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KH561AK Datasheet(PDF) 9 Page - Cadeka Microcircuits LLC. |
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KH561AK Datasheet(HTML) 9 Page - Cadeka Microcircuits LLC. |
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9 / 13 page  KH561 DATA SHEET REV. 1A January 2004 9 With this total value derived, the required external Cx is developed by backing out the effect of the internal 10pF. This, and an expression for the external Cx without the intermediate steps are shown below. The plot in Figure 6 shows the required Cx vs. gain for several desired output impedances using the equations shown above. Note that for lower Ro’s, Cx can get very large. But, since the total compensation is actually the series combination of Cx and 10pF, going to very high Cx’s is increasingly ineffective as the total compensation is only slightly changed. This, in part, sets the lower limits on allowable Ro. Figure 6: External Compensation Capacitance (Cx) A 0% small signal overshoot response can be achieved by increasing Cx slightly from the maximally flat value. Note that this applies only for small signals due to slew rate effects coming into play for large, fast edge rates. Beyond the nominal compensation values developed thus far, this external Cx provides a very flexible means for tailoring the frequency response under a wide variety of gain and loading conditions. It is oftentimes useful to use a small adjustable cap in development to determine a Cx suitable to the application, then fixing that value for production. An excellent 5pF to 20pF trimmer cap for this is a Sprague-Goodman part #GKX20000. When the KH561 is used to drive a capacitive load, such as an ADC or SAW device, the load will act to compen- sate the response along with Cx. Generally, considerably lower Cx values are required than the earlier develop- ment would indicate. This is advantageous in that a low Ro would be desired to drive a capacitive load which, without the compensating effect of load itself, would otherwise require very large Cx values. Gain and Output Impedance Range Figure 7 shows a plot of the recommended gain and output impedances for the KH561. Operation outside of this region is certainly possible with some degradation in performance. Several factors contribute to set this range. At very low output impedances, the required value of feedback resistor becomes so low as to excessively load the output causing a rapid degradation in distortion. The maximum Ro was set somewhat arbitrarily at 200Ω. This allows the KH561 to drive into a 2:1 step down transformer matching to a 50 Ω load. (This offers some advantages from a distortion standpoint. See Kota Application Note KAN-01 for details.) Figure 7: Recommended Gain and Output Impedance Range For a given Ro, the minimum gain shown in Figure 7 has been set to keep the equivalent input noise voltage less than 4nV/ √Hz. Generally, the equivalent input noise volt- age decreases with higher signal gains. The high gain limit has been set by targeting a minimum Rg of 10Ω or a minimum Rf of 100Ω. Amplifier Configurations The KH561 is intended for a fixed, non-inverting, gain configuration as shown in Figure 1. The KH560 offers the better pulse fidelity with its improved thermal tail in the pulse response (vs. the KH561). Due to its low internal forward gain, the inverting node does not present a low impedance, or virtual ground, node. Hence, in an inverting configuration, the signal’s source impedance will see a finite load whose value depends on the output loading. Inverting mode operation can be best achieved using a wideband, unity gain buffer with low output impedance, to isolate the source from this varying load. A DC level can, however, be summed into the inverting node to offset the output either for offset correction or signal conditioning. Accuracy Calculations Several factors contribute to limit the achievable KH561 accuracy. These include the DC errors, noise effects, and the impact internal amplifier characteristics have on the signal gain. Both the output DC error and noise model may be developed using the equivalent model of Figure 5. Generally, non-inverting input errors show up C 10 C 10 C or C 1 R 300 1 2 R 0.08 pF x t t x o g = − = − − No Load Voltage Gain K 0 2 4 6 8 10 12 14 16 18 20 5 10 15 20 25 30 35 40 45 50 55 Maximally Flat Response into a Matched Load Ro = 50Ω Ro = 75Ω Ro = 100Ω Output Impedance ( Ω) 0 10 20 30 40 50 60 70 80 90 100 0 20 40 60 80 100 120 140 160 180 200 Low Rf or Rg Region Recommended Region High Noise Region |
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