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LMV722MX Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LMV722MX Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 15 page Application Notes (Continued) loop. Increased capacitive drive is possible by increasing the value of C F. This in turn will slow down the pulse response. 3.0 Input Bias Current Cancellation The LMV721/722 family has a bipolar input stage. The typi- cal input bias current of LMV721/722 is 260nA with 5V sup- ply. Thus a 100k Ω input resistor will cause 26mV of error voltage. By balancing the resistor values at both inverting and non-inverting inputs, the error caused by the amplifier’s input bias current will be reduced. The circuit in Figure 4 shows how to cancel the error caused by input bias current. 4.0 Typical Single-Supply Application Circuits 4.1 Difference amplifier The difference amplifier allows the subtraction of two volt- ages or, as a special case, the cancellation of a signal com- mon to two inputs. It is useful as a computational amplifier, in making a differential to single-ended conversion or in reject- ing a common mode signal. 4.2 Instrumentation Circuits The input impendance of the previous difference amplifier is set by the resistor R 1,R2,R3 and R4. To eliminate the prob- lems of low input impendance, one way is to use a voltage follower ahead of each input as shown in the following two instrumentation amplifiers. 4.2.1 Three-op-amp Instrumentation Amplifier The LMV721/722 can be used to build a three-op-amp in- strumentation amplifier as shown in Figure 6 The first stage of this instrumentation amplifier is a differential-input, differential-output amplifier, with two volt- age followers. These two voltage followers assure that the input impedance is over 100M Ω. The gain of this instrumen- tation amplifier is set by the ratio of R 2/R1.R3 should equal R 1 and R4 equal R2. Matching of R3 to R1 and R4 to R2 af- fects the CMRR. For good CMRR over temperature, low drift resistors should be used. Making R 4 slightly smaller than R2 and adding a trim pot equal to twice the difference between R 2 and R4 will allow the CMRR to be adjusted for optimum. 4.2.2 Two-op-amp Instrumentation Amplifier A two-op-amp instrumentation amplifier can also be used to make a high-input impedance DC differential amplifier ( Fig- ure 7). As in the two-op-amp circuit, this instrumentation am- plifier requires precise resistor matching for good CMRR. R 4 should equal to R 1 and R3 should equal R2. DS100922-19 FIGURE 3. Indirectly Driving A Capacitive Load with DC Accuracy DS100922-20 FIGURE 4. Cancelling the Error Caused by Input Bias Current DS100922-21 FIGURE 5. Difference Application DS100922-30 FIGURE 6. Three-op-amp Instrumentation Amplifier www.national.com 9 |
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