AD523_15 Datasheet, PDF (1/3 Pages) Analog Devices – ELECTROMETER-ULTRlOAW BIASCURRENT

ELECTROMETER-ULTRlOAW BIASCURRENT
- ^{MODELS310, 311,41,42, AD523}
GENERAL DESCRIPTION
Amplifiers with bias currents less than 1pA are classified
as suitable for electrometer use where frequency response
and voltage drift are usually secondary requirements. Both
varactor bridge and FET input designs are employed to
achieve these bias currents ranging from one pico amp
(10-12 A) to ten femptoamps (10-14 A).
Available with either inverring, noninvening or differential
inputs, these amplifiers are used as current to voltage
OBSOLETE conveners with high impedance transducers such as photo-
multiplier tUbes, flame detectors, pH cells and radiation
detectors. To minimize RFI and other noise pickup problems,
the varactor modulated amplifiers, operating at 10fA, are
available with shielded cases.
VARACTOR BRIDGE ELECTROMETERS
MODEL 310 (INVERTING), MODEL 311 (NONINVERTING)
These operational amplifiers featUre extremely low input
bias currents and high input impedances. They are appli-
cable to a wide range of electrometer applications which
have been traditionally fulfilled using vacuum tUbe types.
Because of varactor bridge inputs, the solid state models
310 and 311 are best suited for applications characterized
by extremely high source impedance or where infinitesimal
currents must be measured or amplified accurately. In
principle, the varactor bridge amplifier design is similar to
that of the vibrating reed electrometers (parametric), but
Model
Open Loop Gain
DC Rated Load, min
Rated Output, mm
Frequency Kesponse
Unity Gain, Small Signal
Lowest Cost
High Gain FET
42
J
K
300,000
:tIOV@SmA
lMHz
L
with the inherent advantages of solid state circuitry.
Full Power Response, min
Slewing Rate, min
4kHz
0.2SV//ls
Typical specifications for models 310 and 311 include open
Overload Recovery
IOms
loop gain of 100dB, 2kHz unity gain response, 0.4V/msec
slew rate, initial bias current of 10-14 A, with 10-15 Alc
Input Offset Voltage
Initial, 25°C, (adj. to zero)
Avg. vs. Temp (0 to 70°C) max
:t2mV'
:tSO
I ^:tiS
1t2SI1V/C
current stability, and low current and voltage noise of
10-15 A and 10~V pop (1Hz bandwidth) respectively. Two
voltage drift selections are available: 310], 311J with
30~vlc, and 310K, 311K with lO~vlc. Each is housed
in an aluminum enclosure for improved shielding.
vs. Supply Voltage
vs. Time
Input Bias Current
Initial, 2SoC, max
Inverting Input (VaractOr)
Non-Inverting Input (Varactor)
Avg. vs. Temp (0 to 70°C)
3SOfA
:t2S/lV/%
:t2S0/lV/mo.
1100f_-A
---
I ^7SfA
0, -4oA3
Model 310, with inverting input only, is most appropriate
for use with current source signals such as gas chromatographs
flame detectors and photomultiplier tUbes. It is also useful
for precision long term integrators or where extremely wide
dynamic current range is needed as in log compression
amplifiers. Current to voltage conveners may also be
developed using a feedback resistor for setting the conversion
scale factor.
Model 311 has a single noninvening input for measuring
voltage from very high source impedances where bias currents
would create substantial offset errors. Such sources include
Input Impedance
Differential
Invetting Input (to common)
Non-Inverting Input (to common)
Common Mode (FET)
Input Noise
Voltage, 0.01 to 1Hz, pop
SHz to SOkHz, rms
Current, 0.1 to 10Hz, pop
1 to 100Hz, rms (Varactor)
Input Voltage Range
Common Mode Voltage, min
Common Mode Rejection
Max Safe Differential Voltage
Power Supply Range (VDC)
Rated Specification (VDC)
10'3 _-n_-l_-l_-3pF
----
n 1013
6/lV
8/lV
Sf A
---
:!-lOV
66dB@:tlV
+ISV
:t(l2 to 18)V
:tlSV@2mA
pH cells or stored capacitor charge as found in long term
track and hold applications. Common mode rejection is
100dB at :!:25V with 1014[2 impedance to ground for
reduced source loading errors.
FET INPUT ELECTROMETERS
Temperature Range
Operating, Rated Specifications
Package Outline
Case Dimensions
Price
1-9
10-24
0 to +70°C
QB-l
1.1" x 1.1" x 0.57"
$32
$30
I ^$.'³"⁸
$42
I .,0
MODELS 41,42, AD523 (MONOLITHIC): This family of
FET input amplifiers fully complements the varactor bridge
(1 )With exwnol4.99k trim.
(2)With trim terminal open.
(J L\h~ bias at 70°C
(4)Slgnal input only.
48 AMPLIFIERS