The HFA1112 is a closed-loop buffer with user-programmable gain and ultra-high-speed performance. Manufactured on Intersil's proprietary complementary bipolar UHF-1 process, these devices feature a wide -3dB bandwidth of 850MHz, very fast switching speeds, excellent gain flatness, low distortion and high output current. A unique feature of the pinout allows the user to select a voltage gain of +1, -1 or +2 without using any external components. Gain selection is done by connecting to the input, as described in the "Application Information" section. The result is a more flexible product, inventory of fewer parts types, and more efficient use of board space. Compatibility with existing op amp outputs provides the flexibility to upgrade low-gain amplifiers while reducing component count. Unlike most buffers, the standard output provides an upgrade path if higher closed-loop gain is required in the future. This amplifier has programmable output limits like the HFA1113. For applications requiring standard buffer output, see the HFA1110 datasheet.

• The HFA1112 uses a novel design that allows the user to choose from three closed-loop gains without requiring any external components. The result is a more flexible product, inventory of fewer parts types, and more efficient use of board space. This "buffer" operates with a closed loop gain of -1, +1, or +2, and the selection of the gain is done by connecting to the input. A gain of +1 is selected when applying the input signal to +IN and float -IN, and a gain of +2 is selected for ground. Apply the input signal to IN with +IN grounded for a gain of -1. The following table summarizes these connections:

The frequency response of such an amplifier depends to a large extent on the degree of care taken in designing the PC board. It is highly recommended to use low inductance components such as chip resistors and chip capacitors, and must have a solid ground! Care should be taken to decouple the power supply. Large value (10F) tantalum in parallel with small value (0.1F) chip capacitors work well in most cases.

It is recommended to use terminated microstrip signal lines at the input and output of the device. Capacitance applied directly to the output must be minimized or isolated as discussed in the next section. For unity-gain applications, care must also be taken to minimize the amplifier's inverting input-to-ground capacitance. At higher frequencies, this capacitance will tend to shorten the GND-input, causing the closed-loop gain to increase with frequency. This will cause excessive high frequency peaking and potentially other problems as well. The evaluation board shown in Figure 2 is an example of a good high-frequency layout. Driving capacitive loads Capacitive loads, such as A/D inputs, or improperly terminated transmission lines, can reduce the phase margin of the amplifier, resulting in frequency response peaking and possible oscillation. In most cases, oscillation can be avoided by placing a resistor (RS) in series with the output before the capacitor. Figure 1 details the starting point for choosing this resistor. The points on the curve represent the RS and CL combinations for optimal bandwidth, stability, and settling time, but experimental fine-tuning is recommended. Selecting points above or to the right of the curve produces an overdamped response, while points below or to the left of the curve represent areas of underdamped performance. RS and CL form a low-pass network at the output, limiting the system bandwidth to well below the amplifier bandwidth of 850MHz