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KH560AK Datasheet(PDF) 11 Page - Cadeka Microcircuits LLC. |
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KH560AK Datasheet(HTML) 11 Page - Cadeka Microcircuits LLC. |
11 / 13 page KH560 DATA SHEET REV. 1A January 2004 11 For the circuit of Figure 1, the equivalent input noise voltage may be calculated using the data sheet spot noises and Rs = 25Ω, RL = ∞. Recall that 4kT = 16E-21J. All terms cast as (nV/ √Hz)2 Gain Accuracy (DC): A classical op amp’s gain accuracy is principally set by the accuracy of the external resistors. The KH560 also depends on the internal characteristics of the forward current gain and inverting input impedance. The performance equations for Av and Ro along with the Thevinin model of Figure 5 are the most direct way of assessing the absolute gain accuracy. Note that internal temperature drifts will decrease the absolute gain slightly as the part warms up. Also note that the para- meter tolerances affect both the signal gain and output impedance. The gain tolerance to the load must include both of these effects as well as any variation in the load. The impact of each parameter shown in the performance equations on the gain to the load (AL) is shown below. Increasing current gain G Increases AL Increasing inverting input Ri Decreases AL Increasing Rf lncreases AL Increasing Rg Decreases AL Applications Suggestions Driving a Capacitive Load: The KH560 is particularly suitable for driving a capacitive load. Unlike a classical op amp (with an inductive output impedance), the KH560’s output impedance, while starting out real at the programmed value, goes some- what capacitive at higher frequencies. This yields a very stable performance driving a capacitive load. The over- all response is limited by the (1/RC) bandwidth set by the KH560’s output impedance and the load capacitance. It is therefore advantageous to set a low Ro with the constraint that extremely low Rf values will degrade the distortion performance. Ro = 25Ω was selected for the data sheet plots. Note from distortion plots into a capacitive load that the KH560 achieves better than 60dBc THD (10-bits) driving 2Vpp into a 50pF load through 30MHz. Improving the Output Impedance Match vs. Frequency - Using Rx: Using the loop gain to provide a non-zero output impedance provides a very good impedance match at low frequencies. As shown on the Output Return Loss plot, however, this match degrades at higher frequencies. Adding a small external resistor in series with the output, Rx, as part of the output impedance (and adjusting the programmed Ro accordingly) provides a much better match over frequency. Figure 9 shows this approach. Figure 9: Improving Output Impedance Match vs. Frequency Increasing Rx will decrease the achievable voltage swing at the load. A minimum Rx should be used consistent with the desired output match. As discussed in the thermal analysis discussion, Rx is also very useful in limiting the internal power under an output shorted condition. Interpreting the Slew Rate: The slew rate shown in the data sheet applies to the volt- age swing at the load for the circuit of Figure 1. Twice this value would be required of a low output impedance amplifier using an external matching resistor to achieve the same slew rate at the load. Layout Suggestions: The fastest fine scale pulse response settling requires careful attention to the power supply decoupling. Generally, the larger electrolytic capacitor ground connections should be as near the load ground (or cable shield connection) as is reasonable, while the higher frequency ceramic de-coupling caps should be as near the KH560’s supply pins as possible to a low inductance ground plane. Evaluation Boards: An evaluation board (showing a good high frequency lay- out) for the KH560 is available. This board may be ordered as part #730019. Thermal Analysis and Protection A thermal analysis of a chip and wire hybrid is directed at determining the maximum junction temperature of all the internal transistors. From the total internal power dissipation, a case temperature may be developed using the ambient temperature and the case to ambient thermal impedance. Then, each of the dominant power dissipating paths are considered to determine which has the maximum rise above case temperature. The thermal model and analysis steps are shown below. As is typical, the model is cast as an electrical model where the temperatures are voltages, the power dissipa- tors are current sources, and the thermal impedances are resistances. Refer to the summary design equations and Figure 1 for a description of terms. e 2.1 .07 .632 1.22 .759 .089 2.62nV/ Hz n 22 2 2 2 2 = () + () + () + () + () + () = Rg Vi RL Vo Rx Rf KH560 + - Rs R'o = Rx + Ro Cx Ro = R'o - Rx With: Ro = KH560 output impedance and Ro + Rx = RL generally A |
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