AD549

Rev. H | Page 11 of 20

The case of 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.

2

3

6

8

AD549

+

–

GUARD

Figure 28. Inverting Amplifier with Guard

3

2

6

8

AD549

+

–

+

–

GUARD

Figure 29. Noninverting Amplifier with Guard

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. Use low noise coaxial

or triaxial cables for remote connections to the input signal lines.

OFFSET NULLING

The AD549 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 maxi-

mum 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).

2

3

6

5

1

7

4

AD549

+

–

10kΩ

Figure 30. Standard Offset Null Circuit

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 positive

input terminal of the amplifier. The amplifier input offset

voltage drift with temperature is not affected. However,

variation of the power supply voltages causes offset shifts.

2

3

6

AD549

+

–

+

–

200Ω

100kΩ

499kΩ

499kΩ

0.1µF

Figure 31. Alternate Offset Null Circuit for Inverter

Table 3. Insulating Materials and Characteristics

Material

Volume Resistivity

(V to CM)

Minimal

Triboelectric Effect1

Minimal

Piezoelectric Effect1

Resistance to

Water Absorption1

Teflon

W

W

G

Kel-F®

W

M

G

Sapphire

M

G

G

Polyethylene

M

G

M

Polystyrene

W

M

M

Ceramic

W

M

W

Glass Epoxy

W

M

W

PVC

G

M

G

Phenolic

W

G

W