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AD648BH Folha de dados(PDF) 6 Page - Analog Devices |
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AD648BH Folha de dados(HTML) 6 Page - Analog Devices |
6 / 12 page AD648 REV. C –6– 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 0.3 mV and 3 µV/°C, respectively (AD648C). 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 re- duce self-heating or “warm-up” effects on input offset voltage, making the AD648 ideal for on/off battery powered applica- tions. The power dissipation due to the AD648’s 400 µA supply current has a negligible effect on input current, but heavy out- put loading will raise the chip temperature. Since a JFET’s input current doubles for every 10 °C rise in chip temperature, this can be a noticeable effect. The amplifier is designed to be functional with power supply voltages as low as ±4.5 V. It will exhibit a higher input offset voltage than at the rated supply voltage of ±15 V, due to power supply rejection effects. Common-mode range extends from 3 V more positive than the negative supply to 1 V more negative than the positive supply. Designed to cleanly drive up to 10 k Ω and 100 pF loads, the AD648 will drive a 2 k Ω load with re- duced open-loop gain. Figure 21 shows the recommended crosstalk test circuit. A typi- cal value for crosstalk is –120 dB at 1 kHz. 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 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 (with a resistance exceeding 1017 Ω) should be used to isolate high impedance input lines from adjacent lines carrying high voltages. The insulator should be kept clean, since contaminants will degrade the surface resistance. A metal guard completely surrounding the high impedance nodes and driven by a voltage near the common-mode input potential can also be used to reduce some parasitic leakages. The guarding pattern in Figure 22 will reduce parasitic leakage due to finite board surface resistance; but it will not compensate for a low volume resistivity board. Figure 22. Board Layout for Guarding Inputs INPUT PROTECTION The AD648 is guaranteed to withstand input voltages equal to the power supply potential. Exceeding the negative supply volt- age on either input will forward bias the substrate junction of the chip. The induced current may destroy the amplifier due to excess heat. Input protection is required in applications such as a flame de- tector in a gas chromatograph, where a very high potential may be applied to the input terminals during a sensor fault condi- tion. Figures 23a and 23b show simple current limiting schemes that can be used. RPROTECT should be chosen such that the maximum overload current is 1.0 mA (for example 100 k Ω for a 100 V overload). Figure 23a. Input Protection of l-to-V Converter Figure 23b. Voltage Follower Input Protection Method Figure 23b shows the recommended method for protecting a voltage follower from excessive currents due to high voltage breakdown. The protection resistor, RP, limits the input current. A nominal value of 100 k Ω will limit the input current to less than 1 mA with a 100 volt input voltage applied. The stray capacitance between the summing junction and ground will produce a high frequency roll-off with a corner frequency equal to: fcorner = 1 2 π R P Cstray Accordingly, a 100 k Ω value for R P with a 3 pF Cstray will cause a 3 dB corner frequency to occur at 531 kHz. |
Nº de peça semelhante - AD648BH |
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Descrição semelhante - AD648BH |
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