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VCA610 Datasheet(PDF) 11 Page - Texas Instruments |
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VCA610 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 14 page VCA610 11 SBOS013A FIGURE 10. Signal Drive of the VCA610 Gain Control Pin Produces an Exponential Response, Re-expand- ing Signal Companded by Figure 9. R 2 330 Ω VCA610 R 1 470 Ω V O = –VR10 –1.925 [R1 VIN /(R1 + R2) + 1] V IN V R –10mV V C FIGURE 11. This Voltage-Tuneable Low-Pass Filter Pro- duces a Variable Cutoff Frequency with a 3,000:1 Range. fP = G/2πR2C G = 10 –1.925 (VC + 1) = – R 2 R 1 1 1 + R2CS/G OPA680 VCA610 V C V O V OA R 2 330 Ω R 1 330 Ω C 0.047 µF V I Examination of this result illustrates several circuit charac- teristics. First, the argument of the log term, –VIN/VR, reveals an option and a constraint. In Figure 9, VR represents a DC reference voltage. Optionally, making this voltage a second signal produces log-ratio operation. Either way, the log term’s argument constrains the polarities of VR and VIN. These two voltages must be of opposite polarities to ensure a positive argument. This polarity combination results when VR connects to the inverting input of the VCA610. Alter- nately, switching VR to this amplifier’s noninverting input removes the minus sign of the log term’s argument. Then, both voltages must be of the same polarity to produce a positive argument. In either case, the positive polarity re- quirement of the argument restricts VIN to a unipolar range. The above VOL expression reflects a circuit gain introduced by the presence of R1 and R2. This feature adds a convenient scaling control to the circuit. However, a practical matter sets a minimum level for this gain. The voltage divider formed by R1 and R2 attenuates the voltage supplied to the VC terminal by the op amp. This attenuation must be great enough to prevent any possibility of an overload voltage at the VC terminal. Such an overload saturates the VCA610’s gain-control circuitry, reducing the amplifier’s gain. For the feedback connection of Figure 9, this overload condition permits a circuit latch. To prevent this, choose R1 and R2 to ensure that the op amp can not possibly deliver more than 2.5V to the VC terminal. LOW-DRIFT WIDEBAND EXPONENTIAL AMP A common use of the log amp above involves signal companding. The inverse function, signal expanding, re- quires an exponential transfer function. The VCA610 pro- duces this latter response directly as shown in Figure 10. DC reference VR again sets the amplifier’s input voltage and the input signal VIN now drives the gain control point. Resistors R1 and R2 attenuate this drive to prevent overload- ing the gain control input. Setting these resistors at the same values as in the preceding log amp produces an exponential amplifier with the inverse function of the log amp. VOLTAGE-CONTROLLED LOW-PASS FILTER In the circuit of Figure 11, the VCA610 serves as the variable-gain element of a voltage-controlled low-pass filter. As will be described, this implementation expands the circuit’s voltage swing capability over that normally achieved with the equivalent multiplier implementation. The circuit’s re- sponse pole responds to control voltage V C according to the relationship f P = G/2πR2C where G = 10 –1.925 (V C + 1). With the components shown, the circuit provides a linear variation of the low-pass cutoff from 300Hz to 1MHz. The response control results from amplification of the feed- back voltage applied to R 2. Consider first the case where the VCA610 produces G = 1. Then, the circuit performs as if this amplifier were replaced by a short circuit. Visually doing so leaves a simple voltage amplifier with a feedback resistor bypassed by a capacitor. This basic circuit produces a response pole at f P = 1/2πR2C. For G > 1, the circuit applies a greater voltage to R 2, increasing the feedback current this resistor supplies to the summing junction of the OPA620. The increased feedback current produces the same result as if R 2 had been decreased in value in the basic circuit described above. Decreasing the effective R 2 resistance moves the circuit’s pole to a higher frequency, producing the f P = G/2πR2C response control. Finite loop gain and a signal-swing limitation set perfor- mance boundaries for the circuit. Both limitations occur when the VCA610 attenuates rather than amplifies the feedback signal. These two limitations reduce the circuit’s utility at the lower extreme of the VCA610’s gain range. For –1 ≤ V C ≤ 0, this amplifier produces attenuating gains in the range from 0dB to –38.5dB. This directly reduces the net gain in the circuit’s feedback loop, increasing gain error V O V I |
Similar Part No. - VCA610_08 |
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Similar Description - VCA610_08 |
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