In many fiber applications, OUT is connected directly to

an ADC that operates from a supply voltage that is less

than

the

voltage

at

BIAS.

Connecting

the

MAX4004/MAX4006’s clamping diode output, CLAMP,

to the ADC power supply helps avoid damage to the

ADC. Without this protection, voltages can develop at

OUT that can destroy the ADC. This protection is less

critical when OUT is connected directly to subsequent

transimpedance amplifiers (linear or logarithmic) that

have low-impedance, near-ground-referenced inputs. If

a transimpedance amp is used on the low side of the

photodiode, its voltage drop must also be considered.

Leakage from the clamping diode is most often insignif-

icant over nominal operating conditions, but grows with

temperature.

To maintain low levels of wideband noise, lowpass filter-

ing the output signal is suggested in applications where

only DC measurements are required. Determining the

required filtering components is straightforward, as the

MAX4004 exhibits a very high output impedance

(>5M

Ω), while the MAX4006 exhibits an output resis-

tance of 10k

Ω.

In some applications where pilot tones are used to iden-

tify specific fiber channels, higher bandwidths are

desired at OUT to detect these tones. Consider the mini-

mum and maximum currents to be detected, then con-

sult the frequency response and noise typical operating

curves. If the minimum current is too small, insufficient

monitor bandwidth could result, while too high of a cur-

rent could result in excessive monitor noise across the

desired bandwidth. See the MAX4009 family for devices

intended for fiber applications using pilot tones.

Bypassing and External Components

In applications where power-supply noise can interfere

with DC diode measurements, additional filtering is sug-

gested. Such noise is commonly seen when switching

power supplies are used to generate the photodiode

bias voltage. As shown in the typical operating circuit, a

pi filter (two 0.22µF capacitors and one 2.2µH inductor)

greatly suppress power-supply switching noise. If such

a filter is already present in the bias-generating circuit,

only a simple bypass capacitor at the BIAS pin is sug-

gested. The output lowpass filter, a 10k

Ω resistor and a

10nF capacitor, further reduce permeating power-sup-

ply noise as well as other wideband noise that might

otherwise restrict measurements at low signal levels.

Again, reducing the bandwidth of the OUT signal can

affect performance of pilot-tone systems.

To restrict high-frequency photodiode signals from

affecting the current monitor and BIAS power supply, an

RF choke and 10nF capacitor can be added. The

capacitance presented to REF should not exceed 10nF;

larger values increase startup time and could cause the

thermal shutdown circuit to activate during startup.

High-Accuracy, High-Side Current Monitors

in SOT23

8

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