ADL5304ACPZ-R7 datasheet(21/32 Pages) AD

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Data Sheet

ADL5304

Rev. A | Page 21 of 32

The C1D capacitor effectively reduces the bandwidth of the

denominator input stage. A few picofarads of capacitance

(<5 pF) reduce the bandwidth significantly for currents below

approximately 1 µA, though whereas 1 nF to 10 nF are normally

enough to reduce the bandwidth up to the maximum 10 mA of

input current. When measurement speed is of primary importance,

it is better to add filtering after the FET amp outputs, in which

case, C2D, RD, and C3D are the best locations. The resistor in this

case should not be much larger than 1 kΩ because there is a

bias current that is approximately 35 µA that flows from the

temperature compensation block into each of the VDEN and

VNUM pins. Inserting a resistor, as shown in Figure 48, lifts up

the voltages at the INNM and/or INDN pins and potentially causes

headroom problems in the temperature compensation block.

When IDEN is used as the reference, as is normally done, then it

is recommended that C1D is zero, C2D is a 0.1 µF ceramic

decoupling capacitor, RD is a short, and C3D is not placed.

Adding a capacitor, CFB, adds additional filtering at the buffer

output. This capacitor also helps to optimize the pulse response

by placing a zero across the feedback resistor (2.5 kΩ in the

default configuration). A good value to start with is 22 pF, this

introduces a zero at 2.9 MHz that can improve the pulse responses

for input currents above approximately 100 µA.

Photodiode Bias

The ADL5304 provides for adaptive photodiode bias. A

monitoring transistor, Q3, connected in parallel with Q1 (see

Figure 49), samples 1/10th the input current, INUM. This sampled

current is multiplied by a factor of 11 to give an effective output

current at the IMON pin of 1.1 times INUM. Because the photodiode

produces INUM, the additional current has to flow in an external

resistor, RMNTR, equal to 10 × RS, where RS is the value of the

internal parasitic series resistance of the photodiode. This

ensures that the actual junction of the photodiode is biased as

close as possible to 0 V to minimize dark current. The CMON

capacitor provides potential filtering and dynamic currents

during fast transients. The value for best bias response depends

on the photodiode used and should be determined

experimentally. Nominally, CMON = 0.

If the adaptive bias is not used, the IMON pin must be connected

to ground. It is easy to provide a 0.5 V reverse bias across the

diode by using the 2VLT reference and connecting it to the

cathode. Because the ADL5304 forces the voltage at INUM very

close to 1.500 V, the trimmed 2.000 V ensures a precise 0.5 V

reverse bias for the PD.

MONITOR AND

PD BIAS

(1.1× INUM)

VSM2

VSM1

RMNTR

VSM3

VSM4

IREF

IDEN

INUM

100nA

1.5V

VDEN

VNUM

IMON

SHIELD

CMON

FROM 1.5V VREF

Figure 49. Adaptive Photodiode Bias

One example of dual-supply operation is shown in Figure 50, where

the 2.000 V (the 2VLT pin) reference ensures a precisely controlled,

reverse bias across the PD. The user can use other reverse bias

voltages but needs to provide them separately. Note that when the

VSMx pins are grounded, the DCBI and INPS pins must also be

grounded.

MONITOR AND

PD BIAS

(1.1× INUM)

VSM2

VSM1

VSM3

VSM4

IREF

IDEN

INUM

100nA

VDEN

VNUM

IMON

SHIELD

FROM 2.0V

VREF

Figure 50. PD Bias with VSUM at Ground and Using 2.000 V

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