ADA4530-1ARZ-R7 datasheet(34/52 Pages) AD

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Data Sheet

ADA4530-1

HIGH IMPEDANCE MEASUREMENTS

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Rev. C | 34 of 52

This error is significant because it is very difficult to maintain high

insulation resistance values in glass epoxy (such as FR-4) PCB

materials. Resistance values of 10 TΩ to 100 TΩ are achievable.

A 10 TΩ insulation resistance creates a 1% error with the 100 GΩ

sensor used in previous examples. Insulation resistance does not

have an exponential temperature dependence like the amplifier

errors previously discussed in the Input Bias Current section and

the Input Resistance section, which makes insulation resistance the

dominate error source at lower temperatures (less than 70°C).

The effect on the insulation resistance on the TIA circuit depends

on the leakage path. The insulation resistance between the A

terminal and B terminal of the current sensor affects the circuit

in the same way as the amplifier input resistance. This error is

extremely small because the voltage across the insulation is equal

to the offset voltage of the amplifier. A much more significant error

is created from insulation paths to conductors with significantly

different potentials. This type of leakage path is shown as a lumped

element, RSHUNT, in the TIA circuit (see Figure 106). In this exam-

ple, the leakage path is created from the positive supply voltage

(V+) to the A terminal. If the positive supply voltage is 5 V relative to

signal ground, 500 fA flows through the insulation resistance of 10

TΩ. This large error dominates the amplifier input bias current and

input resistance errors over the entire temperature range.

Leakage paths to high voltages can also affect the buffer circuit with

equally ruinous results.

GUARDING

High source impedances and low error requirements can create in-

sulation resistance requirements that are unrealistically high. Fortu-

nately, a technique called guarding can reduce these requirements

to a reasonable level. The concept of guarding is to surround the

high impedance conductor with another conductor (guard) that is

driven to the same voltage potential. If there is no voltage across

the insulation resistance (between high impedance conductor and

guard), there cannot be any current flowing through it.

The ADA4530-1 uses guarding techniques internally, and it has a

very high performance guard buffer integrated. The output of this

buffer is made available externally to simplify the implementation of

guarding at the circuit level.

The voltage buffer circuit (see Figure 105) has been modified to

show the implementation of the guard (see Figure 107). In this

model, a conductor (VGRD) is added, and it completely separates

the high impedance (A) node from the low impedance (B) node

at a different voltage. The insulation resistance is modeled as two

resistances: all of the insulation between the A conductor and the

guard conductor (RSHUNT1), and all of the insulation between the

guard conductor and the B conductor (RSHUNT2). The ADA4530-1

guard buffer then drives this guard conductor (through Pin 2 and

Pin 7) to the A terminal voltage. If the A node and VGRD node are

exactly the same voltage, no current flows through the RSHUNT1

insulation resistance.

In practice, the voltage across RSHUNT1 cannot be 0 V, the guard

buffer offset voltage contributes to the difference in voltage potential

between the A node and VGRD node. For the ADA4530-1, this offset

voltage is trimmed to provide offsets less than 100 µV when the

input common-mode voltage is 1.5 V from the supply rails. The

guard buffer offset voltage and drift are specified in Table 1, Table 2,

and Table 3.

For example, assume that the voltage sensor produces an output

of 1 V. Without guarding, the 10 TΩ insulation resistance creates

an error current of 100 fA. With the guard, the voltage across the

insulation resistance is limited to 100 µV. The guard limits the error

current to 0.01 fA. In this example, the guard reduces the error by a

factor of 104 to an insignificant level.

Figure 107. Voltage Buffer Circuit with Guard

DIELECTRIC RELAXATION

Dielectric relaxation (also known as dielectric absorption or soak-

age) is a property of all insulating materials that can limit the

performance of electrometer circuits that need to settle to a few

femtoamperes.

Dielectric relaxation is the delay in polarization of the dielectric

molecules in response to a changing electric field. This delay is

a property of all insulating materials. The magnitude and the time

constant of the delay depend on the specific dielectric material. The

delays in some materials can be minutes or even hours.

Dielectric relaxation is a problem for electrometer circuits because

small displacement currents flow through the insulator in response

to the polarization of the molecules. Delays in polarization cause

delays in the dissipation of these currents, which can dominate the

settling time in these circuits.

In the context of capacitors, dielectric relaxation is called dielectric

absorption. Capacitors are specified with a test that measures

the residual open-circuit voltage after a specific charge/discharge

cycle. For electrometer circuits, it is more useful to consider the

short-circuit currents produced from step changes in a test voltage.

A simple lumped circuit model of an insulator is connected to the

test voltage source (see Figure 108). The majority of the dielectric

polarizes instantly; this is modeled as Capacitor C1. A small per-

centage of the dielectric polarizes slowly with a time constant of τ2,

modeled as Capacitor C2 and Resistor R2.

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