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AD515A Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer AD515A
Beschreibung Monolithic Precision/ Low Power FET-Input Electrometer Op Amp
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 6 Seiten
AD515A Datasheet, Funktion
a
FEATURES
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
Monolithic Precision, Low Power
FET-Input Electrometer Op Amp
AD515A
PIN CONFIGURATION
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.
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 common-mode input
impedance ensures that the input bias current is essentially
independent of common-mode voltage.
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.
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.
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.
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.
4. The combination of low input noise voltage and very low
input noise current is such that for source impedances from
1M to 1011 , the Johnson noise of the source will easily
dominate the noise characteristic.
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 World Wide Web Site: http://www.analog.com
Fax: 617/326-8703
© Analog Devices, Inc., 1997






AD515A Datasheet, Funktion
AD515A
the total noise by about 40%. Noise currents driving the resistors
also add, but in the AD515A are significant only above 1011 .
this, it may be desirable to use a circuit configuration with
output gain, as in Figure 13. The drawback is that input errors
of offset voltage drift and noise are multiplied by the same gain,
but the precision performance of the AD515A makes the trade-
off easier.
Figure 11. Very High Impedance Instrumentation
Amplifier
Figure 13. Picoampere to Voltage Converter with Gain
One of the problems with low level leakage current testing or
low level current transducers (such as Clark oxygen sensors) is
finding a way to apply voltage bias to the device while still
grounding the device and the bias source. Figure 14 shows a
technique in which the desired bias is applied at the noninvert-
ing terminal thus forcing that voltage at the inverting terminal.
The current is sensed by RF, and the AD524 instrumentation
amplifier converts the floating differential signal to a single-
ended output.
Figure 12. Low Drift Integrator and Low Leakage Guarded
Reset
LOW LEVEL CURRENT-TO-VOLTAGE CONVERTERS
Figure 2 shows a standard low level current-to-voltage converter.
To obtain higher sensitivity, it is obvious to simply use a higher
value feedback resistor. However, high value resistors above
109 tend to be expensive, large, noisy and unstable. To avoid
Figure 14. Current-to-Voltage Converters with Grounded
Bias and Sensor
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
TO-99
–6– REV. A

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