BUF04

a FEATURES Bandwidth – 110 MHz Slew Rate – 3000 V/ s Low Offset Voltage – 2 Vbes, these transistors turn on and literally “brute-force” the output to follow the input. When the input signal drops below 2 Vbes, the transistors return to their normally “OFF” state. REV. 0 –9– BUF04 A two-terminal equivalent circuit of the BUF04 is shown in Figure 30 where the transistor-level equivalent circuit is reduced to its essential elements. The input stage develops a signal current, IIN, that is replicated by an internal current conveyor so as to flow through Rt, the transimpedance of the BUF04. The voltage developed across Rt is buffered by a unity-gain output voltage follower. With an open-loop Rt of 400 kΩ and an RIN of 30 Ω, the voltage gain of the BUF04, given by the ratio Rt/RIN is approximately 13,000—accurate to approximately 13.5 bits. The BUF04’s open-loop ac transimpedance response is determined by the open-loop pole formed by Rt and Ct. Since Ct is typically 8 pF, the open-loop pole occurs at approximately 50 kHz. To minimize the effects of high-frequency coupling, circuits must be built with short interconnect leads, and large ground planes should he used whenever possible to provide a low resistance, low-inductance circuit path. Sockets should be avoided because the increased interlead capacitance can degrade bandwidth and stability. If sockets are necessary, individual pin sockets (oftentimes called “cage jacks,” AMP Part No. 5-330808-3 or 5-330808-6) should be used. They contribute far less stray reactance than molded socket assemblies. Offset Voltage Nulling VIN X1 IIN RIN RFB RIN = 30 Ω Rt = 400 k Ω Ct = 8pF RFB = 100 Ω Rt IIN Ct XI VOUT Although the offset voltage of the BUF04 is very low (1 mV, maximum) for such a high speed buffer, the circuit shown in Figure 32 can be used if additional offset voltage nulling is required. A potentiometer ranging from 1 k to 10 k can be used for VOS nulling; with a 10 kΩ potentiometer, the trim range is ± 30 mV. V+ TRIM RANGE ±30mV 1 VIN 3 10µF 0.1µF 10k 8 7 6 0.1µF 10µF BUF04 4 VOUT Figure 30. Current-Feedback Functional Equivalent Circuit of the BUF04 Grounding and Bypassing Considerations V– To take full advantage of the BUF04’s very wide bandwidth, high slew rates, and dynamic range capabilities requires due diligence with regard to supply bypassing. In high speed circuits, the supply bypassing network must provide a very low impedance return path for currents flowing to and from the load network. As with any high speed application, multiple bypassing is always recommended. A 10 µF tantalum electrolytic in parallel with a 0.1 µF ceramic capacitor is sufficient for most applications. For those high speed applications where output load currents approach 50 mA, small valued resistors (1.1 Ω to 4.7 Ω) in series with the tantalum capacitors may improve circuit transient response by damping out the capacitor’s selfinductance. Figure 31 illustrates bypassing recommendations. V+ 10µF R1 0.1µF KELVIN RETURN FOR LOAD CURRENT 6 RL 0.1µF VOUT Figure 32. Optional Offset Voltage Nulling Scheme APPLICATIONS Output Short-Circuit Protection To optimize the transient response and output voltage swing of the BUF04, internal output short-circuit current limiting was omitted. Although the BUF04 can provide continuous output currents of 50 mA without protection, direct connection of the BUF04’s output to ground or to the supplies will destroy the device. An active current limit technique, illustrated in Figure 33, provides the necessary short-circuit protection while retaining full dc output voltage swing to the load. +15V 10µF RSC1 ≥10Ω 2N2905 2N2905 0.1µF 7 VIN 3 BUF04 4 6 0.1µF 6.2k Ω SET ISC +(ISC–)