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AD546 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.

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.

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.

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.

A protection scheme for the amplifier as an inverter is shown in Figure 35. The protection resistor, RP, is chosen to limit the current through the inverting input to 1 mA for expected transient (less than 1 second) overvoltage conditions, or to 100 µA for a continuous overload. Since RP is inside the feedback loop, 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, shown in Figure 36, RP and the capacitance at the positive input terminal will produce a pole in the signal frequency response at a f = 1/2 π RC. Again, the Johnson noise of RP will add to the amplifier’s input voltage noise. Figure 37 is a schematic of the AD546 as an inverter with an input voltage clamp. Bootstrapping the clamp diodes at the inverting input minimizes the voltage across the clamps and keeps the leakage due to the diodes low. Low leakage diodes (less Figure 38. Sample and Difference Circuit for Measuring Electrometer Leakage Currents MEASURING ELECTROMETER LEAKAGE CURRENTS

Figure 36. Follower with Input Current Limit

Figure 37. Input Voltage Clamp with Diodes

There are a number of methods used to test electrometer leakage 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 techniques and care in physical layout are essential for making accurate 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 converters with high value (1010 Ω) sense resistors (RSa and RSb). R1 and R2 provide for an overall circuit sensitivity of 10 fA/mV (10 pA full scale). CC and CF provide noise suppression and loop compensation. CC should be a low leakage polystyrene capacitor. An ultralow-leakage Kel-F test socket is used for con–10–

REV. A


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