ADA4530-1ARZ-R7 datasheet(36/51 Pages) AD

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Data Sheet

ADA4530-1

Rev. A | Page 35 of 50

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

INSULATOR

MODEL

VSRC

ISRC

Figure 108. Dielectric Relaxation Model Test Circuit

VSRC

ISRC

TIME

1 = RS × C1

2 = R2 × C2

Figure 109. Step Response of Dielectric Relaxation Model

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 proper-

ties of the insulator. The magnitude of the current depends on

the magnitude of the voltage change across the insulator.

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 electrometer circuits.

125

–125

–100

–75

–50

–25

100

–20

–16

–12

–8

–4

04.0

3.0

3.5

2.5

2.0

1.5

1.0

0.5

ELAPSED TIME (Hours)

VSRC

ISRC

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 produc-

tion techniques 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

Rogers 4350B laminates with the ADA4530-1 in the highest

performance applications. All of the critical characterization

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

125

–125

–100

–75

–50

–25

100

–10

–8

–6

–4

–2

0200

120

140

160

180

100

ELAPSED TIME (Seconds)

VSRC

ISRC

Figure 111. Rogers 4350B Dielectric Relaxation Performance

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