AD546JN AD [Analog Devices], AD546JN Datasheet - Page 8

• Manufacturer Part Number: AD546JN
• Description: 1 pA Monolithic Electrometer Operational Amplifier
• Manufacturer: AD [Analog Devices]

AD546
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 ) 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:
or by injection of parasitic currents by changes in capacitance
due to mechanical vibration:
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.
dCp
dV
dT
dT
C
V
P
Figure 28. Input Pin to Insulating Standoff
–8–
Material
Teflon*
Kel-F**
Sapphire
Polyethylene
Polystyrene
Ceramic
Glass Epoxy
PVC
Phenolic
G–Good with Regard to Property.
M–Moderate with Regard to Property.
W–Weak with Regard to Property.
1
**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 V/ C per millivolt of nulled offset.
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
Electronic Measurements, pp.15-17, Keithley Instruments, Inc., Cleveland,
Ohio, 1977.
*Teflon is a registered trademark of E.I. du Pont Co.
Table I. Insulating Materials and Characteristics
1
Figure 29. Standard Offset Null Circuit
10
Volume
Resistivity
( –CM)
10
10
10
10
10
10
10
10
17
17
16
14
12
12
10
10
5
–10
–10
–10
–10
–10
–10
–10
–10
–10
12
18
18
18
18
18
14
17
15
Minimal
Triboelectric
Effects
W
W
M
M
W
W
W
G
W
Minimal
Piezoelectric to Water
Effects
W
M
G
G
M
M
M
M
G
Resistance
Absorption
G
G
G
M
M
W
W
G
W
REV. A