ADA4530-1ARZ-R7 datasheet(37/52 Pages) AD

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Data Sheet

ADA4530-1

HIGH IMPEDANCE MEASUREMENTS

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Rev. C | 37 of 52

In this environment, measurements of the effective input bias cur-

rent appear to drift with time because the leakage currents depend

on the relative humidity for the previous week. This long-term mem-

ory due to the absorption process may need to be taken in account

in certain circumstances (such as long-term product storage in an

unconditioned high humidity environment prior to use).

The rapid adsorption response can change the effective bias cur-

rent in response to local fluctuations in humidity. These current

fluctuations can be much larger than the low frequency current

noise of the amplifier and thermal noise of the resistors. The

sensitive circuitry can be isolated from these local humidity fluctu-

ations by restricting the airflow around the circuitry with an air

baffle. Electrostatic shielding added to reduce interference can

also function as an air baffle. Remove or reduce the sources of

humidity fluctuations whenever possible. Avoid breathing on the

high impedance circuitry, for example.

Figure 113. Effective Input Bias Current Transient Response to Humidity Step

It is important to note that all electrometer circuits are subject

to humidity effects. The legacy circuits constructed with TO-99

packages using air wiring techniques have insulator leakage paths

such as the epoxy between the pins and the Teflon® standoffs that

support the air wired components. The input bias currents of legacy

amplifiers are high enough to mask the humidity effects.

In summary, the ADA4530-1 can be designed using the specified

performance for normal laboratory (<60%) relative humidity condi-

tions. In applications that must operate in uncontrolled or high

humidity environments, some additional derating of the input bias

current is prudent. Characterize the amount of derating on a per

product basis because the net leakage depends on the material

types and physical dimensions of the insulators.

CONTAMINATION

The effective insulation resistance of an electrometer circuit can be

substantially degraded if the insulators are contaminated. Solder

flux, body oils, dust, and dirt are all possible sources of contami-

nation. Some of these contaminants form a parallel leakage path

across the surface of the existing insulator, effectively lowering the

insulation resistance. Guarding techniques help to suppress these

effects.

The effects are more severe when the source of contamination

contains ionic compounds. In the presence of humidity, these con-

taminants act as an electrolyte, which can form a weak battery. Flux

residue and body oils are particularly effective at creating these

parasitic batteries.

As an example, the PCB insulation between two high impedance

nodes was purposefully contaminated with a 3 mm drop of rosin

mildly activated (RMA) type solder flux. This sample was dried and

allowed to stabilize in laboratory conditions (25°C, 40% RH) for

several days. After this time, the voltage vs. current relationship

was measured with an electrometer grade SMU (see Figure 114).

Figure 114. Current to Voltage Response of RMA Contaminated Insulation

This contamination formed a weak battery with an open circuit

voltage (VBATT) of 15 mV and an output resistance (RBATT) of

300 GΩ. This sort of contamination is disastrous in electrometer cir-

cuits because guarding techniques cannot suppress it. A simplified

model is made with the contamination battery applied across the

A terminal and B terminal of a TIA circuit (see Figure 115). The A

terminal and B terminal are both driven to the same voltage, which

creates an error current (IBATT) because the open circuit battery

voltage is dropped across the output resistance as follows:

IBATT = VBATT ÷ RBATT

(10)

This battery current flows through the feedback resistance, where

it is summed with the signal and other error currents in the circuit.

The error current in this example is 50 fA. The battery characteris-

tics are subject to the environmental conditions; therefore, the error

current drifts with time, temperature, and humidity.

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