Datasheet AD843 (Analog Devices) - 8

AD843 GROUNDING AND BYPASSING GROUNDING AND BYPASSING
Like most high bandwidth amplifiers, the AD843 is sensitive to In designing practical circuits using the AD843, the user must capacitive loading. Although it will drive capacitive loads up to keep in mind that some special precautions are needed when 20 pF without degradation of its rated performance, both an dealing with high frequency signals. Circuits must be wired us- increased capacitive load drive capability and a “cleaner” ing short interconnect leads. Ground planes should be used (nonringing) pulse response can be obtained from the AD843 whenever possible to provide both a low resistance, low induc- by using the circuits illustrated in Figures 20 to 23. The addi- tance circuit path and to minimize the effects of high frequency tion of a 5 pF feedback capacitor to the unity gain inverter con- coupling. IC sockets should be avoided, since their increased nection (Figure 20a) substantially reduces the circuit’s interlead capacitance can degrade the bandwidth of the device. overshoot, even when it is driving a 110 pF load. This can be Power supply leads should be bypassed to ground as close as seen by comparing the waveforms of Figures 20b through 20e. possible to the pins of the amplifier. Again, the component leads To drive capacitive loads greater than 100 pF, the load should should be kept very short. As shown in Figure 24, a parallel be decoupled from the amplifier’s output by a 10 Ω resistor and combination of a 2.2 µF tantalum and a 0.1 µF ceramic disc ca- the feedback capacitor, CF, should be connected directly be- pacitor is recommended. tween the amplifier’s output and its inverting input (Figure 21a). When using a 15 pF feedback capacitor, this circuit can drive 400 pF with less than 20% overshoot, as illustrated in Fig- ures 21b and 21c. Increasing capacitor CF to 47 pF also in- creases the capacitance drive capability to 1000 pF, at the expense of a 10:1 reduction in bandwidth compared with the simple unity gain inverter circuit of Figure 20a. Unity gain voltage followers (buffers) are more sensitive to capacitive loads than are inverting amplifiers because there is no attenuation of the feedback signal. The AD843 can drive 10 pF to 20 pF when connected in the basic unity gain buffer circuit of Figure 22a. The 1 kΩ resistor in series with the AD843’s noninverting input serves two functions: first, together with the amplifier’s input capacitance, it forms a low-pass filter which slows down the actual signal seen by the AD843. This helps reduce ringing on Figure 24. Recommended Power Supply Bypassing for the amplifier’s output voltage. The resistor’s second function is the AD843 (DIP Pinout) to limit the current into the amplifier when the differential input voltage exceeds the total supply voltage.
USING A HEAT SINK
The AD843 will deliver a much “cleaner” pulse response when The AD843 consumes less quiescent power than most precision connected in the somewhat more elaborate follower circuit of high speed amplifiers and is specified to operate without using a Figure 23a. Note the reduced overshoot in Figure 23b and 23c heat sink. However, when driving low impedance loads, the cur- as compared to Figures 22b and 22c. rent applied to the load can be 4 to 5 times greater than the qui- escent current. This will produce a noticeable temperature rise, For maximum bandwidth, in most applications, input and feed- which will increase input bias currents. The use of a small heat back resistors used with the AD843 should have resistance val- sink, such as the Mouser Electronics #33HS008 is recommended. ues equal to or less than 1.5 kΩ. Even with these low resistance values, the resultant RC time constant formed between them and stray circuit capacitances is large enough to cause peaking in the amplifier’s response. Adding a small capacitor, CF, as shown in Figures 20a to 23a will reduce this peaking and flatten the overall frequency response. CF will normally be less than 10 pF in value. The AD843 can drive resistive loads over the range of 500 Ω to ∞ with no change in dynamic response. While a 499 Ω load was used in the circuits of Figures 20-23, the performance of these circuits will be essentially the same even if this load is removed or changed to some other value, such as 2 kΩ. To obtain the “cleanest” possible transient response when driv- ing heavy capacitive loads, be sure to connect bypass capacitors directly between the power supply pins of the AD843 and ground as outlined in “grounding and bypassing.” Offset Null Configuration (DIP Pinout) –8– REV. D