LT5570IDD-PBF LINER [Linear Technology], LT5570IDD-PBF Datasheet - Page 12

• Manufacturer Part Number: LT5570IDD-PBF
• Description: Fast Responding, 40MHz to 2.7GHz Mean-Squared Power Detector
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT5570
The DEC pin can be tied to the IN
and ac-coupled to ground while the RF signal is applied
to the IN
side of the inputs with a 100Ω resistor to ground in front
of the ac-blocking capacitor and coupling the other side
to ground using a 1nF capacitor, a broadband 50Ω input
match can be achieved with typical input return loss better
than 12dB from 40MHz to 2.7GHz.
Since there is no voltage conversion gain from imped-
ance transformation in this case, the sensitivity of the
detector is reduced by 6dB. The linear dynamic range is
reduced by the same amount correspondingly as shown
in Figure 6.
External Filtering (FLTR) Capacitor C1
This pin is internally biased at V
from voltage supply V
LT5570, an external capacitor C1 with a value of 22nF or
higher is required to connect the FLTR Pin to V
connect this fi ltering capacitor to ground or any other
low voltage reference at any time to avoid an abnormal
start-up condition.
12
Figure 6. Output Voltage and Linearity Error vs
RF Input Power in Single-ended Input Confi guration
–
(or IN
2.4
2.0
1.2
0.0
1.6
0.8
0.4
–50
+
) Pin. By simply terminating the signal
–40
CC
. To assure stable operation of the
–30
INPUT POWER (dBm)
–20
CC
–10
– 0.13V via a 2k resistor
+
(or IN
0
500MHz
880MHz
2140MHz
2700MHz
5570 F06
10
–
) Pin directly
CC
. Don’t
C1’s value has a dominant effect on the output transient
response. The lower the capacitance, the faster the output
rise and fall times as illustrated in Figure 7. For signals
with AM content such as W-CDMA, ripple can be observed
when the loop bandwidth set by C1 is close to the modu-
lation bandwidth of the signal. A 4-carrier W-CDMA RF
signal is used as an example in this case. The trade-offs
of residual ripple vs. output transient time are also as
shown in Figure 7.
In general, the LT5570 output ripple remains relatively
constant regardless of the RF input power level for a fi xed
C1 and modulation format of the RF signal. Typically, C1
must be selected to average out the ripple to achieve the
desired accuracy of RF power measurement. For a two-tone
RF signal with equal power applied to the LT5570 input,
Figure 8 shows the variation of the output dc voltage and
its RMS value of the residual ac voltage as a function of
the delta frequency. Both values are referred to dB by
normalizing them to the output slope (about 37mV/dB).
In this measurement, C1 = 22nF. Increasing C1 will shift
both curves toward a lower frequency.
Figure 7. Residual Ripple, Output Transient
Times vs. Filtering Capacitor C1
40
35
30
25
20
15
10
5
0
0
EXTERNAL FILTERING CAPACITOR C1 (μF)
0.1
0.2
0.3
0.4
AT 2140MHZ, P
0.5
0.6
RESIDUAL RIPPLE
RISE TIME
FALL TIME
0.7
IN
0.8
= 10dBm
0.9
5570 F07
1.0
320
280
240
200
160
120
80
40
0
5570f