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1 pA Monolithic Electrometer Operational Amplifier - Analog Devices

Número de pieza AD546
Descripción 1 pA Monolithic Electrometer Operational Amplifier
Fabricantes Analog Devices 
Logotipo Analog Devices Logotipo
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AD546 datasheet

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AD546 pdf
300
250
200
150
+25oC
100
0
5 10 15
SUPPLY VOLTAGE ± VOLTS
20
Figure 10. Input Bias Current
vs. Supply Voltage
160
140
120
100
80
60
40
20
10
100
1k 10k
FREQUENCY – Hz
Figure 11. Input Voltage Noise
Spectral Density vs. Frequency
AD546
100k WHENEVER JOHNSON NOISE IS GREATER THAN
AMPLIFIER NOISE, AMPLIFIER NOISE CAN BE
CONSIDERED NEGLIGIBLE FOR THE APPLICATION.
10k
1 kHz BANDWIDTH
1k
RESISTOR JOHNSON NOISE
100
10
10 Hz
BANDWIDTH
1
0.1
100k
AMPLIFIER GENERATED NOISE
1M 10M 100M 1G 10G
SOURCE RESISTANCE – Ohms
100G
Figure 12. Noise vs. Source
Resistance
100 100
80 80
60 60
40 40
20 20
00
–20 –20
–40
10 100 1k
–40
10k 100k 1M 10M
FREQUENCY – Hz
Figure 13. Open Loop Frequency
Response
40
35
30
25
20
15
10
5
0
10 100 1k 10k 100k 1M
FREQUENCY – Hz
Figure 14. Large Signal Frequency
Response
Figure 15. CMRR vs. Frequency
Figure 16. PSRR vs. Frequency
Figure 17. Output Settling Time vs.
Output Swing and Error Voltage
REV. A
–5–

5 Page

AD546 arduino
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 µA.
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, RP, is chosen to limit the
current through the inverting input to 1 mA for expected tran-
sient (less than 1 second) overvoltage conditions, or to 100 µA
for a continuous overload. Since RP is inside the feedback loop,
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.
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 35. Inverter with Input Current Limit
In the corresponding version of this scheme for a follower,
shown in Figure 36, RP and the capacitance at the positive input
terminal will produce a pole in the signal frequency response at
a f = 1/2 π RC. Again, the Johnson noise of RP will add to the
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 38. Sample and Difference Circuit for Measuring
Electrometer Leakage Currents
Figure 36. Follower with Input Current Limit
Figure 37. Input Voltage Clamp with Diodes
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 (1010 ) sense resistors (RSa and RSb). R1
and R2 provide for an overall circuit sensitivity of 10 fA/mV
(10 pA full scale). CC and CF provide noise suppression and
loop compensation. CC should be a low leakage polystyrene ca-
pacitor. An ultralow-leakage Kel-F test socket is used for con-
–10–
REV. A

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