ad549sh-883b Analog Devices, Inc., ad549sh-883b Datasheet - Page 10

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

AD549
FUNCTIONAL DESCRIPTION
MINIMIZING INPUT CURRENT
The AD549 has been optimized for low input current and
offset voltage. Careful attention to how the amplifier is used
reduces input currents in actual applications.
The amplifier operating temperature should be kept as low
as possible to minimize input current. Like other JFET input
amplifiers, the AD549’s input current is sensitive to chip tem-
perature, rising by a factor of 2.3 for every 10°C. Figure 25 is
a plot of the AD549 input current vs. ambient temperature.
On-chip power dissipation raises the chip operating tempera-
ture, causing an increase in input bias current. Due to the
AD549’s low quiescent supply current, 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.
100pA
100fA
10pA
Figure 26. Input Bias Current vs. Additional Power Dissipation
10fA
1nA
1pA
1fA
6
5
4
3
2
1
Figure 25. Input Bias Current vs. Ambient Temperature
–55
0
ADDITIONAL INTERNAL POWER DISSIPATION (mW)
25
–25
50
5
TEMPERATURE (°C)
75
BASED ON
TYPICAL I
100
35
B
125
= 40fA
65
150
95
175
200
125
Rev. G | Page 10 of 20
CIRCUIT BOARD NOTES
There are a number of physical phenomena that generate
spurious currents that degrade the accuracy of low current
measurements. Figure 27 is a schematic of an I-to-V converter
with these parasitic currents modeled.
Finite resistance from input lines to voltages on the board,
modeled by Resistor R
resistance of more than 10
the amplifier’s signal and supply lines in order to capitalize on
the AD549’s low input currents. Standard PC board material
does not have high enough insulation resistance. Therefore, the
AD549’s input leads should be connected to standoffs made of
insulating material with adequate volume resistivity (that is,
Teflon®). The insulator’s surface 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.
These effects are modeled by Variable Capacitor C
Table 3 lists various insulators and their properties.
Guarding the input lines by completely surrounding them with
a metal conductor biased near the input lines’ potential 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. The case of
1
Electronic Measurements, pp. 15–17, Keithley Instruments, Inc., Cleveland,
Ohio, 1977.
Figure 27. Sources of Parasitic Leakage Currents
R
V
S
P
f
S
P
, results in parasitic leakage. Insulation
C p
15
Ω must be maintained between
2
3
I
I'
AD549
=
R
V
P
+
8
dC p
dT
C
R
F
F
V +
6
dV
dT
C p
P
V
OUT
in Figure 27.
+
–
1