ADA4530-1ARZ-R7 datasheet(36/52 Pages) AD

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Data Sheet

ADA4530-1

HIGH IMPEDANCE MEASUREMENTS

analog.com

Rev. C | 36 of 52

the difficulty maintaining a consistently low relative humidity at low

temperatures.

To evaluate the humidity sensitivity of insulation resistance, there

are two mechanisms which must be considered: adsorption and

absorption.

Adsorption is a process where thin films of molecules adhere to the

surface of a material. Water molecules are subject to this process.

The magnitude of the effect depends on the insulating material

and the relative humidity. Thin films of moisture are conductive,

and they act as leakage resistances in parallel with the insulation

resistance of the material. Because this is a surface effect, guard

ring techniques are effective at reducing it.

Absorption is a process where molecules enter the bulk of a

material. Water molecules can diffuse into a material and affect the

bulk conductivity of that material. Because the leakage paths are

through the bulk of the material, guard rings are not effective at

reducing it.

It is not possible to completely guard all the leakage paths: bulk

or surface. A relevant example of this limitation is the molding

compound of the SOIC package housing the ADA4530-1. Surface

and bulk paths exist from the input pins to all other pins of the pack-

age. The nature of the resulting current depends on the specific

leakage path: paths to V+ increase the bias current flowing out of

the amplifier, paths to V− increase the bias current flowing into the

amplifier, and paths to VOUT lower the effective feedback resistance

in TIA circuits.

Consider the example of a circuit powered from ±5 V power sup-

plies with an input common-mode voltage of 0 V. Assume that all

leakage resistance between the input and V+ is effectively 100 TΩ.

This resistance creates a current equal to 50 fA flowing from V+.

Assume that the leakage resistance between the input and V− is

effectively 250 TΩ. This resistance creates a current equal to 20 fA

flowing to V−. The net current equals −30 fA flowing out of the input

pin.

All of these leakage currents can be combined with the amplifier

input bias current and treated as an effective input bias current.

The effective input bias current sensitivity to relative humidity of the

ADA4530-1 is characterized for several units. The test amplifiers

are configured in TIA and unity buffer circuits with 100 GΩ, hermeti-

cally sealed resistors (RX-1M1009FE) as the feedback and source

resistors, respectively. These glass bodied resistors have a silicone

coating (glass has poor humidity adsorption properties).

The ADA4530-1 amplifiers are mounted on Rogers 4350B PCBs

(glass epoxy boards have poor humidity absorption properties).

Figure 112 shows the effective input bias current vs. relative humidi-

ty for seven characterization units. Figure 112 is plotted with a split

log axis to effectively show the magnitude and polarity of the bias

current. The magnitude of the leakage currents changes by more

than a factor of 100 across the relative humidity span from 5% to

80%. The effective bias current is much less than 1 fA for typical

conditioned environments (RH < 50%).

Figure 112. Effective Input Bias Current vs. Relative Humidity

The magnitude of the effective input bias current becomes very

sensitive to the relative humidity at higher humidity levels (>60%).

Some of the units show an exponential dependence on humidity

(see the blue curve in Figure 112). Other units show a less predicta-

ble dependence; the leakage current magnitude increases rapidly,

but the polarity can change. The net leakage current is the sum of

the currents sourced from higher voltages (like V+) with the currents

sunk by lower voltages (like V−). As the humidity changes, the

relative magnitudes of each of these leakage paths can change,

which can result in changes in the polarity of the leakage current

(see the red and green curves in Figure 112).

The response time of these leakage currents depends on the phys-

ical process that causes them. Because adsorption is a surface

effect, the film thickness rapidly achieves equilibrium with changes

in the relative humidity of the air. Because absorption is a bulk diffu-

sion process, it is very slow compared to the adsorption process.

These widely different time constants mean that the effective input

bias current responds quickly to a step change in relative humidity,

but has a very long settling time. The step response of one amplifier

to a 50% to 60% relative humidity change is shown in Figure 113.

The high frequency response of the initial humidity step (and the

overshoot recovery) is on the order of seconds to tens of seconds.

Complete settling takes over a week as the moisture slowly diffuses

through the PCB insulation and package molding compound. Each

data point in Figure 112 was taken after one week of settling time.

In practical applications, the relative humidity of the air changes

rapidly with daily and seasonal variations. The effective input bias

current response to these humidity changes has two parts. The

response due to the adsorption process follows the rapid changes

immediately. The response due to the absorption process low-pass

filters the humidity changes. This low-pass response causes the

effective input bias current to have long-term memory of the relative

humidity fluctuations.

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