Datasheet LT1886 (Analog Devices) - 10

LT1886
U U W U APPLICATIO S I FOR ATIO
6V Typical Performance Curve of Frequency Response vs Capacitive Load shows the peaking for various capacitive loads. + This stability is useful in the case of directly driving a – coaxial cable or twisted pair that is inadvertently 909Ω unterminated. For best pulse fidelity, however, a termina- 100 4V tion resistor of value equal to the characteristic impedance Ω 50Ω of the cable or twisted pair (i.e., 50Ω/75Ω/100Ω/135Ω) –4V 1K f = 1MHz should be placed in series with the output. The other end 100Ω of the cable or twisted pair should be terminated with the – same value resistor to ground. +
Compensation
–6V 1886 F01 The LT1886 is stable in a gain 10 or higher for any supply
Figure 1. Thermal Calculation Example
and resistive load. It is easily compensated for lower gains with a single resistor or a resistor plus a capacitor. Figure␣ 2 shows that for inverting gains, a resistor from the The dissipation for the amplifiers is: inverting node to AC ground guarantees stability if the PD = (63.5mA)(12V) – (4V/√2)2/(50) = 0.6W parallel combination of RC and RG is less than or equal to R The total package power dissipation is 0.6W. When a 2500 F/9. For lowest distortion and DC output offset, a series capacitor, C sq. mm PC board with 2oz copper on top and bottom is C, can be used to reduce the noise gain at lower frequencies. The break frequency produced by R used, the thermal resistance is 80°C/W. The junction C and CC should be less than 15MHz to minimize peaking. The temperature TJ is: Typical Curve of Frequency Response vs Supply Voltage, TJ = (0.6W)(80°C/W) + 85°C = 133°C AV = –1 shows less than 1dB of peaking for a break The maximum junction temperature for the LT1886 is frequency of 12.8MHz. 150°C so the heat sinking capability of the board is RF adequate for the application. V –R R o F G – = If the copper area on the PC board is reduced to 180 sq. V V R i i G mm the thermal resistance increases to 122°C/W and the RC Vo (RC || RG) ≤ RF/9 + C 1 junction temperature becomes: C (OPTIONAL) < 15MHz 2πRCCC 1886 F02 TJ = (0.6W)(122°C/W) + 85°C = 158°C
Figure 2. Compensation for Inverting Gains
which is above the maximum junction temperature indi- cating that the heat sinking capability of the board is Figure 3 shows compensation in the noninverting configu- inadequate and should be increased. ration. The RC, CC network acts similarly to the inverting case. The input impedance is not reduced because the
Capacitive Loading
network is bootstrapped. This network can also be placed The LT1886 is stable with a 1000pF capacitive load. The between the inverting input and an AC ground. photo of the small-signal response with 1000pF load in a Another compensation scheme for noninverting circuits is gain of 10 shows 50% overshoot. The photo of the large- shown in Figure 4. The circuit is unity gain at low frequency signal response with a 1000pF load shows that the output and a gain of 1 + RF/RG at high frequency. The DC output slew rate is not limited by the short-circuit current. The offset is reduced by a factor of ten. The techniques of 10