ADL5308ACCZ-R7 datasheet(15/22 Pages) AD

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Data Sheet

ADL5308

APPLICATIONS INFORMATION

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Rev. 0 | 15 of 22

INTERFACES DESCRIPTION

INP and IREF Interface

The photocurrent input flows into the INP pin connected to the

anode of the PD, which can operate at a maximum current of 25

mA. An internal reference current input sets the IREF through an

I2C. An external reference current is also an option to connect to

the IREF pin, which requires disabling the internal IREF. The IREF

can operate at a maximum current of 5 mA.

Figure 31. Simplified INP and IREF Interface

SUM Interface

A large voltage difference between nodes can cause a significant

leakage current, even if the impedance between the nodes is

relatively high. Guarding reduces the errors due to leakages. The

concept of guarding is to surround the high-impedance conductor

with another conductor (guard) driven to the same voltage potential.

If there is no voltage across the insulation resistance (between the

high-impedance conductor and guard), there cannot be any current

flowing through it.

Reducing errors from external sources in a current-sensing circuit

requires a different approach than the voltage sensing input of the

typical high-impedance op amp circuit. Leakage can be a significant

source of error for highly sensitive log amps, especially at the low

end of their range. For example, a 1 GΩ leakage path to the ground

from the current lines INP and IREF with a VSUM set to the default

1.53 V generates a 1.53 nA offset.

The ADL5308 uses SUM node as guard pins, which shield the

input current lines INP and IREF. It has an internal 1 kΩ resistor

connected from a voltage reference buffer (VREF) to the SUM node.

Figure 32. Simplified SUM Interface

VLOG and FB Interface

The Logarithmic voltage-output (VLOG) changes logarithmically

with the current applied to INP and IREF. The output can be

buffered using the log output amplifier as the VLOG pin shorted

to the feedback (FB) pin. The nominal slope is 200 mV/dec in the

range of 1 nA to 10 mA of the current input over the entire operating

temperature with an internal trimmed IREF value as shown in

Figure 4.

The logarithmic slope can be changed by connecting external gain

resistors RA and RB as shown in Figure 33. For example, for equal

resistors between FB and VLOG and FB to ground, the gain is 2.

Figure 33. Simplified Schematic of VLOG and FB Interface

Figure 35 shows the logarithmic output response for a 220 µA to

10 nA input current transition (an 87 dB step in magnitude). Two

curves are shown: one without any low-pass filter at the VLOG

output and one with a 220 Ω/1.2 nF low-pass filter network at the

VLOG output. As can be seen, the variation due to noise is lower

with the low-pass filter network but its response is slightly slower.

Figure 34. Simplified Schematic of the Low-Pass Filter Network at the VLOG

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