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LOG112 Datasheet(PDF) 8 Page - Texas Instruments |
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LOG112 Datasheet(HTML) 8 Page - Texas Instruments |
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8 / 20 page  LOG112, 2112 8 SBOS246D www.ti.com FIGURE 7. Precision Current Inverter/Current Source. Figure 5 shows a low-level current source using a series resistor. The low offset op amp reduces the effect of the LOG112 and LOG2112’s input offset voltage. FREQUENCY COMPENSATION Frequency compensation for the LOG112 is obtained by connecting a capacitor between pins 5 and 14. Frequency compensation for the LOG2112 is obtained by connecting a capacitor between pins 2 and 5, or 15 and 12. The size of the capacitor is a function of the input currents, as shown in the Typical Characteristic curves ( Minimum Value of Compensa- tion Capacitor). For any given application, the smallest value of the capacitor which may be used is determined by the maximum value of I2 and the minimum value of I1. Larger values of CC make the LOG112 and LOG2112 more stable, but reduce the frequency response. In an application, highest overall bandwidth can be achieved by detecting the signal level at VOUT, then switching in appropriate values of compensation capacitors. NEGATIVE INPUT CURRENTS The LOG112 and LOG2112 function only with positive input currents (conventional current flows into input current pins). In situations where negative input currents are needed, the circuits in Figures 6, 7, and 8 may be used. FIGURE 5. Current Source with Offset Compensation. 1M Ω +2.5mV 100k Ω –2.5V +2.5V OPA335 100 Ω I 1 = 2.5nA to 1mA I 2 = 2.5nA V LOGOUT C C 14 1 910 V– LOG112 6 5 V+ 8 V REF Chopper Op Amp GND FIGURE 6. Current Inverter/Current Source. Q B National LM394 I IN I OUT D 1 OPA703 D 2 Q A FREQUENCY RESPONSE The frequency response curves seen in the Typical Charac- teristic curves are shown for constant DC I1 and I2 with a small-signal AC current on one input. The 3dB frequency response of the LOG112 and LOG2112 are a function of the magnitude of the input current levels and of the value of the frequency compensation capacitor. See Typical Characteristic curve, 3dB Frequency Response for details. The transient response of the LOG112 and LOG2112 are different for increasing and decreasing signals. This is due to the fact that a log amp is a nonlinear gain element and has different gains at different levels of input signals. Smaller input currents require greater gain to maintain full dynamic range, and will slow the frequency response of the LOG112 and LOG2112. 1.5k Ω Photodiode 10nA to 1mA 10nA to 1mA +5V +5V 1k Ω +3.3V(1) 1/2 OPA2335 1/2 OPA2335 BSH203 TLV271 or 1/2 OPA2335 (+3.3V Back Bias) LOG112 Pin 1 or Pin 14 NOTE: (1) +3.3V bias is an arbitrary ac level < 5V that also appears on the −IN through the op amp where it applies a reverse bias to the photodiode. |
Similar Part No. - LOG112 |
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Similar Description - LOG112 |
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