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OP270GP bảng dữ liệu(PDF) 10 Page - Analog Devices |
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OP270GP bảng dữ liệu(HTML) 10 Page - Analog Devices |
10 / 16 page REV. C OP270 –10– NOISE MEASUREMENTS Peak-to-Peak Voltage Noise The circuit of Figure 6 is a test setup for measuring peak-to-peak voltage noise. To measure the 200 nV peak-to-peak noise specifica- tion of the OP270 in the 0.1 Hz to 10 Hz range, the following precautions must be observed: 1. The device has to be warmed up for at least five minutes. As shown in the warm-up drift curve, the offset voltage typically changes 2 mV due to increasing chip temperature after power-up. In the 10-second measurement interval, these temperature induced effects can exceed tens of nanovolts. 2. For similar reasons, the device has to be well shielded from air currents. Shielding also minimizes thermocouple effects. 3. Sudden motion in the vicinity of the device can also “feed through” to increase the observed noise. 4. The test time to measure noise of 0.1 Hz to 10 Hz should not exceed 10 seconds. As shown in the noise-tester frequency response curve of Figure 7, the 0.1 Hz corner is defined by only one pole. The test time of 10 seconds acts as an additional pole to eliminate noise contribution from the frequency band below 0.1 Hz. FREQUENCY (Hz) 100 0.01 0.1 1.0 10 100 80 60 40 20 0 Figure 7. 0.1 Hz to 10 Hz Peak-to-Peak Voltage Noise Test Circuit Frequency Response 5. A noise-voltage-density test is recommended when measuring noise on a large number of units. A 10 Hz noise-voltage-density measurement will correlate well with a 0.1 Hz to 10 Hz peak-to-peak noise reading, since both results are determined by the white noise and the location of the 1/f corner frequency. 6. Power should be supplied to the test circuit by well bypassed low noise supplies, e.g., batteries. They will minimize output noise introduced via the amplifier supply pins. Noise Measurement — Noise Voltage Density The circuit of Figure 8 shows a quick and reliable method of measuring the noise voltage density of dual op amps. The first amplifier is in unity-gain, with the final amplifier in a noninverting gain of 101. As noise voltages of each amplifier are uncorrelated, they add in rms fashion to yield: ee e OUT nA 2 nB 2 = () +() Ê ËÁ ˆ ¯˜ 101 The OP270 is a monolithic device with two identical amplifi- ers. The noise voltage density of each individual amplifier will match, giving: ee e OUT n 2 n = Ê ËÁ ˆ ¯˜ = () 101 2 101 2 1/2 OP270 – + R1 100 1/2 OP270 – + R2 10k eOUT eOUT (nV/ Hz) = 101 ( 2en) VS = 15V TO SPECTRUM ANALYZER Figure 8. Noise Voltage Density Test Circuit OP270 DUT – + R3 1.24k R2 100k R1 5 OP27E – + R5 8.06k enOUT TO SPECTRUM ANALYZER R4 200 GAIN = 10,000 VS = 15V Figure 9. Current Noise Density Test Circuit Noise Measurement — Current Noise Density The test circuit shown in Figure 9 can be used to measure cur- rent noise density. The formula relating the voltage output to current noise density is: i e G nV Hz R n nOUT S = Ê ËÁ ˆ ¯˜ - () 2 2 40 / where: G = gain of 10,000 RS = 100 k W source resistance |
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