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LOG101AIDE4 Datasheet(PDF) 6 Page - Texas Instruments |
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LOG101AIDE4 Datasheet(HTML) 6 Page - Texas Instruments |
6 / 16 page www.ti.com LOG101 6 SBOS242B FIGURE 6. Current Inverter/Current Source. 10M Ω +25mV 100k Ω –2.5V +2.5V OPA335 100 Ω I 1 = 2.5nA to 1mA I 2 = 2.5nA V OUT C C 1G Ω to 2.5kΩ REF3025 8 3 1 56 V– LOG101 4 3 2.5V V+ V+ Chopper Op Amp GND FIGURE 5. Current Source with Offset Compensation. Figure 5 shows a low-level current source using a series resistor. The low offset op-amp reduces the effect of the LOG101’s input offset voltage. FREQUENCY RESPONSE The frequency response curve seen in the Typical Charac- teristic Curves is shown for constant DC I1 and I2 with a small signal AC current on one input. The 3dB frequency response of the LOG101 is 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 LOG101 is different for in- creasing and decreasing signals. This is due to the fact that a log amp is a nonlinear gain element and has different gains Q B National LM394 I IN I OUT D 1 OPA703 D 2 Q A 2N2905 I REF R REF 2N2905 +15V –15V I REF = 6V R REF 3.6k Ω 6V IN834 FIGURE 3. Temperature Compensated Current Source. SETTING THE REFERENCE CURRENT When the LOG101 is used to compute logarithms, either I1 or I2 can be held constant and becomes the reference current to which the other is compared. VOUT is expressed as: VOUT = (1V) • log (I1/I2)(1) IREF can be derived from an external current source (such as shown in Figure 3), or it may be derived from a voltage source with one or more resistors. When a single resistor is used, the value may be large depending on IREF. If IREF is 10nA and +2.5V is used: RREF = 2.5V/10nA = 250MΩ A 1 + R 2 R 1 +5V R 3 V REF = 100mV R 3 >> R2 I REF – V OS 1 FIGURE 4. T Network for Reference Current. A voltage divider may be used to reduce the value of the resistor, as shown in Figure 4. When using this method, one must consider the possible errors caused by the amplifier’s input offset voltage. The input offset voltage of amplifier A1 has a maximum value of 1.5mV, making VREF a suggested value of 100mV. at different levels of input signals. Smaller input currents require greater gains to maintain full dynamic range, and will slow the frequency response of the LOG101. FREQUENCY COMPENSATION Frequency compensation for the LOG101 is obtained by connecting a capacitor between pins 3 and 8. The size of the capacitor is a function of the input currents, as shown in the Typical Characteristic Curves (Minimum Value of Compen- sation 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 will make the LOG101 more stable, but will 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 LOG101 will function only with positive input currents (conventional current flows into pins 1 and 8). In situations where negative input currents are needed, the circuits in Figures 6, 7, and 8 may be used. (2) |
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