Datasheet

-
+
LMP201X
+Input
-Input
(0V to 5V Range)
430:
LM9140-2.5
+2.5V
+5V
ADC1203X
+V
REF
GND
-V
REF
V
IN
+5V
1M
LMP2011, LMP2012
SNOSA71K –OCTOBER 2004–REVISED MARCH 2013 www.ti.com
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 Equation 1 above, the output noise has a
square-root relationship to the Bandwidth.
In the case of the inverting configuration, it is also possible to increase the input impedance of the overall
amplifier, by raising the value of R1, without having to increase the feed-back resistor, R2, to impractical values,
by utilizing a "Tee" network as feedback. See the LMC6442 data sheet (Application Notes section) for more
details on this.
Figure 37. AC Coupled ADC Driver
LMP201X AS ADC INPUT AMPLIFIER
The LMP201X is a great choice for an amplifier stage immediately before the input of an ADC (Analog-to-Digital
Converter), whether AC or DC coupled. See Figure 37 and Figure 38. 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/√Hz
1/f corner frequency = 100 Hz
A
V
= 2000
Measurement time = 100 sec
Bandwidth = 2 Hz
This example will result in about 2.2 mV
PP
(1.9 LSB) of output noise contribution due to the op amp alone,
compared 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 thermocouple 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.
Below are some typical block diagrams showing the LMP201X used as an ADC amplifier (Figure 37 and
Figure 38).
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