ADA4351-2ACPZ-R7 datasheet(34/36 Pages) AD

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Data Sheet

ADA4351-2

APPLICATIONS INFORMATION

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Rev. 0 | 34 of 36

The feedback pole (1/(2π × 1 MΩ × 3 pF) = 53 kHz) shown

in Figure 97 is well under f0, giving an approximate single-order

pole at that feedback pole frequency. Adding an additional external

CF,EXT can tune this roll-off to lower frequencies; however, it is not

possible to extend the frequency span starting out with an internal

3 pF CF,INT. Increasing the CIN to 100 pF decreases f0 moving the

feedback pole closer to that but still giving an approximate first-or-

der response at the feedback pole frequency (53 kHz). Increasing

the CIN to more than 200 pF with a 1 MΩ feedback and an internal

3 pF CF,INT starts to show a second-order closed-loop response

extending the f−3 dB more than the simple feedback pole.

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 (whether 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. Take care in the design of the system can mitigate

these sources and preserve optimal performance.

An appropriate cleaning process after assembly is essential 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.

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