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THAT1512 Datasheet(PDF) 5 Page - List of Unclassifed Manufacturers |
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THAT1512 Datasheet(HTML) 5 Page - List of Unclassifed Manufacturers |
5 / 8 page These diodes reduce the likelihood that accidental electrostatic discharge (ESD) or electrical over stress (EOS) will damage the ICs. Other diodes across the base-emitter junctions of the input transistors prevent excessive reverse biasing of these junctions (which would degrade the noise performance of the input devices). Other than the protection diodes, the 1510/1512 input pins are connected only to the bases of their respective input devices. For proper operation, the bases must be provided a source of dc bias that will maintain the inputs within the IC's input common- mode range. Figure 3 shows the simplest approach; dc bias is supplied via R1 and R2. At 1 kΩ each, they will minimize pickup of unwanted noise and interfer- ence, as well as generate relatively little noise due to input current noise in the 1510/1512. However, at high gains, their inherent voltage noise, plus the 1510/1512's input current noise drawn across these resistors, adds significantly to the noise at the 1510/1512's output. Because RG is dc coupled in the circuit of Figure 3, the dc level at the output of the 1510/1512 will vary with gain. In most applications, the output should be ac-coupled to the next stage. For applica- tions where RG is variable (via a pot or switched resistors) to allow gain adjustment, RG should be ac-coupled as shown in Figure 4. By adding CG in series with RG, dc gain is fixed (at unity for the 1510, and ½ for the 1512). This constrains the output dc offset to just over +/-5 mV, and prevents it from varying with gain. With this low offset, ac coupling of the output is usually unnecessary. CG must be large enough not to interfere with low-frequency response at the smallest values of RG. For 60 dB gain, RG=10 Ω (1510) or RG=5 Ω (1512). For a -3 dB point of approximately 5 Hz, CG=3,300 μF (1510), or CG=6,800 μF (1512). For other maximum gains or minimum frequencies, scale CG accordingly. Phantom Power Phantom power is required for many condenser microphones. THAT recommends the circuit of Figure 5 when phantom power is included4. R3, R4, and D1 - D6 are used to limit the current that flows through the 1510/1512 inputs when the circuit inputs (-In and +In) are shorted to ground while phantom power is turned on. This causes C4 and/or C5 to discharge through other circuit components, often generating transient currents of several amps. R3 and R4 should be at least 10 Ω to limit destructive THAT 1510/1512 Low-Noise Page 5 of 8 Document 600031 Rev 08 High Performance Audio Preamplifier THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA Tel: +1 508 478 9200; Fax: +1 508 478 0990; Web: www.thatcorp.com Copyright © 2013, THAT Corporation RG +In Out -In RG2 RG1 U1 THAT 1510/1512 -15 +15 -In +In R1 1k R2 1k C1 470p C2 470p C3 47p Out C6 100n C9 100n V+ V- Ref Figure 3. Basic 1510 / 1512 Circuit 4. In revisions 0 and 1 of this data sheet, we recommended using Schottky diodes (1N5819 types) at D 1 ~ D4 to protect the 1510/1512 inputs against overloads. Subse - quently, we discovered that the leakage of these diodes could cause problems with DC fluctuations (hence noise) at the 1510/1512 output. Upon further investigation, we concluded that conventional rectifier diodes like the 1N4004 (the glass-passivated GP version) provide adequate protection and d o not introduce unacceptable leakage. Additionally, 1N4004 diodes are much cheaper and more readily available than the Schottky types. Since publishing revision 4, we determined that the internal reverse-bias diodes between the pins +In/RG2 and -In/RG1 may be damaged by phantom power faults under certain conditions. Small-signal diodes (D5 and D6) avoid this problem by appearing in parallel with the internal diodes, diverting excess current around the 1510/1512. |
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