Data Sheet

ADA4352-2

analog.com

Rev 0

43 of 47

introduce less noise, but also make sure that this RC combination allows the signal to settle within the acquisition

time specified for the desired sample rate of the ADC.

As in the section on RC Tolerancing and Effect of External RC Filter, assume REXT = 200 Ω and CEXT = 180 pF. For the RF0

= 315 Ω gain setting, ADA4352-2’s dominant pole lies beyond the external RC filter’s bandwidth. The RC filter usually

limits the system bandwidth and thus also the noise bandwidth. For the rest of the RFX gain settings, the poles

created by ADA4352-2’s internal resistors and their respective compensation capacitors roll off at a much lower

frequency than the external RC filter. For these higher RFX settings, this external RC filter does not contribute to

noise reduction. See Table 9 for minimum, nominal, and maximum bandwidths for each gain setting.

Using the AD4696 Easy Drive ADC’s Input Settings

The AD4696 Easy Drive ADC provides the option of using high-Z mode at its analog inputs, which precharges the

switched capacitor networks of its MUX and SAR ADC without adding noise. This mode eases driver demands on the

ADA4352-2 output stage. It greatly limits the current that the ADA4352-2's output must provide when the AD4696's

sampling switches close at the beginning of its acquisition period and protects it from nonlinear kickback that can

significantly affect precision and linearity. Reducing this current also reduces the DC voltage error due to REXT.

Enabling high-Z mode in the AD4696 helps the overall signal chain achieve better SNR, THD, and DC accuracy.

Achieving Low Input Bias Current

There are several factors to consider in a low input bias current circuit. Leakage currents into high impedance

signal nodes can easily degrade measurement accuracy of picoamp signals. At the picoamp level, leakage current

can come from unexpected sources, including adjacent traces on the PCB (on the same layer or even from internal

layers), contamination on the PCB (from the assembly process or the environment), or other components on the

signal path. The ADA4352-2’s internal feedback resistors provide an advantage here as they are protected from

external leakage currents into the feedback path. Nevertheless, the system should be designed to mitigate these

sources and preserve optimal performance.

An appropriate cleaning process is essential after assembly to avoid leakages from solder flux and other

contaminants. Relative humidity also must be considered because PCB materials and the plastic mold compound

of the package itself can absorb moisture and cause additional leakage paths.

PCB Layout Cautions and Considerations

TIAs are extremely sensitive to parasitic capacitance at their inputs, as the capacitance directly reduces their

bandwidth and increases noise gain. A stray capacitance as low as 5 pF can greatly impact system performance,

especially if the capacitance of the photodiode used is around the same order of magnitude as the board parasitics.

Stripping the ground plane around the input trace is crucial for minimizing parasitic capacitance at the input, and

also makes it more difficult for leakage currents from the PCB to couple into the signal path and cause output

errors. As the ADA4352-2’s feedback resistors are internal, there is no need to strip ground plane around the

feedback path, saving board space that would be required for both external resistors and the removed ground

plane around them. However, the signal trace at the inverting input is still susceptible to leakage current and must

be protected.

It is important to keep the high impedance signal path as short as possible on the PCB. A high impedance node is

susceptible to picking up any stray signals in the system; therefore, keeping the path as short as possible reduces

this effect. Additionally, the longer the signal trace into the PCB (on the inverting input), the more stray capacitance

at the input of the PGTIA.