ADA4530-1ARZ-R7 datasheet(35/52 Pages) AD

35 / 52 page

Data Sheet

ADA4530-1

HIGH IMPEDANCE MEASUREMENTS

analog.com

Rev. C | 35 of 52

The size of C2 reflects the proportion of slow molecules. The size

depends on the material but it is typically 100 to 10,000 times

smaller than C1. The size of R2 sets the time constant.

τ2 = R2 × C2

(9)

Figure 108. Dielectric Relaxation Model Test Circuit

The current step response (ISRC) of the insulator to a voltage step

is shown in Figure 108. A large initial current charges Capacitor C1

with a fast time constant. This time constant, τ1, equals the source

resistance RS × C1 (see Figure 109). Long after Capacitor C1 is

charged, a much smaller current continues to flow, which charges

Capacitor C2. The time constant of charging is not affected by the

external circuitry; it depends only on the material properties of the

insulator. The magnitude of the current depends on the magnitude

of the voltage change across the insulator.

Figure 109. Step Response of Dielectric Relaxation Model

The dielectric relaxation performance was measured for a variety of

PCB laminates using the test circuit in Figure 108. An electrometer

grade source measurement unit (SMU), Keithley 6430, applies a

±100 V test stimulus and measures the resulting current. The large

alternating polarity test voltage distinguishes the small dielectric

relaxation current from the input offset current of the SMU.

The first PCB laminate tested is the industry-standard FR-4 glass

epoxy. The measurement results are shown in Figure 110. The

glass epoxy laminate requires 1 hour to dissipate the dielectric

relaxation current to less than 10 fA. This result shows that glass

epoxy laminates are unsuitable for the highest performance electro-

meter circuits.

Figure 110. Glass Epoxy Dielectric Relaxation Performance

An alternative PCB laminate to consider is Rogers 4350B. Rogers

4350B is a ceramic laminate designed for RF/microwave circuits.

Rogers 4350B is compatible with standard PCB production techni-

ques and is widely available. The measurement results for the

Rogers 4350B material are shown in Figure 111. This material

requires less than 20 sec to dissipate the dielectric relaxation

current to less than 1 fA.

Based on its superior performance, it is recommended to use Rog-

ers 4350B laminates with the ADA4530-1 in the highest perform-

ance applications. All of the critical characterization measurements

of the ADA4530-1 are taken using Rogers 4350B.

Figure 111. Rogers 4350B Dielectric Relaxation Performance

HUMIDITY EFFECTS

The insulation resistance of the materials used to construct circuits

is sensitive to moisture. At lower temperatures (<70°C) the insu-

lation 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

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