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OPA111BM Datasheet(PDF) 9 Page - Texas Instruments |
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OPA111BM Datasheet(HTML) 9 Page - Texas Instruments |
9 / 14 page 9 ® OPA111 should completely surround the high impedance input leads and should be connected to a low impedance point which is at the signal input potential. The amplifier case should be connected to any input shield or guard via pin 8. This insures that the amplifier itself is fully surrounded by guard potential, minimizing both leak- age and noise pickup (see Figure 3). If guarding is not required, pin 8 (case) should be connected to ground. FIGURE 4. Voltage Noise Spectral Density vs Source Resistance. FIGURE 6. Pyroelectric Infrared Detector. FIGURE 3. Connection of Input Guard. NOISE: FET VERSUS BIPOLAR Low noise circuit design requires careful analysis of all noise sources. External noise sources can dominate in many cases, so consider the effect of source resistance on overall operational amplifier noise performance. At low source impedances, the lower voltage noise of a bipolar operational amplifier is superior, but at higher impedances the high current noise of a bipolar amplifier becomes a serious liability. Above about 15k Ω, the OPA111 will have a lower total noise than an OP-27 (see Figure 4). BIAS CURRENT CHANGE VERSUS COMMON-MODE VOLTAGE The input bias current of most popular BIFET operational amplifiers are affected by common-mode voltage (Figure 5). Higher input FET gate-to-drain voltage causes leakage and ionization (bias) currents to increase. Due to its cascode input stage, the extremely low bias current of the OPA111 is not compromised by common-mode voltage. APPLICATIONS CIRCUITS Figures 6 through 18 are circuit diagrams of various appli- cations for the OPA111. FIGURE 5. Input Bias Currrent vs Common-Mode Voltage. 80 60 40 20 0 –20 –15 –10 –5 0 5 10 15 Common-Mode Voltage (VDC) OPA111 AD547 LF156/157 LF155 T A = 25°C; curves taken from manufacturers' published typical data. OP-15/16/17 "Perfect Bias Current Cancellation" OPA111 2 3 6 Output 8 1000pF Polystyrene 1000M Ω 1000M Ω Pyroelectric Detector NOTE: Pyroelectric detectors respond to rate-of-change (AC signal) only. 3 2 4 5 6 7 8 1 OPA111 2 3 6 8 In Out Inverting TO-99 Bottom View OPA111 2 3 6 In Out Non-Inverting 8 OPA111 2 3 6 In Out Buffer 8 Board layout for input guarding: guard top and bottom of board. Alternate: use Teflon® standoff for sensitive input pins. Teflon® E. I. Du Pont de Nemours & Co. 100 1k 10k 100k 1M 10M 1k 100 10 1 BM OP-27 + Resistor OPA111 + Resistor Resistor Noise Only OPA111 + Resistor Resistor Noise Only OP-27 + Resistor Source Resistance, R S (Ω) E O R S E O = e N 2 + (I NRS) 2 + 4kTR S |
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