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15633I Datasheet(PDF) 11 Page - Linear Technology |
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15633I Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LTC1563-2/LTC1563-3 156323fa APPLICATIONS INFORMATION Output Loading: Resistive and Capacitive The op amps of the LTC1563-X have a rail-to-rail output stage. To obtain maximum performance, the output load- ing effects must be considered. Output loading issues can be divided into resistive effects and capacitive effects. Resistive loading affects the maximum output signal swing and signal distortion. If the output load is excessive, the output swing is reduced and distortion is increased. All of the output voltage swing testing on the LTC1563-X is done with R22 = 100k and a 10k load resistor. For best undistorted output swing, the output load resistance should be greater than 10k. Capacitive loading on the output reduces the stability of the op amp. If the capacitive loading is sufficiently high, the stability margin is decreased to the point of oscillation at the output. Capacitive loading should be kept below 30pF. Good, tight layout techniques should be maintained at all times. These parts should not drive long traces and must never drive a long coaxial cable. When probing the LTC1563-X, always use a 10x probe. Never use a 1x probe. A standard 10x probe has a capacitance of 10pF to 15pF while a 1x probe’s capacitance can be as high as 150pF. The use of a 1x probe will probably cause oscillation. For larger capacitive loads, a series isolation resistor can be used between the part and the capacitive load. If the load is too great, a buffer must be used. Layout Precautions The LTC1563-X is an active RC filter. The response of the filter is determined by the on-chip capacitors and the external resistors. Any external, stray capacitance in par- allel with an on-chip capacitor, or to an AC ground, can alter the transfer function. Capacitance to an AC ground is the most likely problem. Capacitance on the LPA or LPB pins does not affect the transfer function but does affect the stability of the op amps. Capacitance on the INVA and INVB pins will affect the transfer function somewhat and will also affect the stability of the op amps. Capacitance on the SA and SB pins alters the transfer function of the filter. These pins are the most sensitive to stray capacitance. Stray capacitance on these pins results in peaking of the frequency response near the cutoff frequency. Poor layout can give 0.5dB to 1dB of excess peaking. To minimize the effects of parasitic layout capacitance, all of the resistors for section A should be placed as close as possible to the SA pin. Place the R31 resistor first so that it is as close as possible to the SA pin on one end and as close as possible to the INVA pin on the other end. Use the same strategy for the layout of section B, keeping all of the resistors as close as possible to the SB node and first placing R32 between the SB and INVB pins. It is also best if the signal routing and resistors are on the same layer as the part without any vias in the signal path. Figure 1 illustrates a good layout using the LTC1563-X with surface mount 0805 size resistors. An even tighter layout is possible with smaller resistors. 1653 F01 R11 LTC1563-X R12 VOUT VIN Figure 1. PC Board Layout Single Pole Sections and Odd Order Filters The LTC1563 is configured to naturally form even ordered filters (2nd, 4th, 6th and 8th). With a little bit of work, single pole sections and odd order filters are easily achieved. To form a single pole section you simply use the op amp, the on-chip C1 capacitor and two external resistors as shown in Figure 2. This gives an inverting section with the gain set by the R2-R1 ratio and the pole set by the R2-C1 time constant. You can use this pole with a 2nd order section to form a noninverting gain 3rd order filter or as a stand alone inverting gain single pole filter. Figure 3 illustrates another way of making odd order filters. The R1 input resistor is split into two parts with an additional capacitor connected to ground in between the resistors. This “TEE” network forms a single real pole. RB1 |
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