Monolithic Precision, Low Power FET-Input Electrometer Op Amp

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 measurement,

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 electrodes,

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 10 15 W 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.