AD546

REV. A

–8–

Figure 28. Input Pin to Insulating Standoff

Leakage through the bulk of the circuit board will still occur

with the guarding schemes shown in Figures 27a and 27b. Stan-

dard “G10” type printed circuit board material may not have

high enough volume resistivity to hold leakages at the sub-

picoampere level particularly under high humidity conditions.

One option that eliminates all effects of board resistance

is shown in Figure 28. The AD546’s sensitive input pin (either

Pin 2 when connected as an inverter, or Pin 3 when connected

as a follower) is bent up and soldered directly to a Teflon* insu-

lated standoff. Both the signal input and feedback component

leads must also be insulated from the circuit board by Teflon

standoffs or low-leakage shielded cable.

Contaminants such as solder flux on the board’s surface and on

the amplifier’s package can greatly reduce the insulation resis-

tance between the input pin and those traces with supply or sig-

nal voltages. Both the package and the board must be kept clean

and dry. An effective cleaning procedure is to first swab the sur-

face with high grade isopropyl alcohol, then rinse it with deion-

ized water and, finally, bake it at 80

°C for 1 hour. Note that if

either polystyrene or polypropylene capacitors are used on the

printed circuit board, a baking temperature of 70

°C is safer,

since both of these plastic compounds begin to melt at approxi-

mately +85

°C.

Other guidelines include making the circuit layout as compact

as possible and reducing the length of input lines. Keeping cir-

cuit board components rigid and minimizing vibration will re-

duce triboelectric and piezoelectric effects. All precision high

impedance circuitry requires shielding from electrical noise and

interference. For example, a ground plane should be used under

all high value (i.e., greater than 1 M

Ω) feedback resistors. In

some cases, a shield placed over the resistors, or even the entire

amplifier, may be needed to minimize electrical interference

originating from other circuits. Referring to the equation in Fig-

ure 26, this coupling can take place in either, or both, of two

different forms—coupling via time varying fields:

dV

dT

or by injection of parasitic currents by changes in capacitance

due to mechanical vibration:

dCp

dT

V

Both proper shielding and rigid mechanical mounting of compo-

nents help minimize error currents from both of these sources.

Table I lists various insulators and their properties.

Table I. Insulating Materials and Characteristics

Volume

Minimal

Minimal

Resistance

Resistivity

Triboelectric

Piezoelectric to Water

Material

1

(

–CM)

Effects

Effects

Absorption

Teflon*

10

WW

G

Kel-F**

WM

G

Sapphire

10

MG

G

Polyethylene

10

MG

M

Polystyrene

WM

M

Ceramic

10

WM

W

Glass Epoxy

10

WM

W

PVC

GM

G

Phenolic

10

WG

W

G–Good with Regard to Property.

M–Moderate with Regard to Property.

W–Weak with Regard to Property.

Ohio, 1977.

*Teflon is a registered trademark of E.I. du Pont Co.

**Kel-F is a registered trademark of 3M Company.

OFFSET NULLING

The AD546’s input offset voltage can be nulled by using balance

Pins 1 and 5, as shown in Figure 29. Nulling the input offset

voltage in this fashion will introduce an added input offset volt-

age drift component of 2.4

µV/°C per millivolt of nulled offset.

Figure 29. Standard Offset Null Circuit

The circuit in Figure 30 can be used when the amplifier is used

as an inverter. This method introduces a small voltage in series

with the amplifier’s positive input terminal. The amplifier’s