MAX4007/MAX4008
High-Accuracy, 76V, High-Side
Current Monitors in SOT23
_______________________________________________________________________________________ 7
typ) protects the device against short-circuit-to-ground
conditions, and a thermal shutdown feature reduces
both the reference current and the monitor current to
zero if the die temperature reaches +150°C.
The MAX4007/MAX4008 accept a supply voltage of
+2.7V to +76V, suitable for APD photodiode applica-
tions. A clamping diode, shown in the Functional
Diagram, is provided to protect subsequent output cir-
cuitry from an overvoltage condition.
Applications Information
Clamping the Monitor Output Voltage
CLAMP provides a means for diode clamping the volt-
age at OUT; thus, V
OUT
is limited to V
CLAMP
+ 0.6V.
CLAMP can be connected to either an external supply,
to BIAS, or may be left floating if voltage clamping is
not required.
Using APD or PIN Photodiodes in Fiber
Applications
When using the MAX4007/MAX4008 to monitor APD or
PIN photodiode currents in fiber applications, several
issues must be addressed. In applications where the
photodiode must be fully depleted, keep track of volt-
ages budgeted for each component with respect to the
available supply voltage(s). The current monitors require
as much as 1.1V between BIAS and REF, which must be
considered part of the overall voltage budget.
Additional voltage margin can be created if a negative
supply is used in place of a ground connection, as long
as the overall voltage drop experienced by the
MAX4007/MAX4008 is less than or equal to 76V. For
this type of application, the MAX4007 is suggested so
the output can be referenced to “true” ground and not
the negative supply. The MAX4007’s output current can
be referenced as desired with either a resistor to
ground or a transimpedance amplifier. Take care to
ensure that output voltage excursions do not interfere
with the required margin between BIAS and OUT. 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
MAX4007/MAX4008s’ clamping diode output, CLAMP,
to the ADC power supply helps avoid damage to the
ADC. Without this protection, voltages can develop at
OUT that might 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 fil-
tering the output signal is suggested in applications
where only DC measurements are required.
Determining the required filtering components is
straightforward, as the MAX4007 exhibits a very high
output impedance (>5MΩ), while the MAX4008 exhibits
an output resistance of 10kΩ.
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
bandwidth could result, while too high a current could
result in excessive noise across the desired bandwidth.
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 induc-
tor) greatly suppresses power-supply switching noise. If
such a filter is already present in the bias generating cir-
cuit, only a simple bypass capacitor at the BIAS pin is
suggested. The output lowpass filter, a 10kΩ resistor
and a 10nF capacitor, further reduce permeating power-
supply 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 monitors 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.
