LTC5582 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC5582' PDF : 16 Pages View PDF
LTC5582
Applications Information
while the overall dynamic range remains the same. At
high frequency, additional LC elements may be needed
for the input impedance matching due to the parasitics
of the transformer and PCB traces.
J1
RF INPUT
LTC5582
IN+
2
DEC
3
T1
1:8
IN–
4
VCC
200Ω
50pF
200Ω
RF INPUT
MATCHING NETWORK
CS1
6.8pF
CS2
6.8pF
LM
66nH
TO IN+
TO IN–
5582 F05
Figure 5. Single-Ended-to-Differential Conversion
0
–5
–10
5582 F04
Figure 4. Differential Input Configuration
Due to the high input impedance of the LTC5582, a narrow
band L-C matching network can be also used to convert a
single-ended input to differential signal as shown in Figure
5. By this means, the sensitivity and overall linear dynamic
range of the detector will be very similar to the one using 1:8
RF input balun. The conversion gain is strongly dependent
on the loss (or Q) of the matching network, particularly at
high frequency. The lower the Q, the lower the conversion
gain. However, the matching bandwidth is correspondingly
wider. The following formulas are provided to calculate the
input matching network for single-ended-to-differential
conversion at low RF frequency (i.e., below 1GHz).
CS1 = CS2 = πfc
1
50RIN
=
2.25 • 109
fc
(pF)
–15
–20
–25
–30
0
100 200 300 400 500 600 700 800 900 1000
FREQUENCY (MHz)
5582 F06
Figure 6. RF Input Return Loss
2.8
2.4
SINGLE-ENDED-TO-
DIFFERENTIAL INPUTS
2.0
1.6
SINGLE-ENDED
1.2
0.8
LM =
50RIN
2πfc
=
2.25 • 1010
fc
(nH)
where RIN is the differential input resistance (400Ω) and
fc is the center RF operating frequency.
As an example, Figure 6 shows that good input return
loss is achieved from 300MHz to 400MHz when Cs1= Cs2
= 6.8pF and LM = 66nH. Figure 7 show the sensitivity is
also improved by 8dB at 350MHz while the dynamic range
remains the same.
0.4
–75 –65 –55 –45 –35 –25 –15 –5 5
RF INPUT POWER (dBm)
5582 F07
Figure 7. Output Voltage vs RF Input Power
Although these equations give a good starting point,
it is usually necessary to adjust the component values
after building and testing the circuit. As the RF operating
frequency increases, the real values of CS1, CS2, LM will
deviate from the above equations due to parasitics of the
components, device and PCB layout.
For a 50Ω input termination, the approximate RF input
power range of the LTC5582 is from –60dBm to 2dBm,
5582f
11
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