ADL5303 Data Sheet
Rev. A | Page 18 of 24
EVALUATION BOARD
An evaluation board is available for the ADL5303, the sche-
matic for which is shown in Figure 31, and the board layout
is shown in Figure 32 and Figure 33. It can be configured for
a wide variety of experiments. The board is factory set for
photoconductive mode with a buffer gain of unity, providing a
slope of 10 mV/dB and an intercept of 100 pA. By substituting
resistor and capacitor values, all of the application circuits
presented in this data sheet can be evaluated.
The system is completed by the final buffer amplifier, which is
an uncommitted op amp with a rail-to-rail output capability,
a 10 MHz bandwidth, and good load driving capabilities. The
buffer can be used to implement multipole low-pass filters for
noise reduction. The buffer also facilitates modification of the
output scaling and the intercept point using simple resistor
divider networks and the 2 V output provided by the VREF pin.
SHIELDS AND GUARDS
Reducing errors from external sources in a current sensing
circuit requires a different approach then 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 ground from the INPT input with a V
SUM
set
to the default 0.5 V generates a 0.5 nA offset. The ADL5303
evaluation board makes extensive use of guards to reduce the
effects of leakage at low input levels. It is important to carefully
handle and clean the ADL5303 evaluation board to prevent
contaminants from handling or improper washing of the
PCB causing leakage currents. Circuit board designs for
the ADL5303 must connect the EPAD to the VSUM pins
to provide a continuous guard around the sensitive INPT
pin to reduce the influence of surface contaminants.
A common mistake for those unfamiliar with low level current
sensing is to attach a high impedance scope probe or meter to
measure the input for debug. This can cause significant error,
as the typical 1M ~ 100 MΩ impedance of these probes sources/
sinks current from the input, depending on their bias.
In instrumentation applications where measurements <1 nA are
required, the use of triaxial cables and connectors is common to
reduce leakage through the insulating dielectric by carrying a
continuous guard from current source to sensing circuit on the
intermediate conductor. This type of guarding circuit is differ-
ent from a conventional electrostatic shield used in voltage
sensing applications. An electrostatic shield relies on low
impedance and the ability to flow current freely to minimize
voltage induced on the shield that can capacitively couple into
a high impedance input. A guard is actively driven to the same
voltage as the current carrying center conductor eliminating
leakage through the dielectric between the center conductor
and the guard. The guard does not flow current other than the
leakage from the guard to the outer shield. The guard is usually
connected to a single end of the cable only because any signifi-
cant current flow through the guard can couple inductively
to the center conductor. Using the ADL5303 evaluation board,
the guard can be driven either from the guard of an external
current source or from the internal VSUM bias of the ADL5303.
The ADL5303 evaluation board can bias the shield of a coaxial
cable connected to the INPT input to the nominal V
SUM
voltage
with Switch S1 but this requires careful consideration of the
environment on the other side of the cable. For example if
the ADL5303 evaluation board is configured for V
SUM
= 0.5 V
connecting the other end of the INPT coaxial cable to an
instrument with a ground referenced shield pulls V
SUM
to
ground and collapses the input stage of the ADL5303. Floating
the current source end of the shield provides a low leakage
guard but a separate return path for the signal current must
then be provided. If cable dielectric leakage is not a concern,
the INPT can be connected directly to a coaxial cable with
the shield providing a signal ground.
