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9 / 12 page AD549 REV. A –9– DIFFERENTIAL INPUT VOLTAGE – V (VIN– – VIN–) 100µ 10µ 1µ –5 –4 –3 –2 –1 0 1 2 3 4 5 100n 10n 1n 100p 10p 1p 100f 10f IIN– IIN+ Figure 36. Input Current vs. Differential Input Voltage INPUT PROTECTION The AD549 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 Figure 37. RP is chosen to limit the current through the invert- ing 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 37. Inverter with Input Current Limit In the corresponding version of this scheme for a follower, shown in Figure 38, 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 R P will add to the amplifier’s input voltage noise. Figure 38. Follower with Input Current Limit Figure 39 is a schematic of the AD549 as an inverter with an input voltage clamp. Bootstrapping the clamp diodes at the in- verting input minimizes the voltage across the clamps and keeps the leakage due to the diodes low. Low leakage diodes, 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 cur- rent and capacitance. Figure 39. Input Voltage Clamp with Diodes SAMPLE AND DIFFERENCE CIRCUIT TO MEASURE 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 physi-cal layout are essential to making accu- rate leakage measurements. Figure 40 is a schematic of the sample and difference circuit. It uses two AD549 electrometer amplifiers (A and B) as current-to voltage converters with high value (10 10 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 suppres- sion and loop compensation. CC should be a low leakage poly- styrene capacitor. An ultralow leakage Kel-F test socket is used for contacting the device under test. Rigid Teflon coaxial cable is used to make connections to all high impedance nodes. The Figure 40. Sample and Difference Circuit for Measuring Electrometer Leakage Currents |
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