lmc2001 National Semiconductor Corporation, lmc2001 Datasheet - Page 8

• Manufacturer Part Number: lmc2001
• Description: High Precision, 6mhz Rail-to-rail Output Operational Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Notes
In terms of the measured output peak-to-peak noise, the fol-
lowing relationship holds between output noise voltage, e
for different closed loop gain, A
width is BW:
It should be kept in mind that in order to minimize the output
noise voltage for a given closed loop gain setting, one could
TABLE 1. Composite Amplifier Measured Performance
Av
50
100
100
500
1000
FIGURE 6. Non-Inverting Composite Amplifier
R1
(ohm)
200
100
1K
200
100
R2
(ohm)
10K
10K
100K
100K
100K
C2
(pF)
8
10
0.67
1.75
2.2
v
(Continued)
, settings, where -3dB Band-
BW
(MHz)
3.3
2.5
3.1
1.4
0.98
SR
(V/us)
178
174
170
96
64
e
(mVpp)
37
70
70
250
400
npp
DS100058-31
npp
(1)
,
8
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:
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 ).
Opamp flatband noise
1/f
f(max)
Av
Measurement time
0.94
corner frequency
8nV/
100 sec
100Hz
100Hz
100