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LMP7731

SNOSAT6E – JULY 2007 – REVISED MARCH 2013

www.ti.com

Figure 47. Start-Up Input Offset Voltage Drift

During the peak-to-peak noise measurement, the LMP7731 must be shielded. This prevents offset variations due

to airflow. Offset can vary by a few nV due to this airflow and that can invalidate measurements of input voltage

noise with a magnitude which is in the same range. For similar reasons, sudden motions must also be restricted

in the vicinity of the test area. The feed-through which results from this motion could increase the observed noise

value which in turn would invalidate the measurement.

DIODES BETWEEN THE INPUTS

The LMP7731 has a set of anti-parallel diodes between the input pins as shown in Figure 48. These diodes are

present to protect the input stage of the amplifier. At the same time, they limit the amount of differential input

voltage that is allowed on the input pins. A differential signal larger than the voltage needed to turn on the diodes

might cause damage to the diodes. The differential voltage between the input pins should be limited to ±3 diode

drops or the input current needs to be limited to ±20 mA.

Figure 48. Anti-Parallel Diodes between Inputs

DRIVING AN ADC

Analog to Digital Converters, ADCs, usually have a sampling capacitor on their input. When the ADC's input is

directly connected 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 sufficient 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 needed to quickly and smoothly charge the

16

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