Datasheet AD648 (Analog Devices) - 7

AD648 APPLICATION NOTES
The AD648 is a pair of JFET-input op amps with a guaranteed maximum IB of less than 10 pA, and offset and drift laser- trimmed to 1.0 mV and 10 µV/°C, respectively (AD648B). AC specs include 1 MHz bandwidth, 1.8 V/µs typical slew rate and 8 µs settling time for a 20 V step to ± 0.01%—all at a supply current less than 400 µA. To capitalize on the device’s perfor- mance, a number of error sources should be considered. The minimal power drain and low offset drift of the AD648 Figure 22. Board Layout for Guarding Inputs reduce self-heating or “warm-up” effects on input offset voltage, making the AD648 ideal for on/off battery powered applica-
INPUT PROTECTION
tions. The power dissipation due to the AD648’s 400 µA supply The AD648 is guaranteed to withstand input voltages equal to current has a negligible effect on input current, but heavy out- the power supply potential. Exceeding the negative supply volt- put loading will raise the chip temperature. Since a JFET’s age on either input will forward bias the substrate junction of input current doubles for every 10°C rise in chip temperature, the chip. The induced current may destroy the amplifier due to this can be a noticeable effect. excess heat. The amplifier is designed to be functional with power supply Input protection is required in applications such as a flame voltages as low as ± 4.5 V. It will exhibit a higher input offset detector in a gas chromatograph, where a very high potential voltage than at the rated supply voltage of ± 15 V, due to power may be applied to the input terminals during a sensor fault supply rejection effects. Common-mode range extends from 3 V condition. Figures 23a and 23b show simple current limiting more positive than the negative supply to 1 V more negative schemes that can be used. RPROTECT should be chosen such that than the positive supply. Designed to cleanly drive up to 10 kΩ the maximum overload current is 1.0 mA (for example 100 kΩ and 100 pF loads, the AD648 will drive a 2 kΩ load with reduced for a 100 V overload). open-loop gain. Figure 21 shows the recommended crosstalk test circuit. A typical value for crosstalk is –120 dB at 1 kHz. Figure 23a. Input Protection of l-to-V Converter Figure 21. Crosstalk Test Circuit
LAYOUT
To take full advantage of the AD648’s 10 pA max input current, parasitic leakages must be kept below an acceptable level. The practical limit of the resistance of epoxy or phenolic circuit board material is between 1 × 1012 Ω and 3 × 1012 Ω. This can Figure 23b. Voltage Follower Input Protection Method result in an additional leakage of 5 pA between an input of 0 V and a –15 V supply line. Teflon or a similar low leakage material Figure 23b shows the recommended method for protecting a (with a resistance exceeding 1017 Ω) should be used to isolate voltage follower from excessive currents due to high voltage high impedance input lines from adjacent lines carrying high breakdown. The protection resistor, RP, limits the input current. voltages. The insulator should be kept clean, since contaminants A nominal value of 100 kΩ will limit the input current to less will degrade the surface resistance. than 1 mA with a 100 volt input voltage applied. A metal guard completely surrounding the high impedance The stray capacitance between the summing junction and nodes and driven by a voltage near the common-mode input ground will produce a high-frequency roll-off with a corner potential can also be used to reduce some parasitic leakages. frequency equal to: The guarding pattern in Figure 22 will reduce parasitic leakage due to finite board surface resistance; but it will not compensate fcorner = 1 2 π RP Cstray for a low volume resistivity board. Accordingly, a 100 kΩ value for RP with a 3 pF Cstray will cause a 3 dB corner frequency to occur at 531 kHz. REV. E –7–