LMP7731_15 Datasheet, PDF (15/26 Pages) Texas Instruments – 2.9 nV/sqrt(Hz) Low Noise, Precision, RRIO Amplifier

LMP7731
www.ti.com
SNOSAT6E – JULY 2007 – REVISED MARCH 2013
INPUT VOLTAGE NOISE MEASUREMENT
The LMP7731 has very low input voltage noise. The peak-to-peak input voltage noise of the LMP7731 can be
measured using the test circuit shown in Figure 45
0.1 PF
100 k:
-
LMP7731
10:
+
VOLTAGE GAIN = 50,000
2 k:
4.7 PF
+
LMP7731
-
100 k:
4.3 k: 22 PF
2.2 PF 110 k:
SCOPE
x1
R_IN = 1M
24.3 k:
0.1 PF
Figure 45. 0.1 Hz to 10 Hz Noise Test Circuit
The frequency response of this noise test circuit at the 0.1 Hz corner is defined by only one zero. The test time
for the 0.1 Hz to 10 Hz noise measurement using this configuration should not exceed 10 seconds, as this time
limit acts as an additional zero to reduce or eliminate the noise contributions of noise from frequencies below 0.1
Hz.
Figure 46 shows typical peak-to-peak noise for the LMP7731 measured with the circuit in Figure 45 for the
LMP7731.
Figure 46. 0.1 Hz to 10 Hz Input Voltage Noise
Measuring the very low peak-to-peak noise performance of the LMP7731, requires special testing attention. In
order to achieve accurate results, the device should be warmed up for at least five minutes. This is so that the
input offset voltage of the op amp settles to a value. During this warm up period, the offset can typically change
by a few µV because the chip temperature increases by about 30°C. If the 10 seconds of the measurement is
selected to include this warm up time, some of this temperature change might show up as the measured noise.
Figure 47 shows the start-up drift of five typical LMP7731 units.
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