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Data Sheet

ADA4530-1

Rev. A | Page 43 of 50

GUARD

FR-4

ROGERS

4350B

SOLDER MASK

Figure 123. Layout Cross Section with Guard Plane

VIA FENCE

A via fence is an additional structure that guards the lateral

leakage paths in the laminate between the guard ring and the

guard plane (see Figure 123). The fence is implemented by

surrounding the entire guard ring with vias that connect the

guard ring to the guard plane (see Figure 121 and Figure 122).

CABLES AND CONNECTORS

Guarding techniques are required for all of high impedance

wiring; not just on the PCB. Frequently, the high impedance

sensor is not directly mounted on the PCB with the electrome-

ter amplifier and external cables are used to make the connection.

The typical way to guard a cable connecting to a current output

sensor is by using a coaxial cable. A coaxial cable consists of an

inner conductor surrounded with insulation, which is, in turn,

surrounded by a braided conductor. Use the inner conductor

for the high impedance (A) terminal and the outer braided

shield conductor for the low impedance (B) terminal. Conven-

iently, this arrangement effectively guards the coaxial insulation

resistance because the A terminal and B terminal are nominally

at the same voltage (when attached to a TIA interface circuit).

Voltage output sensors are more problematic because the A

terminal and B terminal are not at the same voltage. The typical

way to guard the voltage output sensor cable is to use triaxial

cable. Triaxial cable is constructed with an inner conductor with

two separate braided conductors. Each of these braided

conductors is separated from each other with insulation. Use

the inner conductor for the high impedance (A) terminal and

the inner braided conductor for the guard (VGRD) connection,

and use the outer braided conductor for the low impedance (B)

terminal. All the insulation around the inner conductor is

completely surrounded by the guard conductor, which keeps the

voltage drop across this insulation equal to zero.

ELECTROSTATIC INTERFERANCE

Very high impedance electrometer circuits are susceptible to

interference through capacitive coupling. The amount of capaci-

tance required to couple low frequency signals is surprisingly

small. For example, line frequency (60 Hz) interference is

coupled (with a −3 dB loss) to a 1 TΩ impedance with only 3 fF

of coupling capacitance.

Traditional electrical interferers are not the only sources of concern.

Calculate the displacement current, I, in a capacitor as follows:









(2)

The second term in this equation is frequently ignored in most

circuits, but it can generate some unusual problems in electrome-

ter circuits. The problem is that the movement of any charged

object changes the coupling capacitance between the object and

the electrometer, and this change in capacitance injects small

currents into the circuit. The ADA4530-1 is so sensitive that it

easily detects the movement of a hand or the movement of a

piece of paper. These types of effects are not periodic or

predictable, and they can appear as erratic dc shifts on the time

scales of interest.

Both of these types of interference can be reduced by the

addition of a shield. A shield is a piece of conductive material

placed between the high impedance input and the interference

source. This shield must be electrically connected to a low

impedance source (such as signal ground). If the shield physi-

cally interrupts all of the capacitive coupling paths, all of the

displacement current from the interference source is shunted to

the low impedance source.

Notice that the construction of a shield is almost the same as

the construction of a guard. Because of this similarity, many

guard structures also provide shielding as well. The primary

difference is that the dc voltage of the shield is not important,

whereas the guard must have a voltage equal to that of the high

impedance input. Shields that are driven by the guard buffer

have the added benefit of bootstrapping the capacitance between

the high impedance input and the shield. The disadvantage of

this approach is that the guard buffer output impedance is 1 kΩ,

which makes the shield less effective than a signal ground or a

chassis ground connection. The most effective systems typically

use the box within a box construction: the outer shield is driven

with ground and the inner shield is driven with guard.

There is another capacitive interference effect that typically

cannot be shielded. This displacement current is generated from

a change in capacitance with respect to time (the second term of

Equation 2). This change is due to the mechanical movement of

the circuit components. This movement, which can be caused

by mechanical impact or vibration, generates electrical interference.

This interference typically appears at unexpected frequencies

that are equal to the mechanical resonances of the components.

This effect must be considered when using traditional air wiring

techniques for large feedback resistors or relays. It is important

to ensure solid mechanical connections to Teflon standoffs for

this type of construction.

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