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ICL7650B Datasheet(PDF) 7 Page - Maxim Integrated Products |
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ICL7650B Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 12 page Chopper-Stabilized Op Amps _______________________________________________________________________________________ 7 Nulling Capacitor Connection Separate pins are provided for CRETN and CLAMP in the ICL7650. If you do not need the clamp feature, order the ICL7653; this device only offers the CRETN pin and will produce slightly lower noise and improved AC common-mode rejection. If you need to use the clamp feature, order the ICL7650 and connect the external capacitors to V-. To prevent load-current IR drops and other extraneous signals from being injected into the capacitors, use a separate PC board trace to connect the capacitor commons directly to the V- pin. The out- side foil of the capacitors should be connected to the low-impedance side of the null storage circuit, V- or CRETN. This will act as an ESD voltage shield. Clock Operation The ICL7650’s internal oscillator generates a 200Hz fre- quency, which is available at the CLK OUT pin. The device can also be operated with an external clock, if desired. An internal pull-up permits the INT/EXT pin to be left open for normal operation. However, the internal clock must be disabled and INT/EXT must be tied to V- if an external clock is used. An external clock signal may then be applied to the EXT CLK IN pin. The duty cycle of the external clock is not critical at low frequen- cies. However, a 50% to 80% positive duty cycle is pre- ferred for frequencies above 500Hz, since the capacitors are charged only when EXT CLK IN is high. This ensures that any transients have time to settle before the capacitors are turned off. The external clock should swing between ground and V+ for power sup- plies up to ±6V, and between V+ and (V+ - 6V) for higher supply voltages. To avoid a capacitor imbalance during overload, use a strobe signal. Neither capacitor will be charged if a strobe signal is connected to EXT CLK IN so that it is low while the overload signal is being applied to the amplifier. A typical amplifier will drift less than 10µVs since the leakage of the capacitor pins is quite low at room temperature. Relatively long measurements may be made with little change in offset. Applications Information Device Selection In applications that require lowest noise, Maxim’s ICL7652 may be preferred over the ICL7650/ICL7653. The ICL7650/ICL7653 offer a higher gain-bandwidth product and lower input bias currents, while the ICL7652 reduces noise by using larger input FETs. These larger FETs, however, increase the leakage at the ICL7652’s external null pins. Therefore, the ICL7650/ICL7653 can operate to a higher temperature with 0.1µF capacitors before the clock ripple (due to leakage at the null capacitor pins) becomes excessive and 1µF external capacitors are required. Output Stage/Load Driving The ICL7650/ICL7653 somewhat resemble a transcon- ductance amplifier whose open-loop gain is proportional to load resistance. This behavior is apparent when loads are less than the high-impedance stage (approximately 18k Ω for one output circuit). The open-loop gain, for example, will be 17dB lower with a 1k Ω load than with a 10k Ω load. This lower gain is of little consequence if the amplifier is used strictly for DC since the DC gain is typi- cally greater than 120dB, even with a 1k Ω load. For wideband applications, however, the best frequency response will be achieved with a load resistor of 10k Ω or higher. The result will be a smooth 6dB per octave response from 0.1Hz to 2MHz, with phase shifts of less than 10° in the transition region where the main amplifier takes over from the null amplifier. Component Selection CEXTA and CEXTB, the two required capacitors, have optimum values depending on the clock or chopping frequency. The correct value is 0.1µF for the preset internal clock. When using an external clock, scale this component value in proportion to the relationship between the chopping frequency and the nulling time constant. A low-leakage ceramic capacitor may prove suitable for many applications; however, a high-quality film-type capacitor (such as mylar) is preferred. For lowest settling time at initial turn-on, use capacitors with low dielectric absorption (such as polypropylene types). With low-dielectric-absorption capacitors, the ICL7650/ICL7653 will settle to 1µV offset in 100ms, but several seconds may be required if ceramic capacitors are used. Thermoelectric Effects Thermoelectric effects developed in thermocouple junctions of dissimilar materials (metals, alloys, silicon, etc.) ultimately limit precision DC measurements. Unless all junctions are at the same temperature, ther- moelectric voltages (typically around 10µV/°C, but up to hundreds of µV/°C for some materials) will be gener- ated. In order to realize the extremely low offset volt- ages that the chopper amplifier can provide, take special precautions to avoid temperature gradients. To eliminate air movement, enclose all components (par- ticularly those caused by power-dissipating elements in the system). Minimize power-supply voltages and power dissipation, and use low-thermoelectric-coeffi- cient connections where possible. It is advisable to separate the device surrounding heat-dissipating ele- ments, and to use high-impedance loads. |
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