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| 1 TC7650 CHOPPER-STABILIZED OPERATIONAL AMPLIFIER FEATURES s s s s s s s s s s Low Input Offset Voltage ......................... 0.7V Typ Low Input Offset Voltage Drift ......... 0.05V/C Max Low Input Bias Current ............................ 10pA Max High Impedance Differential CMOS Inputs .... 1012 High Open-Loop Voltage Gain ................ 120dB Min Low Input Noise Voltage ............................ 2.0Vp-p High Slew Rate .......................................... 2.5V/sec Low-Power Operation ..................................... 20mW Output Clamp Speeds Recovery Time Compensated Internally for Stable Unity Gain Operation s Direct Replacement for ICL7650 s Available in 8-Pin Dip and 14-Pin Dip GENERAL DESCRIPTION The TC7650 CMOS chopper-stabilized operational amplifier practically removes offset voltage error terms from system error calculations. The 5V maximum VOS specification, for example, represents a 15 times improvement over the industry-standard OP07E. The 50nV/C offset drift specification is over 25 times lower than the OP07E. The increased performance eliminates VOS trim procedures, periodic potentiometer adjustment and the reliability problems caused by damaged trimmers. The TC7650 performance advantages are achieved without the additional manufacturing complexity and cost incurred with laser or "zener zap" VOS trim techniques. The TC7650 nulling scheme corrects both DC VOS errors and VOS drift errors with temperature. A nulling amplifier alternately corrects its own VOS errors and the main amplifier VOS error. Offset nulling voltages are stored on two user-supplied external capacitors. The capacitors connect to the internal amplifier VOS null points. The main amplifier input signal is never switched. Switching spikes are not present at the TC7650 output. The 14-pin dual-in-line package (DIP has an external oscillator input to drive the nulling circuitry for optimum noise performance. Both the 8 and 14-pin DIPs have an output voltage clamp circuit to minimize overload recovery time. PIN CONFIGURATIONS 8-Pin DIP CA 1 8 CB 7 VDD TC7650CPA 6 OUTPUT 5 CLAMP 2 3 4 5 6 7 ORDERING INFORMATION Part No. TC7650CPA TC7650CPD Package 8-Pin Plastic DIP 14-Pin Plastic DIP Temperature Range 0C to +70C 0C to +70C Max VOS 5V 5V FUNCTIONAL BLOCK DIAGRAM OUTPUT CLAMP OUTPUT CLAMP CIRCUIT MAIN AMPLIFIER INPUTS NULL A B OSCILLATOR 14-PIN DIP ONLY INT/EXT EXT CLK IN CLK OUT (-) INPUT 2 (+) INPUT 3 VSS 4 OUTPUT CEXT 14-Pin DIP CB 1 14 INT/EXT EXT CLK 13 INPUT 12 INT CLK OUTPUT TC7650CPD 11 V DD 10 OUTPUT 9 OUTPUT CLAMP INTERMOD COMPENSATION B B NULL AMPLIFIER A NULL * FOR 8-PIN DIP, CONNECT TO V . SS TC7650 B A CEXT CA 2 NC (GUARD) (-) INPUT (+) INPUT NC (GUARD) VSS 3 4 5 6 7 *CRET 8 CRETN NC = NO INTERNAL CONNECTION TC7650-5 9/11/96 8 3-273 TELCOM SEMICONDUCTOR, INC. CHOPPER-STABILIZED OPERATIONAL AMPLIFIER TC7650 ABSOLUTE MAXIMUM RATINGS* Total Supply Voltage (VDD to VSS) .............................. 18V Input Voltage ........................ (VDD + 0.3V) to (VSS - 0.3V) Storage Temperature Range ................ - 65C to +150C Lead Temperature (Soldering, 10 sec) ................... 300C Voltage on Oscillator Control Pins ................... VDD to VSS Output Short Circuit Duration ............................. Indefinite Current Into Any Pin ................................................. 10mA While Operating (Note 3) .................................. 100A Operating Temperature Range C Device ................................................ 0C to +70C indicated. Symbol Input VOS VOS/T Package Power Dissipation (TA 70C) 8-Pin Plastic DIP ............................................. 730mW 14-Pin Plastic DIP ...........................................800mW *Static-sensitive device. Unused devices must be stored in conductive material. Protect devices from static discharge and static fields. Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only and functional operation of the device at these or any other conditions above those indicated in the operational sections of the specifications is not implied. Exposure to Absolute Maximum Rating Conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS: VDD = +5V, VSS = -5V, CA = CB = 0.1F, TA = 25C, unless otherwise Parameter Input Offset Voltage Input Offset Voltage Average Temperature Coefficient Offset Voltage vs. Time Input Bias Current Test Conditions TA = +25C Over Operating Temp Range Operating Temperature Range Min -- -- -- Typ 0.7 1.0 0.01 Max 5 -- 0.05 Units -- V V/C -- TA = +25C 0C TA +70C -25C TA +85C RS = 100, 0 to 10Hz f = 10 Hz -- -- -- -- -- -- -- -5 120 100 1.5 35 100 0.5 2 0.01 1012 - 5.2 to +2 130 -- 10 150 400 -- -- -- +1.6 -- IBIAS IOS eNP-P IN RIN CMVR CMRR Output A VOUT Input Offset Current Input Noise Voltage Input Noise Current Input Resistance Common-Mode Voltage Range Common-Mode Rejection Ratio Large Signal Voltage Gain Output Voltage Swing (Note 2) Clamp ON Current Clamp OFF Current nV/ month pA pA pA pA VP-P pA/Hz V dB CMVR = -5V to +1.5V RL = 10k RL = 10k RL = 100k RL = 100k (Note 1) - 4V < VOUT < +4V (Note 1) Unity Gain (+1) CL = 50 pF, RL = 10k 120 4.7 -- 25 -- 130 4.85 4.95 70 1 -- -- -- 200 -- dB V V A pA Dynamic BW SR tR fCH Unity-Gain Bandwidth Slew Rate Rise Time Overshoot Internal Chopping Frequency Pins 12-14 Open (DIP) -- -- -- -- 120 2.0 2.5 0.2 20 200 -- -- -- -- 375 MHz V/sec sec % Hz 3-274 TELCOM SEMICONDUCTOR, INC. CHOPPER-STABILIZED OPERATIONAL AMPLIFIER TC7650 ELECTRICAL CHARACTERISTICS: VDD = +5V, VSS = - 5V, CA = CB = 0.1F, TA = 25C, unless otherwise specified. Symbol Supply VDD, VSS IS PSRR Operating Supply Range Supply Current Power Supply Rejection Ratio No Load VS = 3V to 8V 4.5 -- 120 -- 2 130 16 3.5 V mA dB 1 Parameter Test Conditions Min Typ Max Units 2 3 4 5 6 7 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 100A is recommended to avoid latch-up problems. 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 operates 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 chopper 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: 1 VOSE = AN + 1 VOSN (1) 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 (approximately 18k). With loads less than this, the chopper amplifier 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 load than with a 10k 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 load. In wideband applications, the best frequency response will be achieved with a load resistor of 10k 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. 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) Substituting (1) (2) and assuming AN >>1: VOUT = AM AN (V+ - V-) + [ VOSM + VOSN AN ] (3) 8 TELCOM SEMICONDUCTOR, INC. 3-275 CHOPPER-STABILIZED OPERATIONAL AMPLIFIER TC7650 V+ ANALOG INPUT V- B MAIN + AMPLIFIER NULL - GAIN = AM VOUT TC7650 A + NULL - NULL AMPLIFIER B A CB CA GAIN = AN , OFFSET = VOSN Figure 1. TC7650 Contains a Nulling and Main Amplifier. Offset Correction Voltages Are Stored on Two External Capacitors. VDD VSS 4 11 7 TC7650 + 8 2 CA CB 1 10 3 2 VDD 7 6 TC7650 + 8 1 CA 8-PIN PACKAGE CB VSS 4 - - 5 14-PIN PACKAGE Figure 2. Nulling Capacitor Connection Nulling Capacitor Connection The offset voltage correction capacitors are connected to CA and CB. The common capacitor connection is made to VSS (pin 4) on the 8-pin packages and to capacitor return (CR, pin 8) on the 14-pin packages. The common connection should be made through a separate PC trace or wire to avoid voltage drops. The capacitors outside foil, if possible, should be connected to CR or VSS. The 14-pin DIP device can be driven by an external clock. The INT/EXT input (pin 14) has an internal pull-up and may be left open for internal clock operation. If an external clock is used, INT/EXT must be tied to VSS (pin 7) to disable the internal clock. The external clock signal is applied to the external clock input (pin 13). The external clock amplitude should swing between VDD and ground for power supplies up to 6V and between V+ and V+ - 6V for higher supply voltages. At low frequencies the external clock duty cycle is not critical, since an internal divide-by-two gives the desired 50% switching duty cycle. The offset storage correction capacitors are charged only when the external clock input is high. A 50% to 80% external clock positive duty cycle is desired for frequencies above 500Hz to guarantee transients settle before the internal switches open. The external clock input can also be used as a strobe input. If a strobe signal is connected at the external clock input so that it is LOW during the time an overload signal is applied, neither capacitor will be charged. The leakage currents at the capacitors pins are very low. At 25C a typical TC7650 will drift less than 10V/sec. Output Clamp Chopper-stabilized systems can show long recovery times from overloads. If the output is driven to either supply rail, output saturation occurs. The inputs are no longer held at a "virtual ground." The VOS null circuit treats the differential signal as an offset and tries to correct it by charging the external capacitors. The nulling circuit also saturates. Once the input signal returns to normal, the response time is lengthened by the long recovery time of the nulling amplifier and external capacitors. Through an external clamp connection, the TC7650 TELCOM SEMICONDUCTOR, INC. Clock Operation The internal oscillator is set for a 200Hz nominal chopping frequency on both the 8- and 14-pin DIPs. With the 14-pin DIP TC7650, the 200Hz internal chopping frequency is available at the internal clock output (pin 12). A 400Hz nominal signal will be present at the external clock input pin (pin 13) with INT/EXT high or open. This is the internal clock signal before a divide-by-two operation. 3-276 CHOPPER-STABILIZED OPERATIONAL AMPLIFIER TC7650 eliminates the overload recovery problem by reducing the feedback network gain before the output voltage reaches either supply rail. The output clamp circuit is shown in Figure 3, with typical inverting and noninverting circuit connections shown in Figures 4 and 5. Output voltage versus clamp circuit current characteristics are shown in the typical operating curves. For the clamp to be fully effective, the impedance across the clamp output should be greater than 100k. 1 2 3 4 5 6 7 INTERNAL + + POSITIVE CLAMP BIAS V -V T V - 0.7V P-CHANNEL OUTPUT CLAMP PIN N-CHANNEL INTERNAL - NEGATIVE CLAMP BIAS V + V T - V + 0.7V TC7650 OUTPUT PIN Latch-Up Avoidance Junction-isolated CMOS circuits inherently include a parasitic 4-layer (p-n-p-n) structure which has characteristics similar to an SCR. Under certain circumstances this junction may be triggered into a low-impedance state, resulting in excessive supply current. To avoid this condition, no voltage greater than 0.3V beyond the supply rails should be applied to any pin. In general, the amplifier supplies must be established either at the same time or before any input signals are applied. If this is not possible, the drive circuits must limit input current flow to under 0.1mA to avoid latchup. Figure 3. Internal Clamp Circuit Thermoelectric Potentials * CONNECT TO VSS ON 8-PIN DIP. C + - CLAMP R3 R3 + ( R1 / R2 ) 100k FOR FULL CLAMP EFFECT. 0.1F TC7650 * R C R2 OUTPUT INPUT R1 Figure 4. Noninverting Amplifier With Optional Clamp Precision DC measurements are ultimately limited by thermoelectric potentials developed in thermocouple junctions of dissimilar metals, alloys, silicon, etc. Unless all junctions are at the same temperature, thermoelectric voltages, typically around 0.1V/C, but up to tens of V/C for some materials, will be generated. In order to realize the benefits extremely-low offset voltages provide, it is essential to take special precautions to avoid temperature gradients. All components should be enclosed to eliminate air movement, especially those caused by power-dissipating elements in the system. Low thermoelectric-coefficient connections should be used where possible and power supply