ad549sh-883b Analog Devices, Inc., ad549sh-883b Datasheet - Page 11

• Manufacturer Part Number: ad549sh-883b
• Description: Ultralow Input Bias Current Operational Amplifier
• Manufacturer: Analog Devices, Inc.

the AD549 is connected to Pin 8 so that it can be bootstrapped
near the input potential. This minimizes pin leakage and input
common-mode capacitance due to the case. Guard schemes for
inverting and noninverting amplifier topologies are illustrated
in Figure 28 and Figure 29.
Other guidelines include keeping the circuit layout as compact
as possible and keeping the input lines short. Keeping the
assembly rigid and minimizing sources of vibration reduces
triboelectric and piezoelectric effects. All precision, high
impedance circuitry requires shielding against interference
noise. Low noise coaxial or triaxial cables should be used for
remote connections to the input signal lines.
Table 3. Insulating Materials and Characteristics
Material
Teflon®
Kel-F®
Sapphire
Polyethylene
Polystyrene
Ceramic
Glass Epoxy
PVC
Phenolic
1
G: good with regard to property.
M: moderate with regard to property.
W: weak with regard to property.
I
V
N
Figure 29. Noninverting Amplifier with Guard
S
+
–
Figure 28. Inverting Amplifier with Guard
GUARD
GUARD
Volume Resistivity
(V to CM)
10
10
10
10
10
10
10
10
10
17
17
16
14
12
12
10
10
5
to 10
to 10
to 10
to 10
to 10
to 10
to 10
to 10
to 10
12
2
3
18
18
18
18
18
14
17
15
3
2
AD549
AD549
8
8
C
R
F
F
6
6
V
R
R
V
–
OUT
F
I
OUT
+
–
Minimal
Triboelectric Effect
W
W
M
M
W
W
W
G
W
+
Rev. G | Page 11 of 20
1
OFFSET NULLING
The AD549’s input offset voltage can be nulled by using balance
Pin 1 and Pin 5, as shown in Figure 30. Nulling the input offset
voltage in this fashion introduces an added input offset voltage
drift component of 2.4 μV/°C per mV of nulled offset
(a maximum additional drift of 0.6 μV/°C for the AD549K,
1.2 μV/°C for the AD549L, and 2.4 μV/°C for the AD549J).
The approach in Figure 31 can be used when the amplifier is
used as an inverter. This method introduces a small voltage
referenced to the power supplies in series with the amplifier’s
positive input terminal. The amplifier input offset voltage drift
with temperature is not affected. However, variation of the
power supply voltages causes offset shifts.
Minimal
Piezoelectric Effect
W
M
G
G
M
M
M
M
G
Figure 31. Alternate Offset Null Circuit for Inverter
V
Figure 30. Standard Offset Null Circuit
I
+
–
R
I
2
3
200Ω
499kΩ
AD549
–V
4
2
3
S
+V
1
7
AD549
1
S
499kΩ
0.1µF
10kΩ
5
6
R
F
+V
–V
Resistance to
Water Absorption
G
G
G
M
M
W
W
G
W
S
S
6
100kΩ
V
OUT
+
–
V
OUT
+
–
AD549
1