AD8305

Data Sheet

Rev. C | Page 14 of 24

APPLICATIONS INFORMATION

The AD8305 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, which is represented in decibel

terms. Basic connections for measuring a single-current input

are shown in Figure 35, which also includes various nonessential

components.

BIAS

GENERATOR

VLOG

COMM

VNEG

VSUM

IREF

0.5V

80k

Ω

0.5V

TEMPERATURE

COMPENSATION

6.69k

Ω

Q1

COMM

20k

Ω

451

Ω

VREF

VRDZ

14.2k

Ω

Q2

INPT

COMM

2.5V

VPOS

BFIN

SCAL

VOUT

200k

Ω

1nA

1nF

1nF

1k

Ω

1nF

1k

Ω

12k

Ω

+5V

10nF

8k

Ω

–

+

Figure 35. Basic Connections for Fixed Intercept Use

The 2 V difference in voltage between the VREF and INPT pins

VRDZ to VREF raises the voltage at VLOG by 0.8 V, effectively

over 1 mA, may be used. The effect of such changes is shown in

Figure 5.

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

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,

here shown 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.

Because the basic scaling at VLOG is 0.2 V/decade, and a swing

of 4 V at the buffer output would correspond to 20 decades, it is

often useful to raise the slope to make better use of the rail-to-rail

voltage range. For illustrative purposes, the circuit in Figure 35

provides an overall slope of 0.5 V/ decade (25 mV/dB). Thus,

corresponding to a dynamic range of 120 dB (electrical, that is,

60 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 3.5 kHz. Such filtering is useful in minimizing the

more effective in reducing the total noise; examples are provided in

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 35 usually suffice, but some

experimentation may be necessary when the photodiode

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 is recommended for measuring 10 nA at −40°C.

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.