LMP7731_09 NSC [National Semiconductor], LMP7731_09 Datasheet - Page 16

• Manufacturer Part Number: LMP7731_09
• Description: 2.9 nV/sqrt(Hz) Low Noise, Precision, RRIO Amplifier
• Manufacturer: NSC [National Semiconductor]

DRIVING AN ADC
Analog to Digital Converters, ADCs, usually have a sampling
capacitor on their input. When the ADC's input is directly con-
nected to the output of the amplifier a charging current flows
from the amplifier to the ADC. This charging current causes
a momentary glitch that can take some time to settle. There
are different ways to minimize this effect. One way is to slow
down the sampling rate. This method gives the amplifier suf-
ficient time to stabilize its output. Another way to minimize the
glitch caused by the switch capacitor is to have an external
capacitor connected to the input of the ADC. This capacitor is
chosen so that its value is much larger than the internal
switching capacitor and it will hence provide the voltage need-
ed to quickly and smoothly charge the ADC's sampling ca-
pacitor. Since this large capacitor will be loading the output of
the amplifier as well, an isolation resistor is needed between
the output of the amplifier and this capacitor. The isolation
resistor, R
, separates the additional load capacitance from
ISO
the output of the amplifier and will also form a low-pass filter
and can be designed to provide noise reduction as well as
anti-aliasing. The drawback to having R
is that it reduces
ISO
signal swing since there is some voltage drop across it.
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Figure 6 (a) shows the ADC directly connected to the ampli-
FIGURE 6. Driving an ADC
fier. To minimize the glitch in this setting, a slower sample rate
needs to be used. Figure 6 (b) shows R
and an external
ISO
capacitor used to minimize the glitch.
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