High Speed, 200 dB Range,

Logarithmic Converter

Data Sheet

ADL5304

Rev. A

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FEATURES

Optimized for very fast response at all input currents

Overall bandwidth of >4 MHz for inputs >1 µA

Bandwidth: 25 kHz at input of 1 nA and 350 kHz at 10 nA

10 decades of input range: 1 pA to 10 mA

Law conformance: ±0.25 dB from 100 pA to 100 µA

Log ratio or fixed-intercept operation

Precision voltage references and reference current

Adaptive photodiode (PD) bias for low dark current

Programmable log slope and intercept

Default log slope of 10 mV/dB at VLOG pin

Single- or dual-supply operation

APPLICATIONS

High accuracy optical power measurement

Wide range baseband log compression

Versatile detector for high speed APC loops

GENERAL DESCRIPTION

The ADL5304 is a high speed logarithmic converter with fast

response and low noise over a 200 dB (1 pA to 10 mA) measure-

ment range. The ADL5304 provides a nominal logarithmic slope of

10 mV/dB (200 mV/decade); other values are easily configured.

Logarithmic intercept can be programmed over a wide range

with the internal 100 nA current source or externally for log ratio

applications. The default intercept value of 3.162 fA places the

A single positive supply of 5 V is all that is required for operation

over a specified 1 pA to 3 mA input range. Dual-supply operation

extends the specified input current range to 10 mA.

The ADL5304 accepts two current inputs to the logarithmic

NPN transistor, connected in a feedback path around a low offset

same way, which allows for log ratio operation. The input summing

nodes (INUM and IDEN) operate at a constant default voltage of

1.5 V. The VSM1 to VSM4 pins flank the INUM and IDEN inputs

to provide a guard voltage to minimize leakage currents.

Adaptive photodiode biasing is provided for optical measurements.

1st order keeps the internal PD junction at 0 V to minimize

dark current.

The VLOG output is buffered and can be rescaled through

to 10 mA. This corresponds to 0.5 V to 2.5 V at the VLOG pin

in the default configuration shown in Figure 1.

Accurate 1.5 V (Pin 1P5V) and 2.0 V (Pin 2VLT) reference outputs

allow precise repositioning of the intercept using external resistors.

The ADL5304 is available in a 32-lead, 5 mm × 5 mm LFCSP

and specified for operation from −40°C to +85°C.

SIMPLIFIED BLOCK DIAGRAM

ADL5304

2

30

3

4

5

6

8

7

MONITOR AND

PD BIAS

TEMPERATURE

COMPENSATION

1.5V

1.5V

32

31

26

9

IMON

VNUM

INNM

VDEN

HFCP

COMM NMFS VNEG DNFS

INDN

1P5V

2VLT

DCBI

BSDC

VSM1

VSM2

VSM3

IREF

IDEN

INUM

VSM4

10

11

27

29

28

100nA

1.5V

17

24

2V

BIAS AND

12

13

14

ACOM

15

16

INPS

INMS

VLOG

23

22

5kΩ

5kΩ

1kΩ

1kΩ

7.5kΩ

21

20

19

18

SCL1

SCL2

SCL3

ACOM

VPOS

Figure 1.