AD546

REV. A

–10–

input current at either terminal stays below a few hundred

femtoamps until one input terminal is forced higher than 1 V to

1.5 V above the other terminal. Under these conditions, the

input current limits at 30

µA.

INPUT PROTECTION

The AD546 safely handles any input voltage within the supply

voltage range. Subjecting the input terminals to voltages beyond

the power supply can destroy the device or cause shifts in input

current or offset voltage if the amplifier is not protected.

A protection scheme for the amplifier as an inverter is shown in

current through the inverting input to 1 mA for expected tran-

sient (less than 1 second) overvoltage conditions, or to 100

µA

and is much lower in value than the amplifier’s input resistance,

it does not affect the inverter’s dc gain. However, the Johnson

noise of the resistor will add root sum of squares to the

amplifier’s input noise.

Figure 35. Inverter with Input Current Limit

In the corresponding version of this scheme for a follower,

terminal will produce a pole in the signal frequency response at

a f = 1/2

π RC. Again, the Johnson noise of R

amplifier’s input voltage noise.

Figure 37 is a schematic of the AD546 as an inverter with an in-

put voltage clamp. Bootstrapping the clamp diodes at the invert-

ing input minimizes the voltage across the clamps and keeps the

leakage due to the diodes low. Low leakage diodes (less

Figure 36. Follower with Input Current Limit

Figure 37. Input Voltage Clamp with Diodes

than 1 pA), such as the FD333’s should be used, and should be

shielded from light to keep photocurrents from being generated.

Even with these precautions, the diodes will measurably increase

the input current and capacitance.

In order to achieve the low input bias currents of the AD546, it

is not possible to use the same on-chip protection as used in

other Analog Devices op amps. This makes the AD546 sensitive

to handling and precautions should be taken to minimize ESD

exposure whenever possible.

Figure 38. Sample and Difference Circuit for Measuring

Electrometer Leakage Currents

MEASURING ELECTROMETER LEAKAGE CURRENTS

There are a number of methods used to test electrometer leak-

age currents, including current integration and direct current to

voltage conversion. Regardless of the method used, board and

interconnect cleanliness, proper choice of insulating materials

(such as Teflon or Kel-F), correct guarding and shielding tech-

niques and care in physical layout are essential for making accu-

rate leakage measurements.

Figure 38 is a schematic of the sample and difference circuit

which is useful for measuring the leakage currents of the AD546

and other electrometer amplifiers. The circuit uses two AD549

electrometer amplifiers (A and B) as current to voltage convert-

ers with high value (10

10

Ω) sense resistors (RSa and RSb). R1

and R2 provide for an overall circuit sensitivity of 10 fA/mV

pacitor. An ultralow-leakage Kel-F test socket is used for con-