LTC5587
13
5587f
Operation
The LTC5587 combines a proprietary high-speed power
detector with an internal 150kHz lowpass averaging filter
and a true 12-bit successive approximation ADC with a
serial output interface. It can accurately measure the RMS
power of high crest-factor modulated RF signals. The
output voltage of the RF power detector is converted to
a 12-bit digital word that is directly proportional to the
average RF input power in dBm. The part can be oper-
ated from a single supply or dedicated supplies, allowing
the user to select a specific voltage range for the ADC
conversion in addition to interfacing with 1.8V, 2.5V, or
3V digital systems.
Evaluation
Figure 1 shows the simplified evaluation circuit schematic,
and Figure 2 shows the associated board artwork. To en-
sure proper oper ation, good grounding prac tice should be
followed in the board layout, with liberal placement of vias
under the exposed pad of the package and around signal
and digital lines. The evaluation board shown in Figure 2
contains additional support circuitry not shown in Figure 1
that includes an optional 3.3V regulator for the V
DD
, OV
DD
,
and V
CC
supplies and an optional 1.8V regulator for the V
REF
reference. This onboard reference provides good accuracy
(less than ±5mV) over temperature, contributing less than
±0.1dB error to the ADC output. To evaluate the digital
output, the QuickEval PC-based software can be used with
the DC590B USB controller interface board. This board
contains a generic USB to serial peripheral interface (SPI)
controller. A 14-pin ribbon cable connects the evaluation
board to the DC590B board. The DC590B allows the evalu-
ation at approximately a 200Hz sample rate (f
SMPL
). (See
http://cds.linear.com/docs/Reference%20Design/dc590B.
pdf). For higher sample rates the digital I/O pins can be
accessed directly on the board. Contact LTC Applications
for more information on higher sample rate evaluation.
RF Input Matching
The input resistance is about 205Ω. Input capacitance
is 1.6pF. The impedance vs frequency of the RF input is
detailed in the following table.
APPLICATIONS INFORMATION
A shunt 68Ω resistor can be used to provide a broadband
match at low frequencies up to 1GHz and from 4.5GHz to
6GHz. As shown in Figure 3, a nominal broadband input
m a t c h c a n b e a c h i e ve d u p t o 1. 8 G H z b y u si n g a n L C m a t c h -
ing circuit consisting of a series 3.3nH inductor (L1) and
a shunt 1.8pF capacitor (C1). This match will maintain a
return loss of about 10dB across the band. For matching
at higher frequencies, L1 and C1 values are listed in the
table of Figure 1. The input reflection coefficient referenced
to the RF input pin with no external components is shown
on the smith chart in Figure 4. Alternatively, it is possible
to match using an impedance transformation network by
omitting R1 and transforming the 205Ω input to 50Ω. This
narrow band matching will improve sensitivity up to about
6dB max, and the dynamic range remains the same. For
example: by omitting R1 and setting L1 = 1.8nH and C1 =
3pF, a 2:1 VSWR match can be obtained from 1.95GHz to
2.36GHz with a sensitivity improvement of 5dB.
Table 1. RF Input Impedance
FREQUENCY
(MHz)
INPUT
IMPEDANCE (Ω)
S11
MAG ANGLE (°)
10 203.3-j1.4 0.605 –0.7
50 201.8-j7.0 0.605 –3.7
100 197.2-j13.7 0.606 –7.3
200 161.9-j25.8 0.608 –14.6
400 142.5-j43.6 0.614 –28.9
500 125.3-j48.5 0.619 –35.8
800 88.0-j60.4 0.636 –55.6
900 79.2-j62.6 0.643 –61.8
1000 71.8-j64.3 0.650 –67.7
1500 46.6-j68.8 0.685 –94.3
2000 31.1-j69.2 0.715 –116
2100 29.9-j69.0 0.721 –119.9
2500 22.4-j66.8 0.739 –134.1
3000 15.3-j60.7 0.756 –149.6
3500 9.9-j47.3 0.768 –163.2
4000 6.6-j16.9 0.779 –175.5
5000 9.8-j51.7 0.787 162.1
6000 18.5-j69.4 0.792 141.4
