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AMMC-6650 Datasheet(PDF) 9 Page - AVAGO TECHNOLOGIES LIMITED |
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AMMC-6650 Datasheet(HTML) 9 Page - AVAGO TECHNOLOGIES LIMITED |
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9 / 10 page  9 OP AMP 1 insures that the attenuator maintains a good input and output match to 50 Ω, while OP AMP 2 increases the usable control voltage range versus using only direct voltage ramps for V1 and V2 and improves over tempera- ture operation. If optimum VSWR is all that is required, OP AMP 2 may be eliminated however, RL must remain connected to the DCout pad of the AMMC-6650 and the control voltage can be applied directly to V2. CAUTION: Low voltage op-amps must be used so as not to exceed the maximum limit of V1 and V2 control voltages. As shown, a voltage reference (VREF) is fed to the reference circuit DCin pad via a 500 Ωresistor,creatinga500Ωsource. The reference circuit termination RL, is connected to the DCout pad and ideally is also equal to 500 Ω. This voltage is controlled in parallel with the RF attenuator. The chosen value of VREF must be low enough to avoid modifying the FET biasing and lower than the turn-on voltage of the ESD protection diode but high enough such that the attenuat- ed voltage at OP AMP 2 is usable compared to input offsets etc. The optimum value for the positive reference voltage is approximately 0.1 to 0.4 V. At equilibrium, the voltages at nodes A and B of the OP AMP 1 must be equal which implies that the input impedance to the DC reference circuit is equal to RREF. When V2 is changed to a lower value, the voltage at node A becomes greater than that of node B. This voltage difference causes the output voltage of op OP AMP 1 to move toward its positive rail until equilibrium is once again established. When V2 is changed to a higher value the voltage at node A becomes less than that of node B and the output voltage of OP AMP 1 will swing toward its negative rail until equi- librium is reached. If the reference circuit precisely tracks the RF circuit, the voltage output of OP AMP 1 at equilibrium insures that the RF circuit is matched to 50 Ω. If attenuation linearity is required, OP AMP 2 is included as shown in Figure 14 and a positive control voltage is applied to VCONTROL. At equilibrium, voltages at nodes C and D are equal. When VCONTROL is changed, the output of OP AMP 2 adjusts to a value that forces the voltage at node C to equal the voltage at node D. Therefore, the output voltage of the DC reference circuit is proportional to VCONTROL. The input voltage to the reference circuit is being held constant and the log(VCONTROL) is proportional to the reference circuit attenuation 20log(DCout/DCin). If the FET parameters of the DC reference circuit track the FET parameters of the RF circuit, the voltage output of the RF circuit is also proportional to the control voltage. This translates to a linear relationship between the attenuation (in dB) and the log(VCONTROL). Two RF attenuation vs voltage curves corresponding to different values of VREF are shown in Figure 15. These curves were obtained by using the driver circuit shown in Figure 14 and the VREF values 0.1 V and 0.4 V. Values for RL, R1 and R2 were 500 Ω, 10 kΩ and 100Ω re- spectively. Control voltage ranged from 4.5 V to 0 V. Because the FETs in the DC circuit are not identical to those in the RF circuit, the DC circuit does not exactly track the RF circuit. This results in attenuation vs. voltage curves that are not exactly linear. OP AMP 2 provides temperature compensation by adjusting V2 in such a way as to keep voltage at point C equal to that point D. If the attenuation changes over tempera- ture, voltage at point C tries to change, but is corrected by OP AMP 2. Another way to improve performance of the attenuator driver circuit is to adjust RL and RREF. If the reference circuit precisely tracked the RF circuit and the ON resistance of the FETs was zero ohms, then RL and RREF would be exactly 500 Ω. Due to the difference in layout structures, the reference circuit does not track the RF circuit precisely. RL and RREF can be adjusted in order to compensate for these differences. Optimum values of RL and RREF have been found to be between 500 Ω and 650Ω. For maximum dynamic range on the attenuation control circuit, RL should be less than RREF by an amount equal to the “ON resistance” of the reference circuit series FETs. The “ON resistance” of the series FETs is about 95 Ω total. Therefore, the relationship between RL and RREF is as follows: RREF = RL + 95Ω The voltage divider formed by R1 and R2 can be used to adjust the sensitivity of the attenuator versus control voltage. For the driver circuit shown in Figure 14, maximum attenuation is always achieved by setting VCONTROL equal to 0 V. Minimum attenuation is achieved when Vcontrol ≈ x x Vref or Vcontrol ≈ x DCout Therefore, an increase in the resistor ratio R1/R2 increases the value of the control voltage required to produce minimum attenuation. R1 + R2 R2 ⎛ ⎜ ⎝ RL 500 Ω + RL ⎛ ⎜ ⎝ R1 R2 ⎛ ⎜ ⎝ 1 + |
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