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LT1677CS8 Datasheet(PDF) 11 Page - Linear Technology |
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LT1677CS8 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LT1677 Current noise is measured in the circuit shown in Figure 7 and calculated by the following formula: i e nV M n no = () −() ()( ) 2 2 12 130 101 1 101 • / Ω The LT1677 achieves its low noise, in part, by operating the input stage at 120 µA versus the typical 10µA of most other op amps. Voltage noise is inversely proportional while current noise is directly proportional to the square root of the input stage current. Therefore, the LT1677’s current noise will be relatively high. At low frequencies, the low 1/f current noise corner frequency ( ≈90Hz) mini- mizes current noise to some extent. In most practical applications, however, current noise will not limit system performance. This is illustrated in the Total Noise vs Source Resistance plot (Figure 8) where: Total Noise = [(voltage noise)2 + (current noise • RS) 2 + (resistor noise)2]1/2 Three regions can be identified as a function of source resistance: (i) RS ≤ 400Ω. Voltage noise dominates (ii) 400 Ω ≤ RS ≤ 50k at 1kHz 400 Ω ≤ RS ≤ 8k at 10Hz Figure 6a. 0.1Hz to 10Hz Noise Test Circuit Figure 6b. 0.1Hz to 10Hz Peak-to-Peak Noise Tester Frequency Response 1677 F06a 10 Ω 0.1 µF 4.7 µF VOLTAGE GAIN = 50,000 24.3k 100k – + * LT1677 LT1001 2k 4.3k 110k 100k SCOPE × 1 RIN = 1M *DEVICE UNDER TEST NOTE: ALL CAPACITOR VALUES ARE FOR NONPOLARIZED CAPACITORS ONLY 2.2 µF 0.1 µF 22 µF FREQUENCY (Hz) 100 90 80 70 60 50 40 30 0.01 1 10 100 1677 F06b 0.1 APPLICATIO S I FOR ATIO Figure 7 1677 F07 100 Ω 100k LT1677 500k 500k eno SOURCE RESISTANCE (k Ω) 0.1 1 10 100 1000 1 10 100 1677 F08 VS = ±15V TA = 25°C SOURCE RESISTANCE = 2R R R AT 1kHz AT 10Hz RESISTOR NOISE ONLY Figure 8. Total Noise vs Source Resistance (iii) RS > 50k at 1kHz RS > 8k at 10Hz Clearly the LT1677 should not be used in region (iii), where total system noise is at least six times higher than the } Resistor noise dominates } Current noise dominates |
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