ADA4530-1ARZ-R7 datasheet(38/51 Pages) AD

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Data Sheet

ADA4530-1

Rev. A | Page 37 of 50

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.

In this environment, measurements of the effective input bias

current appear to drift with time because the leakage currents

depend on the relative humidity for the previous week. This

long-term memory due to the absorption process may need to

be taken in account in certain circumstances (that is, long-term

product storage in an unconditioned high humidity environ-

ment prior to use).

The rapid adsorption response can change the effective bias

current 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

fluctuations by restricting the airflow around the circuitry with

an air baffle. Electrostatic shielding added to reduce interfer-

ence 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.

–8

–6

–4

–2

175

150

125

100

ELAPSED TIME (Hours)

VSY = 10V

TA = 25°C

VCM = VSY/2

RELATIVE HUMIDITY

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 used

to 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

conditions. 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

contamination. Some of these contaminants form a parallel

leakage path across the surface of the existing insulator effec-

tively 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

contaminates 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 imped-

ance 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).

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 electrome-

ter circuits 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

All of 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

characteristics are subject to the environmental conditions;

therefore, the error current drifts with time, temperature, and

humidity.

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