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AD515AJH
Monolithic Precision, Low Power FET-Input Electrometer Op Amp
REV.A
Monolithic Precision, Low Power
FET-Input Electrometer Op Amp
PIN CONFIGURATIONFEATURES
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 mV/8C max (AD515AK)
Low Noise: 4 mV p-p, 0.1 Hz to 10 Hz
PRODUCT DESCRIPTIONThe 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 HIGHLIGHTSThe 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 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).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.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.
AD515A–SPECIFICATIONS
(typical @ +258C and VS = 615 V dc, unless otherwise noted)NOTES
*Specifications same as AD515AJ.Open Loop Gain is specified with or without pulling of VOS.A conservative design would not exceed 750 pF of load capacitance.Input Offset Voltage specifications are guaranteed after 5 minutes of
operation at TA = +25°C.Bias Current specifications are guaranteed after 5 minutes of operation at
TA = +25°C. For higher temperatures, the current doubles every +10°C.1f it is possible for the input voltage to exceed the supply voltage, a series
protection resistor should be added to limit input current to 0.1 mA.
The input devices can handle overload currents of 0.1 mA indefinitely without
damage. See next page.
Specifications shown in boldface are tested on all production units at final test.
Specifications subject to change without notice.
WARNING!
CAUTIONESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000V readily
accumulate on the human body and test equipment and can discharge without detection.
AD515A
LAYOUT AND CONNECTIONS CONSIDERATIONSThe design of very high impedance measurement systems in-
troduces a new level of problems associated with the reduction
of leakage paths and noise pickup.A primary consideration in high impedance system designs is
to attempt to place the measuring device as near to the signal
source as possible. This will minimize current leakage paths,
noise pickup and capacitive loading. The AD515A, with its
combination of low offset voltage (normally eliminating the
need for trimming), low quiescent current (minimal source
heating, possible battery operation), internal compensation
and small physical size lends itself to installation at the signal
source or inside a probe. As a result of the high load capaci-
tance rating, the AD515A can comfortably drive a long
signal cable.The use of guarding techniques is essential to realizing the
capability of the ultralow input currents of the AD515A.
Guarding is achieved by applying a low impedance bootstrap
potential to the outside of the insulation material surround-
ing the high impedance signal line. This bootstrap potential
is held at the same level as that of the high impedance line;
therefore, there is no voltage drop across the insulation and,
hence, no leakage. The guard will also act as a shield to
reduce noise pickup and serves an additional function of
reducing the effective capacitance to the input line. The case
of the AD515A is brought out separately to Pin 8 so it can
also be connected to the guard potential. This technique
virtually eliminates potential leakage paths across the package
insulation, provides a noise shield for the sensitive circuitry
and reduces common-mode input capacitance to about 0.8
pF. Figure 1 shows a proper printed circuit board layout for
input guarding and connecting the case guard. Figures 2 and
3 show guarding connections for typical inverting and
noninverting applications. If Pin 8 is not used for guarding, it
should be connected to ground or a power supply to reduce
noise.
Figure 1.Board Layout for Guarding Inputs with Guarded
TO-99 PackagePrinted circuit board layout and construction is critical for
achieving the ultimate in low leakage performance that the
AD515A can deliver. The best performance will be realized
by using a teflon IC socket for the AD515A; at a minimum a
teflon standoff should be used for the high impedance lead.
If this is not feasible, the input guarding scheme shown in
Figure 1 will minimize leakage as much as possible; the
circuit; to minimize noise and leakage, they must be carried
in rigid, shielded cables.Another important concern for achieving and maintaining
low leakage currents is complete cleanliness of circuit boards
and components. Completed assemblies should be washed
thoroughly in a low residue solvent such as TMC Freon or
high purity methanol, followed by a rinse with deionized
water and nitrogen drying. If service is anticipated in a high
contaminant or high humidity environment, a high dielectric
conformal coating is recommended. All insulation materials
except Kel-F or teflon will show rapid degradation of surface
leakage at high humidities.
