LTC5540 datasheet(12/18 Pages) AD

12 / 18 page

LTC5582

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 4. Differential Input Configuration

Figure 5. Single-Ended-to-Differential Conversion

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.

RF INPUT

IN–

IN+

VCC

1:8

5582 F04

200

LTC5582

DEC

50pF

Figure 6. RF Input Return Loss

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

bandwidthiscorrespondinglywider.Thefollowingformulas

are provided to calculate the input matching network for

single-ended-to-differentialconversionatlowRFfrequency

(i.e., below 1GHz).

CS1=CS2 =

πfc 50RIN

= 2.25•10

(pF)

LM =

50RIN

2πfc

= 2.25•10

(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.

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.

RF INPUT

CS1

6.8pF

CS2

6.8pF

TO IN–

TO IN+

5582 F05

MATCHING NETWORK

66nH

FREQUENCY (MHz)

–5

–10

–15

–20

–25

–30

5582 F06

1000

900

800

700

600

500

400

300

200

100

RF INPUT POWER (dBm)

–75 –65

–25

–45

–5

5582 F07

–35

–55

–15

SINGLE-ENDED-TO-

DIFFERENTIAL INPUTS

SINGLE-ENDED

2.8

2.4

1.6

0.8

2.0

1.2

0.4

Figure 7. Output Voltage vs RF Input Power

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