AD549

Rev. H | Page 10 of 20

FUNCTIONAL DESCRIPTION

MINIMIZING INPUT CURRENT

The AD549 is optimized for low input current and offset

voltage. Careful attention to how the amplifier is used reduces

input currents in actual applications.

Keep the amplifier operating temperature as low as possible to

minimize input current. Like other JFET input amplifiers, the

AD549 input current is sensitive to chip temperature, rising by

a factor of 2.3 for every 10°C. Figure 25 is a plot of the AD549

input current vs. ambient temperature.

1nA

100pA

10pA

1pA

100fA

10fA

1fA

–55

–25

5

35

65

125

95

TEMPERATURE (°C)

Figure 25. Input Bias Current vs. Ambient Temperature

On-chip power dissipation raises the chip operating tempera-

ture, causing an increase in input bias current. Due to the low

quiescent supply current of the AD549, the chip temperature

is less than 3°C higher than its ambient temperature when the

(unloaded) amplifier is operating with 15 V supplies. The

difference in the input current is negligible.

However, heavy output loads can cause a significant increase in

chip temperature and a corresponding increase in the input

current. Maintaining a minimum load resistance of 10 Ω is

recommended. Input current vs. additional power dissipation

due to output drive current is plotted in Figure 26.

6

5

4

3

2

1

0

25

50

75

100

125

150

175

200

ADDITIONAL INTERNAL POWER DISSIPATION (mW)

BASED ON

Figure 26. Input Bias Current vs. Additional Power Dissipation

CIRCUIT BOARD NOTES

A number of physical phenomena generate spurious currents

that degrade the accuracy of low current measurements. Figure 27

is a schematic of a current to voltage (I-to-V) converter with

these parasitic currents modeled.

2

3

6

8

AD549

+

–

+V +

V

dT

dV

dT

Figure 27. Sources of Parasitic Leakage Currents

Finite resistance from input lines to voltages on the board,

the amplifier signal and supply lines to capitalize on the low

input currents of the AD549. Standard PCB material does not

have high enough insulation resistance; therefore, connect the

input leads of the AD549 to standoffs made of insulating

material with adequate volume resistivity (that is, Teflon®). The

surface of the insulator must be kept clean to preserve surface

resistivity. For Teflon, an effective cleaning procedure consists

of swabbing the surface with high grade isopropyl alcohol,

rinsing with deionized water, and baking the board at 80°C for

10 minutes.

In addition to high volume and surface resistivity, other proper-

ties are desirable in the insulating material chosen. Resistance

to water absorption is important because surface water films

drastically reduce surface resistivity. The insulator chosen

should also exhibit minimal piezoelectric effects (charge

emission due to mechanical stress) and triboelectric effects

(charge generated by friction). Charge imbalances generated

by these mechanisms can appear as parasitic leakage currents.

Table 3 lists various insulators and their properties.1

Guarding the input lines by completely surrounding them with

a metal conductor biased near the potential of the input lines

has two major benefits. First, parasitic leakage from the signal

line is reduced because the voltage between the input line and

the guard is very low. Second, stray capacitance at the input

node is minimized. Input capacitance can substantially degrade

signal bandwidth and the stability of the I-to-V converter.

Ohio, 1977.