Figure 2.Picoampere Current-to-Voltage Converter
Inverting Configuration
Figure 3.Very High Impedance Noninverting Amplifier
INPUT PROTECTIONThe AD515A is guaranteed for a maximum safe input potential
equal to the power supply potential.
Many instrumentation situations, such as flame detectors in gas
chromatographs, involve measurement of low level currents
from high voltage sources. In such applications, a sensor fault
condition may apply a very high potential to the input of the
current-to-voltage converting amplifier. This possibility necessi-
tates some form of input protection. Many electrometer type
devices, especially CMOS designs, can require elaborate Zener
protection schemes that often compromise overall performance.
The AD515A requires input protection only if the source is not
current limited and, as such, is similar to many JFET-input
designs. The failure mode would be overheating from excess
current rather than voltage breakdown. If the source is not
current limited, all that is required is a resistor in series with the
affected input terminal so that the maximum overload current is
AD515A
COAXIAL CABLE AND CAPACITANCE EFFECTSIf it is not possible to attach the AD515A virtually on top of the
signal source, considerable care should be exercised in designing
the connecting lines carrying the high impedance signal. Shielded
coaxial cable must be used for noise reduction, but use of
coaxial cables for high impedance work can add problems from
cable leakage, noise and capacitance. Only the best polyethylene
or virgin teflon (not reconstituted) should be used to obtain the
highest possible insulation resistance.
Cable systems should be made as rigid and vibration free as
possible since cable movement can cause noise signals of three
types, all significant in high impedance systems. Frictional
movement of the shield over the insulation material generates a
charge that is sensed by the signal line as a noise voltage. Low
noise cable with graphite lubricant such as Amphenol 21-537
will reduce the noise, but short rigid lines are better. Cable
movements will also make small changes in the internal cable
capacitance and capacitance to other objects. Since the total
charge on these capacitances cannot be instantly changed, a
noise voltage results, as predicted from: ΔV = Q/ΔC. Noise
voltage is also generated by the motion of a conductor in a
magnetic field.
The conductor-to-shield capacitance of coaxial cable is usually
about 30 pF/foot. Charging this capacitance can cause consider-
able stretching of high impedance signal rise time, thus cancel-
ling the low input capacitance feature of the AD515A. There are
two ways to circumvent this problem. For inverting signals or
low level current measurements, the signal is carried on the line
connected to the inverting input and shielded (guarded) by the
ground line as shown in Figure 2. Since the signal is always at
virtual ground, no voltage change is required and no capaci-
tances are charged. In many circumstances, this will destabilize
the circuit; if so, capacitance from output to inverting input will
stabilize the circuit.
Noninverting and buffer situations are more critical since the
signal line voltage and therefore charge will change, causing
signal delay. This effect can be considerably reduced by
connecting the cable shield to a guard potential instead of
ground, an option shown in Figure 3. Since such a connection
results in positive feedback to the input, the circuit may be
destabilized and oscillate. If so, capacitance from positive input
to ground must be added to make the net capacitance at Pin 3
positive. This technique can considerably reduce the effective
capacitance that must be charged.
Typical Performance CurvesFigure 4.PSRR and CMRR vs. Frequency
Figure 6.Input Common-Mode Range vs. Supply Voltage
ELECTROMETER APPLICATION NOTESThe 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.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.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.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.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
Figure 10.Input Bias Current vs. Differential Input Voltage
AD515A CIRCUIT APPLICATION NOTESThe 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.
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
AD515AC1135a–1–5/97
PRINTED IN U.S.A.
the total noise by about 40%. Noise currents driving the resistors
also add, but in the AD515A are significant only above 1011 Ω.
Figure 11.Very High Impedance Instrumentation
Amplifier
Figure 12.Low Drift Integrator and Low Leakage Guarded
Reset
LOW LEVEL CURRENT-TO-VOLTAGE CONVERTERSFigure 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 above9 Ω tend to be expensive, large, noisy and unstable. To avoid
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 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 14. Current-to-Voltage Converters with Grounded
Bias and Sensor
OUTLINE DIMENSIONSDimensions shown in inches and (mm).
TO-99
