LT5537 データシートの表示（PDF） - Linear Technology

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LT5537

Wide Dynamic Range RF/IF Log Detector

Linear Technology 

LT5537

APPLICATIO S I FOR ATIO

Dynamic Range

The LT5537 is capable of detecting and log-converting an

input signal over a wide dynamic range. The range of the

output voltage may be limited, however, and the monoto-

nicity of the output versus input at high input level may be

affected if the supply voltage is low and the log-linear slope

is set too high. The minimum VCC to support 90dB

dynamic range with 20mV/dB slope is 2.8V under nominal

conditions at 25°C. The data shown in the Typical Perfor-

mance Characteristics plots was taken with VCC = 3V. If

there is difficulty encountered in achieving the desired

dynamic range, then the user is advised to increase the

supply voltage or else to decrease the output slope by

connecting a smaller valued resistor between the output

and ground.

VCC

CAP+ CAP–

7k 7k

IN+

IN–

VBIAS

TO 2ND

STAGE

5537 F04

Figure 3. Simplified Input Circuit

Figure 4. Input Admittance

Input Matching

The LT5537 has a high impedance input (Figure 3). The

differential input impedance is derived from S11 measure-

ment with one of the input pins AC grounded (Figure 4). At

200MHz, the input is equivalent to 1.73k//1.45pF (Table 1).

The input dynamic range is constant in voltage terms,

ranging from approximately –89dBVrms to 1dBVrms at

200MHz. The dynamic range expressed in power is

dependent on the actual impedance selected in the ap-

plication design.

Table 1. Parallel Equivalent RC of the LT5537 Input

FREQUENCY

100MHz

200MHz

400MHz

600MHz

800MHz

1000MHz

R

1.85kΩ

1.73kΩ

1.07kΩ

673Ω

435Ω

303Ω

C

1.51pF

1.45pF

1.48pF

1.52pF

1.65pF

1.78pF

The simplest way of input matching the LT5537 is to

terminate the input signal with a 50Ω resistor and AC

couple it to one of the input pins while AC grounding the

other input pin (Figure 13). The sensitivity (defined as the

minimum input power required for the output to be within

3dB of the ideal log-linear response) is –76.4dBm at

200MHz in this case.

To achieve the best sensitivity, the input termination

impedance should be increased and the input pins should

be differentially driven. An example application circuit is

shown in Figure 5 which uses a transformer to step up the

impedance and perform the balun function. The 240Ω

resistor (R2) sets the impedance at the input of the chip to

200Ω. A 1:4 transformer is used to match the 50Ω signal

source impedance to the circuit input impedance. C1 and

C2 are DC blocking capacitors. This application circuit has

a (3dB error) sensitivity of –82.4dBm at 200MHz.

J1

INPUT

M/A-COM

C1

ETC4-1-2

N/C

R2

240Ω C2

(1:4)

IN+

2

IN–

3

5537 F06

Figure 5. Differential Input Matching to 200Ω

5537fa

9

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