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

for different closed loop gain, A

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

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

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