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ICL7653BCPA Datasheet(PDF) 6 Page - Maxim Integrated Products |
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ICL7653BCPA Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 12 page Chopper-Stabilized Op Amps 6 _______________________________________________________________________________________ Detailed Description Figure 2 shows the major elements of the ICL7650/ ICL7653. Two amplifiers are illustrated, the main amplifi- er and the nulling amplifier, both of which have offset- null capability. The main amplifier is connected full time from the input to the output. The nulling amplifier, under control of the chopper-frequency oscillator and clock circuit, alternately nulls itself and the main amplifier. This nulling arrangement, which is independent of the output level, operates over the full power-supply and common- mode ranges. The ICL7650/ICL7653 exhibit an excep- tionally high CMRR, PSRR, and AVOL. Their nulling connections, which are MOSFET back gates, have inher- ently high impedance. Two external capacitors provide storage for the nulling potentials and the necessary nulling-loop time constants. The ICL7650/ICL7653 minimize chopper-frequency charge injection at the input terminals by carefully bal- ancing the input switches. Feed-forward injection into the compensation capacitor, the main cause of output spikes in this type of circuit, is also minimized. Output Clamp (ICL7650 Only) The output clamp reduces the overload recovery time inherent with chopper-stabilized amplifiers. When tied to the summing junction or inverting input pin, a current path between this point and the output occurs just before the output device saturates. This prevents uncontrolled input differential and the consequent charge build-up on the correction-storage capacitors, while causing only a slight reduction in the output swing. Intermodulation Intermodulation effects can cause problems in older chopper-stabilized amplifier modules. Intermodulation occurs since the amplifier has a finite AC gain, and therefore will have a small AC signal at the input. In a chopper-stabilized module, this small AC signal is detected, chopped, and fed into the offset-correction circuit. This results in spurious outputs at the sum and difference frequencies of the chopping and input signal frequencies. Other intermodulation effects in chopper- stabilized modules include gain and phase anomalies near the chopping frequency. These effects are substantially reduced in the ICL7650/ICL7653, which add to the nulling circuit a dynamic current that compensates for the AC signal on the inputs. Unlike modules, the ICL7650/ICL7653 can precisely compensate for the finite AC gain, since both the AC gain rolloff and the intermodulation compensation current are controlled by internal matched capacitors. ICL7650 ICL7653 OUTPUT C CR C 0.1 µF 0.1 µF R2 1M R1 1M ICL7650 INTERNAL BIAS EXT CLK IN CLK OUT NULL CEXTA CEXTB CAP RETURN B A C P A + - N OUTPUT +IN -IN CLAMP MAIN + - B C INT/EXT A A A = CLK OUT A EXT CLK IN B C OSC Figure 1. ICL7650 Test Circuit Figure 2. Block Diagram |
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