Datenblatt-Suchmaschine für elektronische Bauteile |
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TC7650CPA Datenblatt(PDF) 3 Page - TelCom Semiconductor, Inc |
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TC7650CPA Datenblatt(HTML) 3 Page - TelCom Semiconductor, Inc |
3 / 7 page 3-275 TELCOM SEMICONDUCTOR, INC. 7 6 5 4 3 1 2 8 ELECTRICAL CHARACTERISTICS: VDD = +5V, VSS = – 5V, CA = CB = 0.1µF, TA = 25°C, unless otherwise specified. Symbol Parameter Test Conditions Min Typ Max Units Supply VDD, VSS Operating Supply Range 4.5 — 16 V IS Supply Current No Load — 2 3.5 mA PSRR Power Supply VS = ±3V to ±8V 120 130 dB Rejection Ratio NOTES: 1. See "Output Clamp" discussion. 2. Output clamp not connected. See typical characteristics curves for output swing versus clamp current characteristics. 3. Limiting input current to 100 µA is recommended to avoid latch-up problems. CHOPPER-STABILIZED OPERATIONAL AMPLIFIER TC7650 Theory of Operation Figure 1 shows the major elements of the TC7650. There are two amplifiers (the main amplifier and the nulling amplifier), and both have offset-null capability. The main amplifier is connected full-time from the input to the output. The nulling amplifier, under the control of the chopping frequency oscillator and clock circuit, alternately nulls itself and the main amplifier. Two external capacitors provide the required storage of the nulling potentials and the necessary nulling-loop time constants. The nulling arrangement oper- ates over the full common-mode and power-supply ranges, and is also independent of the output level, thus giving exceptionally high CMRR, PSRR, and AVOL. Careful balancing of the input switches minimizes chop- per frequency charge injection at the input terminals, and the feed-forward-type injection into the compensation capacitor that can cause output spikes in this type of circuit. The circuit's offset voltage compensation is easily shown. With the nulling inputs shorted, a voltage almost identical to the nulling amplifier offset voltage is stored on CA. The effective offset voltage at the null amplifier input is: VOSE = VOSN (1) After the nulling amplifier is zeroed, the main amplifier is zeroed; the A switches open and B switches close. The output voltage equation is: VOUT = AM [VOSM +(V+ – V–) + AN(V+– V–) + AN VOSE](2) (3) As desired, the device offset voltages are reduced by the high open-loop gain of the nulling amplifier. Output Stage/Loading The output circuit is a high-impedance stage (approxi- mately 18k Ω). With loads less than this, the chopper ampli- fier behaves in some ways like a transconductance amplifier whose open-loop gain is proportional to load resistance. For example, the open-loop gain will be 17dB lower with a 1k Ω load than with a 10k Ω load. If the amplifier is used strictly for DC, the lower gain is of little consequence, since the DC gain is typically greater than 120dB, even with a 1k Ω load. In wideband applications, the best frequency response will be achieved with a load resistor of 10k Ω or higher. This results in a smooth 6 dB/octave response from 0.1Hz to 2 MHz, with phase shifts of less than 10 ° in the transition region, where the main amplifier takes over from the null amplifier. The clock frequency sets the transition region. Intermodulation Previous chopper-stabilized amplifiers have suffered from intermodulation effects between the chopper frequency and input signals. These arise because the finite AC gain of the amplifier results in a small AC signal at the input. This is seen by the zeroing circuit as an error signal, which is chopped and fed back, thus injecting sum and difference frequencies, and causing disturbances to the gain and phase versus frequency characteristics near the chopping frequency. These effects are substantially reduced in the TC7650 by feeding the nulling circuit with a dynamic current corresponding to the compensation capacitor current in such a way as to cancel that portion of the input signal due to a finite AC gain. The intermodulation and gain/phase disturbances are held to very low values, and can generally be ignored. 1 AN + 1 Substituting (1) → (2) and assuming AN >>1: VOUT = AM AN (V+ – V–) + ] [ VOSM + VOSN AN |
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