Monolithic Precision, Low Power
FET-Input Electrometer Op Amp
a
AD515A
FEATURES
PIN CONFIGURATION
Ultralow Bias Current: 75 fA max (AD515AL)
Ultralow Bias Current: 150 fA max (AD515AK)
Ultralow Bias Current: 300 fA max (AD515AJ)
Low Power: 1.5 mA max Quiescent Current
Low Power: (0.6 mA typ)
Low Offset Voltage: 1.0 mV max (AD515AK & L)
Low Drift: 15 V/؇C max (AD515AK)
Low Noise: 4 V p-p, 0.1 Hz to 10 Hz
PRODUCT DESCRIPTION
The AD515A is a monolithic FET-input operational amplifier
with a guaranteed maximum input bias current of 75 fA
(AD515AL). The AD515A is a monolithic successor to the
industry standard AD515 electrometer, and will replace the
AD515 in most applications. The AD515A also delivers laser-
trimmed offset voltage, low drift, low noise and low power, a
combination of features not previously available in ultralow bias
current circuits. All devices are internally compensated, protected
against latch-up and are short circuit protected.
PRODUCT HIGHLIGHTS
1. The AD515A provides subpicoampere bias currents in an
integrated circuit amplifier.
• The ultralow input bias currents are specified as the maxi-
mum measured at either input with the device fully warmed
up on ±15 V supplies at +25°C ambient with no heat sink.
This parameter is 100% tested.
• By using ±5 V supplies, input bias current can typically be
brought below 50 fA.
The AD515A’s combination of low input bias current, low
offset voltage and low drift optimizes it for a wide variety of
electrometer and very high impedance buffer applications
including photocurrent detection, vacuum ion-gage measure-
ment, long-term precision integration and low drift sample/hold
applications. This amplifier is also an excellent choice for all forms
of biomedical instrumentation such as pH/pIon sensitive elec-
trodes, very low current oxygen sensors, and high impedance
biological microprobes. In addition, the low cost and pin
compatibility of the AD515A with standard FET op amps will
allow designers to upgrade the performance of present systems
at little or no additional cost. The 1015 Ω common-mode input
impedance ensures that the input bias current is essentially
independent of common-mode voltage.
2. The input offset voltage on all grades is laser trimmed, typically
less than 500 µV.
• The offset voltage drift is 15 µV/°C maximum on the
K grade.
• If additional pulling is desired, the amount required will
have a minimal effect on offset drift (approximately 3 µV/°C
per mV).
3. The low quiescent current drain of 0.6 mA typical and
1.5 mA maximum, keeps self-heating effects to a minimum
and renders the AD515A suitable for a wide range of remote
probe applications.
As with previous electrometer amplifier designs from Analog
Devices, the case is brought out to its own connection (Pin 8)
so it can be independently connected to a point at the same
potential as the input, thus minimizing stray leakage to the case.
This feature will also shield the input circuitry from external
noise and supply transients.
4. The combination of low input noise voltage and very low
input noise current is such that for source impedances from
1M Ω to 1011 Ω, the Johnson noise of the source will easily
dominate the noise characteristic.
The AD515A is available in three versions of bias current and
offset voltage, the “J”, “K” and “L”; all are specified for rated
performance from 0°C to +70°C and supplied in a hermetically
sealed TO-99 package.
REV. A
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
World Wide Web Site: http://www.analog.com
© Analog Devices, Inc., 1997
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