ADL5303

Data Sheet

Rev. 0 | 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

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

with Switch S1 but this requires careful consideration of the

environment on the other side of the cable. For example if

connecting the other end of the INPT coaxial cable to an

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.