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ADA4530-1

Data Sheet

Rev. A | Page 36 of 50

HUMIDITY EFFECTS

The insulation resistance of the materials used to construct

circuits is sensitive to moisture. At lower temperatures (<70°C)

the insulation resistance creates more significant leakage

current errors than the amplifier itself. This means that the

relative humidity of the air is the most important error source at

lower temperatures. The dependence on humidity is evident in

the input bias current vs. temperature graphs (see Figure 34 to

Figure 36). There is significant deviation in the low temperature

measurements due to 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 package. 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

supplies with an input common-mode voltage of 0 V. Assume

that all of the 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, hermetically 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

humidity 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 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%).

100

0.1

–0.1

20406080

RELATIVE HUMIDITY (%)

–1

–10

–100

–1000

VSY = 10V

TA = 25°C

VCM = VSY/2

7 UNITS

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 predictable 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

physical 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 diffusion 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. Note that each data point in

Figure 112 was taken after one week of settling time.

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