ad549sh-883b Analog Devices, Inc., ad549sh-883b Datasheet - Page 13

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

DIFFERENTIAL INPUT VOLTAGE OVERLOAD
A plot of the AD549 input currents vs. differential input
voltage (defined as V
input current at either terminal stays below a few hundred
femtoamps until one input terminal is forced higher than 1 V
to 1.5 V above the other terminal. Under these conditions, the
input current limits at 30 μA.
INPUT PROTECTION
The AD549 safely handles any input voltage within the supply
voltage range. Subjecting the input terminals to voltages beyond
the power supply can destroy the device or cause shifts in input
current or offset voltage if the amplifier is not protected.
A protection scheme for the amplifier as an inverter is shown
in Figure 37. R
inverting input to 1 mA for expected transient (less than 1 sec)
overvoltage conditions, or to 100 μA for a continuous overload.
Because R
value than the amplifier’s input resistance, it does not affect the
inverter’s dc gain. However, the Johnson noise of the resistor
adds root sum of squares to the amplifier input noise.
In the corresponding version of this scheme for a follower,
shown in Figure 38, R
terminal produce a pole in the signal frequency response at a
f = ½πRC. Again, the Johnson noise, R
input voltage noise.
100n
100p
100µ
100f
10µ
10n
10p
10f
1n
1p
1µ
–5
Figure 36. Input Current vs. Differential Input Voltage
P
SOURCE
is inside the feedback loop and is much lower in
Figure 37. Inverter with Input Current Limit
–4
P
DIFFERENTIAL INPUT VOLTAGE (V) (V
is chosen to limit the current through the
–3
IN
P
+ − V
R
and the capacitance at the positive input
I
–2
PROTECT
IN
–
R
F
IN
–1
−) appears in Figure 36. The
0
2
3
AD549
P
1
, adds to the amplifier’s
C
2
F
IN
I
IN
+ – V
+
3
6
IN
–)
4
5
Rev. G | Page 13 of 20
Figure 39 is a schematic of the AD549 as an inverter with an
input voltage clamp. Bootstrapping the clamp diodes at the
inverting input minimizes the voltage across the clamps and
keeps the leakage due to the diodes low. Low leakage diodes,
such as the FD333s, should be used and should be shielded
from light to keep photocurrents from being generated. Even
with these precautions, the diodes measurably increase input
current and capacitance.
SAMPLE AND DIFFERENCE CIRCUIT TO MEASURE
ELECTROMETER LEAKAGE CURRENTS
There are a number of methods used to test electrometer leak-
age currents, including current integration and direct current-
to-voltage conversion. Regardless of the method used, board
and interconnect cleanliness, proper choice of insulating
materials (such as Teflon or Kel-F®), correct guarding and
shielding techniques, and care in physical layout are essential to
making accurate leakage measurements.
Figure 40 is a schematic of the sample and difference circuit. It
uses two AD549 electrometer amplifiers (A and B) as current-
to-voltage converters with high value (10
(RSa and RSb). R1 and R2 provide for an overall circuit sensitiv-
ity of 10 fA/mV (10 pA full scale). C
suppression and loop compensation. C
polystyrene capacitor. An ultralow leakage Kel-F test socket is
used for contacting the device under test. Rigid Teflon coaxial
cable is used to make connections to all high impedance
nodes. The use of rigid coaxial cable affords immunity to
error induced by mechanical vibration and provides an outer
conductor for shielding. The entire circuit is enclosed in a
grounded metal box.
SOURCE
SOURCE
Figure 38. Follower with Input Current Limit
Figure 39. Input Voltage Clamp with Diodes
PROTECT
DIODES
R
PROTECT
3
2
2
3
C
and C
AD549
AD549
C
should be a low leakage
10
Ω) sense resistors
F
R
provide noise
F
6
6
AD549