacitor between the V_DD and GND. It should be placed
as close as possible to the V_DD and GND pins of the LMH2100
as indicated in Figure 14. Be aware that the resonance fre-
quency of the capacitor itself should be above the highest RF
frequency used in the application, since the capacitor acts as
an inductor above its resonance frequency.
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