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LMC2001AIM Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LMC2001AIM Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 11 page Application Notes (Continued) TABLE 1. Composite Amplifier Measured Performance Av R1 R2 C2 BW SR e npp (ohm) (ohm) (pF) (MHz) (V/us) (mVpp) 50 200 10K 8 3.3 178 37 100 100 10K 10 2.5 174 70 100 1K 100K 0.67 3.1 170 70 500 200 100K 1.75 1.4 96 250 1000 100 100K 2.2 0.98 64 400 In terms of the measured output peak-to-peak noise, the fol- lowing relationship holds between output noise voltage, e npp, for different closed loop gain, A v, settings, where -3dB Band- width is BW: (1) It should be kept in mind that in order to minimize the output noise voltage for a given closed loop gain setting, one could minimize the overall bandwidth. As can be seen from Equa- tion 1 above, the improvement in output noise has a square law relationship to the reduction in BW. In the case of the inverting configuration, it is also possible to increase the input impedance of the overall amplifier, by rais- ing the value of R1, without having to increase the feedback resistor, R2, to impractical values, by utilizing a “T” network as feedback. See the LMC6442 data sheet (Application Notes section) for more details on this. LMC2001 as ADC Input Amplifier The LMC2001 is a great choice for an amplifier stage imme- diately before the input of an A/D converter (AC or DC coupled) see Figure 7 and Figure 8 because of the following important characteristics: a) Very low offset voltage and offset voltage drift over time and temperature allow a high closed loop gain setting with- out introducing any short term or long term errors. For ex- ample, when set to a closed loop gain of 100 as the analog input amplifier of a 12 bit A/D converter, the overall conver- sion error over full operation temperature and 30 years life of the part (operating at 50˚C) would be less than 5LSB. b) Fast large signal settling time to 0.01% of final value (1.4 us) allows 12 bit accuracy at 100KHz or more sampling rate. c) No flicker (1/f) noise means unsurpassed data accuracy over any measurement period of time, no matter how long. Consider the following opamp performance, based on a typi- cal commercially available device, for comparison: Opamp flatband noise 8nV/ 1/f 0.94 corner frequency 100Hz f(max) 100Hz Av 100 Measurement time 100 sec The example above, will result in about 3mVpp (2.5LSB) of output noise contribution due to the opamp alone, compared to about 420 uVpp (less than 1LSB) when that opamp is re- placed with the LMC2001 which has no 1/f contribution. If the measurement time is increased from 100 sec. to 1 hr., the improvement realized by using the LMC2001 would be a fac- tor of about 44 times (18.5mVpp compared to 420uV when LMC2001 is used) mainly because the LMC2001 accuracy is not compromised by increasing the observation time. d) Copper lead frame construction minimizes any thermo- couple effects which would degrade low level/high gain data conversion application accuracy (see discussion under “The Benefits of the LMC2001” section above). e) Rail-to-Rail output swing maximized the ADC dynamic range in 5V single supply converter applications. Below are some typical block diagrams showing the LMC2001 used as an ADC amplifier ( Figure 7 and Figure 8). DS100058-31 FIGURE 6. Non-Inverting Composite Amplifier www.national.com 8 |
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