ADA4530-1ARZ-R7 datasheet(43/52 Pages) AD

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Data Sheet

ADA4530-1

LAYOUT GUIDELINES

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Rev. C | 43 of 52

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 high impedance wiring—

not just on the PCB. Frequently, the high impedance sensor is not

directly mounted on the PCB with the electrometer 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. Conveniently, 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 termi-

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

to guard the voltage output sensor cable is to use a triaxial cable.

A 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 conduc-

tor 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 inter-

ference through capacitive coupling. The amount of capacitance

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:

I=C∂V∂t+V∂C∂t

(15)

The second term in this equation is frequently ignored in most

circuits, but it can generate some unusual problems in electrometer

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 be-

tween 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 physically interrupts all of the

capacitive coupling paths, all of the displacement current from the

interference source is shunted to the low impedance source.

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

15). 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 interfer-

ence 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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