LMV221SDX National Semiconductor, LMV221SDX Datasheet - Page 27

• Manufacturer Part Number: LMV221SDX
• Manufacturer: National Semiconductor

Specifications of LMV221SDX

• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 6
• Package Type: LLP EP
• Lead Free Status / RoHS Status: Not Compliant

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 LMV221 (Figure 13).
It can be seen that the output of the amplifier is set by the
detection current I
the reference voltage V
V
I
RF input power. The equation shows that temperature varia-
tions in V
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
A differential topology also connects the reference pin, which
is the value of reference voltage V
- V
Differential: V
The resulting equation doesn’t contain the reference voltage
V
age are therefore not measured by the ADC.
3.4.3 Output Behavior in Shutdown
In order to save power, the LMV221 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 LMV221 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 LMV221 and a low pass output filter (see Figure 11, left
side), as discussed below.
In active mode, the filter capacitor C
voltage of the LMV221 — which in this mode has a low output
DET
OUT
REF
REF
represents the detector current that is proportional to the
anymore. Temperature variations in this reference volt-
= I
:
DET
FIGURE 13. Output Stage of the LMV221
REF
R
are also present in the output V
TRANS
ADC
ADC
= V
DET
+ V
= I
OUT
multiplied by the resistor R
DET
REF
REF
- V
R
:
REF
TRANS
= I
+ V
REF
DET
S
is charged to the output
REF
. The ADC reads V
R
TRANS
OUT
20173778
. In case of a
TRANS
plus
OUT
27
impedance to enable fast settling. During shutdown-mode,
the capacitor should preserve this voltage. Discharge of C
through any current path should therefore be avoided in shut-
down. The output impedance of the LMV221 becomes high
in shutdown, such that the discharge current cannot flow from
the capacitor top plate, through R
pin to GND. This is realized by the internal shutdown mech-
anism of the output amplifier and by the switch depicted in
Figure 13. Additionally, it should be ensured that the ADC in-
put 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 LMV221.
Electrical signals (voltages / currents) need a finite time to
travel 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 neighbor-
ing lines, may result in crosstalk between lines, due to para-
sitic capacitive or inductive coupling. This crosstalk is further
enhanced by the fact that all traces on the PCB are suscep-
tible to resonance. The resonance frequency depends on the
trace geometry. Traces are particularly sensitive to interfer-
ence 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 LMV221 is finite, variations of the sup-
ply 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
In order to minimize the injection of RF interference into the
LMV221 through the supply lines, the phase difference be-
tween the PCB traces connecting to V
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
as close as possible to the V
Due to the presence of the RF input, the best possible position
would be to extend the GND plane connecting to the DAP
slightly beyond the short edge of the package, such that the
capacitor can be placed directly to the V
aware that the resonance frequency of the capacitor itself
should be above the highest RF frequency used in the appli-
cation, since the capacitor acts as an inductor above its
resonance frequency.
DD
)
DD
DD
and GND pins of the LMV221.
and GND. It should be placed
S
, and the LMV221's OUT
DD
DD
and GND should be
pin (Figure 14). Be
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