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AD546

1 pA Monolithic Electrometer Operational Amplifier

Analog Devices 

AD546

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 MΩ) 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

CP

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

Material1

Volume

Resistivity

(⍀–CM)

Minimal

Triboelectric

Effects

Minimal

Resistance

Piezoelectric to Water

Effects

Absorption

Teflon*

1017–1018 W

Kel-F**

1017–1018 W

Sapphire

1016–1018 M

Polyethylene 1014–1018 M

Polystyrene 1012–1018 W

Ceramic

1012–1014 W

Glass Epoxy 1010–1017 W

PVC

1010–1015 G

Phenolic

105–1012

W

W

G

M

G

G

G

G

M

M

M

M

W

M

W

M

G

G

W

G–Good with Regard to Property.

M–Moderate with Regard to Property.

W–Weak with Regard to Property.

1Electronic Measurements, pp.15-17, Keithley Instruments, Inc., Cleveland,

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

–8–

REV. A

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