LMV2011 datasheet(13/15 Pages) NSC

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Application Information (Continued)

LMV2011 AS ADC INPUT AMPLIFIER

The LMV2011 is a great choice for an amplifier stage imme-

diately before the input of an ADC (Analog-to-Digital Con-

verter), whether AC or DC coupled. See Figure 7 and Figure

8. This is 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

without introducing any short-term or long-term errors.

For example, when set to a closed-loop gain of 100 as

the analog input amplifier for a 12-bit A/D converter, the

overall conversion error over full operation temperature

and 30 years life of the part (operating at 50˚C) would be

less than 5 LSBs.

B) Fast large-signal settling time to 0.01% of final value

(1.4µs) allows 12 bit accuracy at 100KH

Z or more sam-

pling 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 typical low-noise, high-performance commercially-

available device, for comparison:

Opamp flatband noise = 8nV/

1/f corner frequency = 100Hz

V = 2000

Measurement time = 100 sec

Bandwidth = 2Hz

This example will result in about 2.2 mV

PP (1.9 LSB) of

output noise contribution due to the opamp alone, com-

pared to about 594µV

PP (less than 0.5 LSB) when that

opamp is replaced with the LMV2011 which has no 1/f

contribution. If the measurement time is increased from

100 seconds to 1 hour, the improvement realized by

using the LMV2011 would be a factor of about 4.8 times

(2.86mV

PP compared to 596µV when LMV2011 is used)

mainly because the LMV2011 accuracy is not compro-

mised by increasing the observation time.

D) Copper leadframe construction minimizes any thermo-

couple effects which would degrade low level/high gain

data conversion application accuracy (see discussion

under "The Benefits of the LMV2011" section above).

E) Rail-to-Rail output swing maximizes the ADC dynamic

range in 5-Volt single-supply converter applications. Be-

low are some typical block diagrams showing the

LMV2011 used as an ADC amplifier (Figure 7 and Figure

8).

20051522

FIGURE 8.

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