LT5538

8

5538f

The LT5538 is a 40MHz to 3.8 GHz logarithmic RF power

detector. It consists of cascaded limiting amplifiers and

RF detectors. The output currents from every RF detector

are combined and low-pass filtered before applied to the

output buffer amplifier. As a result, the final DC output

voltage approximates the logarithm of the amplitude of the

input signal. The LT5538 is able to accurately measure an

RF signal over a 70dB dynamic range (–68dBm to 2dBm

at 2.1GHz) with 50Ω single-ended input impedance. The

slope of linear to log transfer function is about 17.7mV/dB

at 2.1GHz. Within the linear dynamic range, very stable

output is achieved over the full temperature range from

–40°C to 85°C and over the full operating frequency

range from 40MHz to 3.8GHz. The absolute variation over

temperature is typically within ±1dB over 65dB dynamic

range at 2.1GHz.

RF INPUT

The simplified schematic of the input circuit is shown in

20pF capacitor. An external capacitor of 1nF is needed to

connect this pin to ground for low frequency operation.

to ground or other matching components. A 56Ω resistor

(R1) connected to ground will provide better than 10dB

input return loss over the operating frequency range up

to 1.5GHz. At higher operating frequency, additional LC

matching elements are needed for a proper impedance

matching to a 50Ω source as shown in Figure 2. Refer to

Figure 6 for the circuit schematic of the input matching

network. The input impedance vs frequency of the RF input

Table 1. RF Input Impedance

FREQUENCY

(MHz)

RF INPUT

IMPEDANCE (Ω)

S11

MAG

ANGLE(°)

40

47.3 + j129.7

0.800

38.5

100

246.6 + j210.7

0.790

11.5

200

408.7 – j37.8

0.785

–1.5

400

192.9 – j190.9

0.772

–14.9

600

105.6 – j158.4

0.756

–25.3

800

69.3 – j127.4

0.737

–34.4

1000

51.8 – j106.2

0.720

–42.7

1200

41.5 – j90.9

0.707

–50.6

1400

34.2 – j78.7

0.697

–58.2

1600

29.2 – j60.0

0.687

–65.6

1800

25.4 – j60.7

0.678

–73.1

2000

22.6 – j53.8

0.669

–80.4

2200

20.5 – j47.7

0.659

–87.7

2400

18.9 – j42.4

0.649

–94.6

2600

17.9 – j37.6

0.638

–101.5

2800

17.1 – j33.4

0.627

–108.2

3000

16.4 – j29.5

0.615

–114.7

3200

16.1 – j26.0

0.602

–121.0

3400

15.9 – j22.8

0.589

–127.0

3600

15.9 – j20.0

0.574

–132.8

3800

15.9 – j17.5

0.560

–137.9

APPLICATIONS INFORMATION

5.3k

394

20p

5.3k

+

–

5538 F01

Figure 1. Simplified Schematic of the Input Circuit

Figure 2. Input Return Loss with Additional LC Matching Network

5538 F02

FREQUENCY (GHz)

0

0.4 0.8 1.2 1.6

2

2.4 2.8 3.2 3.6 4.0

–30

–25

–20

–15

–10

0

–5

W/O L1 AND C8

L1 = 1.5nH, C8 = 1pF

C4, C11 = 12pF, C8 = 0.7pF