voltages and power dissipation should be kept to a minimum. High-impedance loads are preferable, and separation from surrounding heat-dissipating elements is advised. R2 CLAMP R1 - INPUT + C R* C ( R1 OUTPUT R2 ) 100k FOR FULL CLAMP EFFECT. TC7650 Pin Compatibility On the 8-pin mini-DIP TC7650, the external null storage capacitors are connected to pins 1 and 8. On most other operational amplifiers these are left open or are used for offset potentiometer or compensation capacitor connections. For OP05 and OP07 operational amplifiers, the replacement of the offset null potentiometer between pins 1 and 8 by two capacitors from the pins to VSS will convert the OP05/ 07 pin configurations for TC7650 operation. For LM108 devices, the compensation capacitor is replaced by the external nulling capacitors. The LM101/748/709 pinouts are modified similarly by removing any circuit connections to pin 5. On the TC7650, pin 5 is the output clamp connection. 3-277 - * CONNECT TO VR ON 8-PIN DIP. 0.1 F 0.1 F Figure 5. Inverting Amplifier with Optional Clamp 8 TELCOM SEMICONDUCTOR, INC. CHOPPER-STABILIZED OPERATIONAL AMPLIFIER TC7650 Other operational amplifiers may use this pin as an offset or compensation point. The minor modifications needed to retrofit a TC7650 into existing sockets operating at reduced power supply voltages make prototyping and circuit verification straightforward. Inverting Amplifier R1 INPUT - R2 OUTPUT + Input Guarding High impedance, low leakage CMOS inputs allow the TC7650 to make measurements of high-impedance sources. Stray leakage paths can increase input currents and decrease input resistance unless inputs are guarded. A guard is a conductive PC trace surrounding the input terminals. The ring connects to a low-impedance point at the same potential as the inputs. Stray leakages are absorbed by the low-impedance ring. The equal potential between ring and inputs prevents input leakage currents. Typical guard connections are shown in Figure 6. The 14-pin DIP configuration has been specifically designed to ease input guarding. The pins adjacent to the inputs are unused. In applications requiring low leakage currents, boards should be cleaned thoroughly and blown dry after soldering. Protective coatings will prevent future board contamination. R 3* Noninverting Amplifier R2 R 3* - OUTPUT R1 + INPUT R1 R 2 NOTE: R 3 = R1 + R 2 SHOULD BE LOW IMPEDANCE FOR OPTIMUM GUARDING. Follower R 3* - OUTPUT INPUT + Component Selection The two required capacitors, CA and CB, have optimum values, depending on the clock or chopping frequency. For the preset internal clock, the correct value is 0.1F. To maintain the same relationship between the chopping frequency and the nulling time constant, the capacitor values should be scaled in proportion to the external clock, if used. High-quality film-type capacitors (such as Mylar) are preferred; ceramic or other lower-grade capacitors may be suitable in some applications. For fast settling on initial turnon, low dielectric absorption capacitors (such as polypropylene) should be used. With ceramic capacitors, several seconds may be required to settle to 1V. Figure 6. Input Guard Connection 3-278 TELCOM SEMICONDUCTOR, INC. CHOPPER-STABILIZED OPERATIONAL AMPLIFIER TC7650 TYPICAL CHARACTERISTICS Positive Clamp Current vs. Output Voltage 1 mA T = +25C 0.1 mA VA = 5V S 0.01 mA CLAMP CURRENT CLAMP CURRENT 1 Negative Clamp Current vs. Output Voltage 1 mA T = +25C 0.1 mA VA = 5V S 0.01 mA 1 A 0.1 A 0.01 A 1 nA 0.1 nA 0.01 nA 1 pA -4.0 -4.1 -4.2 -4.3 -4.4 -4.5 -4.6 -4.7 -4.8 -4.9 -5.0 2 3 4 5 PHASE (deg) 1 A 0.1 A 0.01 A 1 nA 0.1 nA 0.01 nA 1 pA 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) Supply Current vs. Supply Voltage 3.0 TA = +25C SUPPLY CURRENT (mA) Gain/Phase vs. Frequency 30 20 10 0 GAIN GAIN (dB) 225 180 135 90 45 PHASE 0 -45 -90 CLOSED-LOOP GAIN = 20 -135 -180 2.6 2.2 -10 -20 -30 6 7 1.8 1.4 -40 -50 10 -60 8 TELCOM SEMICONDUCTOR, INC. 3-279 |
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