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PDF AD515A Data sheet ( Hoja de datos )
| Número de pieza | AD515A | |
| Descripción | Monolithic Precision/ Low Power FET-Input Electrometer Op Amp | |
| Fabricantes | Analog Devices | |
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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 Ω 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 Ω to 1011 Ω, 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
1 page  
AD515A
ELECTROMETER APPLICATION NOTES
The AD515A offers subpicoampere input bias currents available
in an integrated circuit package. This design will open up many
new application opportunities for measurements from very high
impedance and very low current sources. Performing accurate
measurements of this sort requires careful attention to detail;
the notes given here will aid the user in realizing the full
measurement potential of the AD515A and perhaps extending
its performance limits.
1. As with all junction FET input devices, the temperature of
the FETs themselves is critical in determining the input bias
currents. Over the operating temperature range, the input
bias currents closely follow a characteristic of doubling every
10°C; therefore, every effort should be made to minimize
device operating temperature.
2. The heat dissipation can be reduced initially by careful
investigation of the application. First, if it is possible to
reduce the required power supplies, this should be done
since internal power consumption contributes the largest
component of self-heating. To minimize this effect, the
quiescent current of the AD515A has been reduced to less
than 1 mA. Figure 8 shows typical input bias current and
quiescent current versus supply voltage.
3. Output loading effects, which are normally ignored, can
cause a significant increase in chip temperature and therefore
bias current. For example, a 2 kΩ load driven at 10 V at the
output will cause at least an additional 25 mW dissipation in
the output stage (and some in other stages) over the typical
24 mW, thereby at least doubling the effects of self-heating.
The results of this form of additional power dissipation are
demonstrated in Figure 9, which shows normalized input
bias current versus additional power dissipated. Therefore,
although many dc performance parameters are specified
driving a 2 kΩ load, to reduce this additional dissipation, we
recommend restricting the load resistance to be at least 10 kΩ.
4. Figure 10 shows the AD515A’s input current versus differen-
tial input voltage. Input current at either terminal stays below
a few hundred fA until one input terminal is forced higher
than 1 V to 1.5 V above the other terminal. Input current
limits at 30 µA under these conditions.
Figure 8. Input Bias Current and Supply Current vs.
Supply Voltage
Figure 9. Input Bias Current vs. Additional Power
Dissipation
AD515A CIRCUIT APPLICATION NOTES
The AD515A is quite simple to apply to a wide variety of
applications because of the pretrimmed offset voltage and
internal compensation, which minimize required external
components and eliminate the need for adjustments to the
device itself. The major considerations in applying this device
are the external problems of layout and heat control which have
already been discussed. In circuit situations employing the use of
very high value resistors, such as low level current to voltage
converters, electrometer operational amplifiers can be destabi-
lized by a pole created by the small capacitance at the negative
input. If this occurs, a capacitor of 2 pF to 5 pF in parallel with
the resistor will stabilize the loop. A much larger capacitor may
be used if desired to limit bandwidth and thereby reduce wide-
band noise.
Selection of passive components employed in high impedance
film or deposited ceramic oxide to obtain the best in low noise
Figure 10. Input Bias Current vs. Differential Input Voltage
and high stability performance. The best packaging for high
MΩ resistors is a glass body sprayed with silicone varnish to
minimize humidity effects. These resistors must be handled
very carefully to prevent surface contamination. Capacitors for
any high impedance or long-term integration situation should
be of a polystyrene formulation for optimum performance.
Most other types have too low an insulation resistance, or high
dielectric absorption.
Unlike situations involving standard operational amplifiers with
much higher bias currents, balancing the impedances seen at
the input terminals of the AD515A is usually unnecessary and
probably undesirable. At the large source impedances, where
these effects matter, obtaining quality matched resistors will be
difficult. More important, instead of a cancelling effect, as with
bias current, the noise voltage of the additional resistor will add
by root-sum-of-squares to that of the other resistor thus increasing
REV. A
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