LMP2011MFX datasheet(13/16 Pages) NSC

13 / 16 page

Application Information (Continued)

LMP201X AS ADC INPUT AMPLIFIER

The LMP201X 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 (1.4

µs) allows 12 bit accuracy at 100 KH

Z 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 op amp performance, based

on a typical low-noise, high-performance commercially-

available device, for comparison:

Op amp flatband noise = 8nV/

1/f corner frequency = 100 Hz

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 op amp alone, com-

pared to about 594 µV

PP (less than 0.5 LSB) when that

op amp is replaced with the LMP201X which has no 1/f

contribution. If the measurement time is increased from

100 seconds to 1 hour, the improvement realized by

using the LMP201X would be a factor of about 4.8 times

(2.86 mV

PP compared to 596 µV when LMP201X is

used) mainly because the LMP201X accuracy is not

compromised 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 LMP201X" 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

LMP201X used as an ADC amplifier (Figure 7 and Figure

8).

20071522

FIGURE 8.

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