lmp8100 National Semiconductor Corporation, lmp8100 Datasheet - Page 26

• Manufacturer Part Number: lmp8100
• Description: Programmable Gain Amplifier
• Manufacturer: National Semiconductor Corporation

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POWER SUPPLY PURITY AND BYPASSING
Particular attention to power supply purity is needed in order
to preserve the LMP8100's gain accuracy and low noise. The
LMP8100 worst-case PSRR is 85 dB or 56.2 µV/V. Never-
theless, the usable dynamic range, gain accuracy and inher-
ent low noise of the amplifier can be compromised through
the introduction and amplification of power supply noise.
To decouple the LMP8100 from supply line AC noise, a 0.1
µF capacitor should be located on each supply line, close to
the LMP8100. Adding a 10 µF capacitor in parallel with the
0.1 µF capacitor will reduce the noise introduced to the
LMP8100 even more by providing an AC path to ground for
most frequency ranges.
A power supply dropout (V
tended reset of the register. If a dropout occurs, the register
will need to be reprogrammed with the correct values.
+
-V
−
< 2.7V) can cause an unin-
FIGURE 16. Data Acquisition System Using the LMP8100
FIGURE 15. Daisy Chain Configuration
26
SCALING AMPLIFIER
The LMP8100 is ideally suited for use as an amplifier between
a sensor that has a wide output range and an ADC. As the
signal from the sensor changes the gain of the LMP8100 can
be changed so that the entire input range of the ADC is being
used at all times. Figure 16 shows a data acquisition system
using the LMP8100 and the ADC121S101. The 100Ω resistor
and 390 pF capacitor form an antialiasing filter for the AD-
C121S101. The capacitor also stores and delivers charge to
the switched capacitor input of the ADC. The capacitive load
on the LMP8100 created by the 390pF capacitor is decreased
by the 100Ω resistor.
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