ADL5310

Rev. A | Page 13 of 20

APPLICATIONS

TEMPERATURE

COMPENSATION

REFERENCE

GENERATOR

451

Ω

14.2k

Ω

80k

Ω

665k

Ω

665k

Ω

20k

Ω

1k

Ω

1nF

1k

Ω

1nF

2k

Ω

4.7nF

2k

Ω

4.7nF

1nF

6.69k

Ω

12k

Ω

8k

Ω

8k

Ω

10 nF

10 nF

COMM

COMM

VREF

VNEG

COMM

VREF

VRDZ

VPOS

VNEG

VSUM

INP2

IRF2

2.5V

0.5V

OUT2

SCL2

BIN2

LOG2

TEMPERATURE

COMPENSATION

451

Ω

14.2k

Ω

6.69k

Ω

12k

Ω

COMM

5V

VNEG

VSUM

INP1

IRF1

OUT1

SCL1

BIN1

LOG1

)

1nA

)

1nA

Figure 34. Basic Connections for Fixed Intercept Use

The ADL5310 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—that is, represented

in decibel terms. Basic connections for measuring a single

current at each input are shown in Figure 34, which also

includes various nonessential components, as explained next.

The 2 V difference in voltage between the VREF and Input Pins

INP1 and INP2, in conjunction with the external 665 kΩ resis-

into Pins IRF1 and IRF2. Connecting VRDZ to VREF raises the

voltage at LOG1 and LOG2 by 0.8 V, effectively lowering each

used. The effect of such changes is shown in Figure 5 and

Figure 8.

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

In fixed-intercept applications there is little benefit in using a

large reference current, because doing so only compresses the

low-current-end of the dynamic range when operated from a

single supply. The capacitor between VSUM and ground is

strongly recommended to minimize the noise on this node, to

reduce channel-to-channel crosstalk, and to help provide clean

reference currents.

In addition, each input and reference pin (INP1, INP2, IRF1,

and IRF2) has a compensation network made up of a series

resistor and capacitor. The junction capacitance of the photo-

diode along with the network capacitance of the board artwork

around the input system creates a pole that varies widely with

input current. The RC network stabilizes the system by simul-

taneously reducing this pole frequency and inserting a zero to

compensate an additional pole inherent in the input system. In

general, the 1 nF, 1 kΩ network handles almost any photodiode

interface. In situations where larger active area photodiodes are

used, or when long input traces are used, the capacitor value

may need to be increased to ensure stability. Although the signal

and reference input systems are similar, additional care is

required to ensure stable operation of the reference inputs at

It is recommended that filter components of 4.7 nF and 2 kΩ

should be used from Pin IRF1 (IRF2) to ground. Temperature-

stable components should always be used in critical locations

such as the compensation networks; Y5V-type chip capacitors

are to be avoided due to their poor temperature stability.