AD795KN Analog Devices, AD795KN Datasheet - Page 10

• Manufacturer Part Number: AD795KN
• Description: Low Power/ Low Noise Precision FET Op Amp
• Manufacturer: Analog Devices

AD795
Leakage through the bulk of the circuit board will still occur
with the guarding schemes shown in Figures 31 and 32. 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 33. The AD795’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*
insulated standoff. Both the signal input and feedback compo-
nent 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
signal voltages. Both the package and the board must be kept
clean and dry. An effective cleaning procedure is to first swab
the surface with high grade isopropyl alcohol, then rinse it with
deionized water and, finally, bake it at 100 C for 1 hour. Poly-
propylene and polystyrene capacitors should not be subjected to
the 100 C bake as they will be damaged at temperatures greater
than 80 C.
Other guidelines include making the circuit layout as compact
as possible and reducing the length of input lines. Keeping
circuit board components rigid and minimizing vibration will
reduce 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
Figure 30, 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:
*Teflon is a registered trademark of E.I. du Pont Co.
Figure 33. Input Pin to Insulating Standoff
1
2
3
4
AD795
7
6
5
8
dV
dCp
dT
dT
BOARD
INPUT SIGNAL
C
PC
V
P
LEAD
TEFLON INSULATED STANDOFF
INPUT PIN:
PIN 2 FOR INVERTER
OR PIN 3 FOR FOLLOWER
AD795
–10–
Both proper shielding and rigid mechanical mounting of
components help minimize error currents from both of these
sources.
OFFSET NULLING
The AD795’s input offset voltage can be nulled (mini-DIP
package only) by using balance Pins 1 and 5, as shown in
Figure 34. Nulling the input offset voltage in this fashion will
introduce an added input offset voltage drift component of
2.4 V/ C per millivolt of nulled offset.
The circuit in Figure 35 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
input offset voltage drift with temperature is not affected.
However, variation of the power supply voltages will cause
offset shifts.
Figure 35. Alternate Offset Null Circuit for Inverter
Figure 34. Standard Offset Null Circuit
+
–
V
I
R
I
2
3
–V
AD795
4
S
200
+V
499k
7
2
3
1
S
100k
AD795
5
0.1 F
499k
R
F
6
6
V
+V
–V
OUT
S
S
100k
+
–
V
OUT
+
–
REV. A