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AD542SH Datasheet(PDF) 8 Page - Analog Devices |
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AD542SH Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page AD542/AD544/AD547 REV. B –8– current-to-voltage converting amplifier. This possibility necessi- tates some form of input protection. Many electrometer type devices, especially CMOS designs, can require elaborate Zener protection schemes which often compromise overall perfor- mance. The BiFET series requires input protection only if the source is not current-limited, and as such is similar to many JFET-input designs. The failure mode would be overheating from excess current rather than voltage breakdown. If the source is not current-limited, all that is required is a resistor in series with the affected input terminal so that the maximum overload current is 1.0 mA (for example, 100 k Ω for a 100 volt overload). This simple scheme will cause no significant reduc- tion in performance and give complete overload protection. Fig- ure 30 shows proper connections. Figure 30. Input Protection D/A CONVERTER APPLICATIONS The BiFET series of operational amplifiers can be used with CMOS DACs to perform both 2-quadrant and 4-quadrant operation. The output impedance of a CMOS DAC varies with the digital word, thus changing the noise gain of the amplifier circuit. The effect will cause a nonlinearity the magnitude of which is dependent on the offset voltage of the amplifier. The BiFET series with trimmed offset will minimize this effect. Ad- ditionally, the Schottky protection diodes recommended for use with many older CMOS DACs are not required when using one of the BiFET series amplifiers. Figure 31a shows the AD547 and AD7541 configured for uni- polar binary (2-quadrant multiplication) operation. With a dc reference voltage or current (positive or negative polarity) ap- plied at pin 17, the circuit operates as a unipolar converter. With an ac reference voltage or current, the circuit provides 2-quadrant multiplication (digitally controlled attenuation). Figure 31a. AD547 Used as DAC Output Amplifier BiFET Application Hints APPLICATION NOTES The BiFET series was designed for high performance op amp applications that require true dc precision. To capitalize on all of the performance available from the BiFETs there are some practical error sources that should be considered. The bias currents of JFET input amplifiers double with every 10 °C increase in chip temperature. Therefore, minimizing the junction temperature of the chip will result in extending the performance limits of the device. 1. Heat dissipation due to power consumption is the main contributor to self-heating and can be minimized by reducing the power supplies to the lowest level allowed by the application. 2. The effects of output loading should be carefully considered. Greater power dissipation increases bias currents and de- creases open loop gain. GUARDING The low input bias current (25 pA) and low noise characteristics of the high performance BiFET op amp make it suitable for electrometer applications such as photo diode preamplifiers and picoampere current-to-voltage converters. The use of guarding techniques in printed circuit board layout and construction is critical for achieving the ultimate in low leakage performance available from these amplifiers. The input guarding scheme shown in Figure 29 will minimize leakage as much as possible; the guard ring is connected to a low impedance potential at the same level as the inputs. High impedance signal lines should not be extended for any unnecessary length on a printed circuit. Figure 29. Board Layout for Guarding Inputs INPUT PROTECTION The BiFET series is guaranteed for a maximum safe input potential equal to the power supply potential. The input stage design also allows differential input voltages of up to ±1 volt while maintaining the full differential input resistance of 10 12 Ω. This makes the BiFET series suitable for comparator situations employing a direct connection to high impedance source. Many instrumentation situations, such as flame detectors in gas chromatographs, involve measurement of low level currents from high-voltage sources. In such applications, a sensor fault condition may apply a very high potential to the input of the |
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