AD546JN AD [Analog Devices], AD546JN Datasheet - Page 10

• Manufacturer Part Number: AD546JN
• Description: 1 pA Monolithic Electrometer Operational Amplifier
• Manufacturer: AD [Analog Devices]

AD546
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 A.
INPUT PROTECTION
The AD546 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 35. The protection resistor, R
current through the inverting input to 1 mA for expected tran-
sient (less than 1 second) overvoltage conditions, or to 100 A
for a continuous overload. Since R
and is much lower in value than the amplifier’s input resistance,
it does not affect the inverter’s dc gain. However, the Johnson
noise of the resistor will add root sum of squares to the
amplifier’s input noise.
In the corresponding version of this scheme for a follower,
shown in Figure 36, R
terminal will produce a pole in the signal frequency response at
a f = 1/2
amplifier’s input voltage noise.
Figure 37 is a schematic of the AD546 as an inverter with an in-
put voltage clamp. Bootstrapping the clamp diodes at the invert-
ing input minimizes the voltage across the clamps and keeps the
leakage due to the diodes low. Low leakage diodes (less
Figure 36. Follower with Input Current Limit
Figure 37. Input Voltage Clamp with Diodes
Figure 35. Inverter with Input Current Limit
RC. Again, the Johnson noise of R
P
and the capacitance at the positive input
P
is inside the feedback loop,
P
, is chosen to limit the
P
will add to the
–10–
than 1 pA), such as the FD333’s should be used, and should be
shielded from light to keep photocurrents from being generated.
Even with these precautions, the diodes will measurably increase
the input current and capacitance.
In order to achieve the low input bias currents of the AD546, it
is not possible to use the same on-chip protection as used in
other Analog Devices op amps. This makes the AD546 sensitive
to handling and precautions should be taken to minimize ESD
exposure whenever possible.
Figure 38. Sample and Difference Circuit for Measuring
Electrometer Leakage Currents
MEASURING 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 tech-
niques and care in physical layout are essential for making accu-
rate leakage measurements.
Figure 38 is a schematic of the sample and difference circuit
which is useful for measuring the leakage currents of the AD546
and other electrometer amplifiers. The circuit uses two AD549
electrometer amplifiers (A and B) as current to voltage convert-
ers with high value (10
and R2 provide for an overall circuit sensitivity of 10 fA/mV
(10 pA full scale). C
loop compensation. C
pacitor. An ultralow-leakage Kel-F test socket is used for con-
C
C
and C
10
should be a low leakage polystyrene ca-
) sense resistors (RSa and RSb). R1
F
provide noise suppression and
REV. A