AD549

REV. A

–6–

Figure 18. Unity Gain

Follower

Figure 21. Unity Gain Inverter

Figure 19. Unity Gain Follower

Large Signal Pulse Response

Figure 22. Unity Gain Inverter

Large Signal Pulse Response

Figure 20. Unity Gain Follower

Small Signal Pulse Response

Figure 23. Unity Gain Inverter

Small Signal Pulse Response

MINIMIZING INPUT CURRENT

The AD549 has been optimized for low input current and offset

voltage. Careful attention to how the amplifier is used will reduce

input currents in actual applications.

The amplifier operating temperature should be kept as low as pos-

sible to minimize input current. Like other JFET input amplifiers,

the AD549’s input current is sensitive to chip temperature, rising

by a factor of 2.3 for every 10

°C rise. This is illustrated in Figure

24, a plot of AD549 input current versus ambient temperature.

TEMPERATURE –

°C

1nA

100pA

10pA

1fA

–55

–25

5

35

65

95

125

1pA

100fA

10fA

Figure 24. AD549 Input Bias Current vs.

Ambient Temperature

On-chip power dissipation will raise chip operating temperature

causing an increase in input bias current. Due to the AD549’s

low quiescent supply current, chip temperature when the (un-

loaded) amplifier is operated with 15 V supplies, is less than 3

°C

higher than ambient. The difference in input current is negligible.

However, heavy output loads can cause a significant increase

in chip temperature and a corresponding increase in input

current. Maintaining a minimum load resistance of 10

Ω is rec-

ommended. Input current versus additional power dissipation

due to output drive current is plotted in Figure 25.

ADDITIONAL INTERNAL POWER DISSIPATION – mW

6.0

5.0

4.0

1.0

0

25

50

75

100

125

150 175

200

3.0

2.0

BASED ON

Figure 25. AD549 Input Bias Current vs.

Additional Power Dissipation

CIRCUIT BOARD NOTES

There are a number of physical phenomena that generate

spurious currents that degrade the accuracy of low current

measurements. Figure 26 is a schematic of an I-to-V converter

with these parasitic currents modeled.

Finite resistance from input lines to voltages on the board,

Ω must be maintained between the

amplifier’s signal and supply lines in order to capitalize on the

AD549’s low input currents. Standard PC board material