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AD515A Datasheet(PDF) 5 Page - Analog Devices

Part No. AD515A
Description  Monolithic Precision, Low Power FET-Input Electrometer Op Amp
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD515A Datasheet(HTML) 5 Page - Analog Devices

   
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AD515A
–5–
REV. A
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 k
Ω 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 k
Ω load, to reduce this additional dissipation, we
recommend restricting the load resistance to be at least 10 k
Ω.
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
Figure 10. Input Bias Current vs. Differential Input Voltage
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
and high stability performance. The best packaging for high
M
Ω 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


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