ADL5306

Rev. 0 | Page 11 of 16

APPLICATIONS

The ADL5306 is easy to use in optical supervisory systems and

in similar situations where a wide-ranging current is to be

converted to its logarithmic equivalent (i.e., represented in

decibel terms). Basic connections for measuring a single current

input are shown in Figure 22, which includes various

nonessential components, as will be explained.

TEMPERATURE

COMPENSATION

BIAS

GENERATOR

451

Ω

8k

Ω

12k

Ω

14.2k

Ω

80k

Ω

20k

Ω

200k

Ω

6.69k

Ω

COMM

COMM

COMM

VNEG

VSUM

VPOS

INPT

VREF

NC

IREF

0.5V

+5V

2.5V

0.5V

VOUT

SCAL

BFIN

VLOG

( )

1nA

1k

Ω

1k

Ω

1nF

1nF

1nF

Q2

Q1

10nF

03727-0-022

Figure 22. Basic Connections for Fixed Intercept Use

The 2 V difference in voltage between VREF and INPT, in

reference raises the voltage at VLOG by 0.8 V, effectively

account when estimating the stability of the intercept. Also, the

fixed-intercept applications, there is little benefit in using a large

reference current, since this only compresses the low current

end of the dynamic range when operated from a single supply,

shown here as 5 V. The capacitor between VSUM and ground is

recommended to minimize the noise on this node and to help

provide a clean reference current.

Since the basic scaling at VLOG is 0.2 V/dec and a swing of 4 V

at the buffer output would therefore correspond to 20 decades,

it will often be useful to raise the slope to make better use of the

rail-to-rail voltage range. For illustrative purposes, the circuit in

Figure 22 provides an overall slope of 0.5 V/dec (25 mV/dB).

2.0 V, corresponding to a dynamic range of 60 dB electrical

(30 dB optical) power.

The optional capacitor from VLOG to ground forms a single-

pole low-pass filter in combination with the 4.55 kΩ resistance

frequency is 3.2 kHz. Such filtering is useful in minimizing the

more effective in reducing the total noise. For examples, see the

AD8304 Data Sheet.

The dynamic response of this overall input system is influenced

by the external RC networks connected from the two inputs

(INPT, IREF) to ground. These are required to stabilize the

input systems over the full current range. The bandwidth

changes with the input current due to the widely varying pole

frequency. The RC network adds a zero to the input system to

ensure stability over the full range of input current levels. The

network values shown in Figure 22 will usually suffice, but some

experimentation may be necessary when the photodiode’s

capacitance is high.

Although the two current inputs are similar, some care is

needed to operate the reference input at extremes of current

(<100 nA) and temperature (<0°C). Modifying the RC network

to 4.7 nF and 2 kΩ will allow operation to –40°C at 10 nA. By

representative current levels, the capacitor value can be adjusted

to provide fast rise and fall times with acceptable settling. To

fine-tune the network zero, the resistor value should be

adjusted.

USING A NEGATIVE SUPPLY

Most applications of the ADL5306 require only a single supply

of 3.0 V to 5.5 V. However, to provide further versatility, dual

supplies may be employed, as illustrated in Figure 23.

TEMPERATURE

COMPENSATION

BIAS

GENERATOR

451

Ω

8k

Ω

12k

Ω

14.2k

Ω

80k

Ω

20k

Ω

200k

Ω

6.69k

Ω

COMM

COMM

COMM

VNEG

VSUM

VPOS

INPT

VREF

NC

IREF

0.5V

+5V

2.5V

0.5V

VOUT

SCAL

BFIN

VLOG

( )

1nA

1k

Ω

1k

Ω

1nF

1nF

Q2

Q1

10nF

03727-0-023

Figure 23. Negative Supply Application