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VCA2613 Datasheet(PDF) 9 Page - Texas Instruments |
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VCA2613 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 20 page VCA2613 9 SBOS179D www.ti.com Achieving the best active feedback architecture is difficult with conventional op amp circuit structures. The overall gain A must be negative in order to close the feedback loop, the input impedance must be high to maintain low current noise and good gain accuracy, but the gain ratio must be set with very low value resistors to maintain good voltage noise. Using a two-amplifier configuration (noninverting for high impedance plus inverting for negative feedback reasons) results in excessive phase lag and stability problems when the loop is closed. The VCA2613 uses a patented architec- ture that achieves these requirements, with the additional benefits of low power dissipation and differential signal han- dling at both input and output. For greatest flexibility and lowest noise, the user may wish to shape the frequency response of the LNP. The COMP1 and COMP2 pins for each channel (pins 10 and 11 for channel A, pins 26 and 27 for channel B) correspond to the drains of Q3 and Q8, see Figure 4. A capacitor placed between these pins will create a single-pole low-pass response, in which the effective R of the RC time constant is approximately 186 Ω. COMPENSATION WHEN USING ACTIVE FEEDBACK The typical open-loop gain versus frequency characteristic for the LNP is shown in Figure 9. The –3dB bandwidth is approximately 180MHz and the phase response is such that when feedback is applied the LNP will exhibit a peaked response or might even oscillate. One method of compensat- ing for this undesirable behavior is to place a compensation capacitor at the input to the LNP, as shown in Figure 10. This method is effective when the desired –3dB bandwidth is much less than the open-loop bandwidth of the LNP. This compensation technique also allows the total compensation capacitor to include any stray or cable capacitance that is associated with the input connection. Equation 4 relates the bandwidth to the various impedances that are connected to the LNP. BW A1 R R 2 C(R )(R ) IF IF = + ( ) + π AVOIDING UNSTABLE PERFORMANCE The VCA2612 and the VCA2613 are very similar in perfor- mance in all respects, except in the area of noise perfor- mance. See Figure 4 for a schematic of the LNP. The improvement in noise performance is because the input wiring resistor (RW) of the VCA2613, see Figure 4, has been considerably reduced compared to the VCA2612. This brings the input noise of the VCA2613 down to 1.0nV/ √Hz com- pared to VCA2612’s 1.25nV/ √Hz. The input impedance at the gate of either Q4 or Q7 can be approximated by the network shown in Figure 11. The resistive component shown in Figure 11 is negative, which gives rise to unstable behav- ior when the signal source resistance has both inductive and capacitive elements. It should be noted that this negative resistance is not a physical resistor, but an equivalent resis- tance that is a function of the devices shown in Figure 4. Normally, when an inductor and capacitor are placed in series or parallel, there is a positive resistance in the loop that prevents unstable behavior. FIGURE 9. Open-Loop Gain Characteristic of LNP. FIGURE 10. LNP with Compensation Capacitor. 25dB –3dB Bandwidth 180MHz Output Input R F R I C A FIGURE 11. VCA2613 Input Impedance. 24pF 57pF –93 Ω For the VCA2613, the situation can be remedied by placing an external resistor with a value of approximately 15 Ω or higher in series with the input lead. The net series resistance will be positive, and there will be no observed instability. Although this technique will prevent oscillations, it is not recommended, as it will also increase the input noise. A 4.7pF external capacitor must be placed between pins COMP2A (pin 11) and LNPINPA (pin 16), and between pins COMP2B (pin 26) and LNPINPB (pin 21). This has the result of making the input impedance always capacitive due to the feedback effect of the compensation capacitor and the gain of the LNP. Using capacitive feedback, the LNP becomes unconditionally stable, as there is no longer a negative component to the input impedance. The compensation capacitor mentioned above will be reflected to the input by the formula: CIN = (A + 1)CCOMP (5) (4) |
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