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

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

A

Measurement time = 100 sec

Bandwidth=2Hz

This example will result in about 2.2 mV

output noise contribution due to the op amp alone, com-

pared to about 594 µV

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

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.

www.national.com

13