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.
Application Notes (Continued)
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:
DS100058-31
FIGURE 6. Non-Inverting Composite Amplifier
TABLE 1. Composite Amplifier Measured Performance
Opamp flatband noise
8nV/
Av
R1
R2
C2
BW
SR
enpp
(ohm) (ohm) (pF) (MHz) (V/us) (mVpp)
1/f0.94 corner frequency
100Hz
100Hz
100
50
200
100
1K
10K
8
3.3
2.5
178
174
170
96
37
f(max)
100
100
500
10K
10
70
Av
100K
100K
100K
0.67 3.1
1.75 1.4
70
Measurement time
100 sec
200
250
400
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.
1000 100
2.2
0.98
64
In terms of the measured output peak-to-peak noise, the fol-
lowing relationship holds between output noise voltage, enpp
for different closed loop gain, Av, settings, where -3dB Band-
width is BW:
,
(1)
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).
It should be kept in mind that in order to minimize the output
noise voltage for a given closed loop gain setting, one could
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).
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