Radiation Hardened, Ultra High Speed Current Feedback Amplifier with Offset Adjust
The is a radiation hardened, high speed, wideband, fast settling current feedback amplifier. These devices are QML approved and are processed and screened in full compliance with MIL-PRF-38535. Built with Intersil' proprietary, complementary bipolar UHF-1 (DI bonded wafer) process, it is the fastest monolithic amplifier available from any semiconductor manufacturer. The HS-1120RH's wide bandwidth, fast settling characteristic, and low output impedance, make this amplifier ideal for driving fast A/D converters. Additionally, it offers offset voltage nulling capabilities as described in the "Offset Adjustment" section of this datasheet. Component and composite video systems will also benefit from this amplifier's performance, as indicated by the excellent gain flatness, and 0.03%/0.05 Degree Differential Gain/Phase specifications (RL = 75). Detailed electrical specifications are contained in SMD 5962F9675601VPA, available on the Intersil Website or AnswerFAX systems (document #967560) A Cross Reference Table is available on the Intersil Website for conversion of Intersil Part Numbers to SMDs. The address is (http://www.intersil.com/datasheets/smd/smd_xref. html). SMD numbers must be used to order Radiation Hardened Products.
· Electrically Screened to SMD 5962F9675601VPA· MIL-PRF-38535 Class V Compliant· Low Distortion 30MHz). -84dBc (Typ)· Wide -3dB Bandwidth. 850MHz (Typ)· Very High Slew Rate. 2300V/µs (Typ)· Fast Settling (0.1%). 11ns (Typ)· Excellent Gain Flatness (to 50MHz). 0.05dB (Typ)· High Output Current. 65mA (Typ)· Fast Overdrive Recovery. <10ns (Typ)· Total Gamma Dose. 300K RAD (Si)· Latch Up. None (DI Technology)
· Video Switching and Routing· Pulse and Video Amplifiers· Wideband Amplifiers· RF/IF Signal Processing· Flash A/D Driver· Imaging Systems
PART NUMBER 5962F9675601VPA HFA1100IJ (Sample) HFA11XXEVAL TEMP. RANGE (oC) to 85 PACKAGE 8 Ld CERDIP 8 Ld CERDIP PKG. NO. GDIP1-T8 F8.3A
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 321-724-7143 | Copyright © Intersil Corporation 1999
Optimum Feedback Resistor The enclosed plots of inverting and non-inverting frequency response illustrate the performance of the HS-1120RH in various gains. Although the bandwidth dependency on closed loop gain isn't as severe as that of a voltage feedback amplifier, there can be an appreciable decrease in bandwidth at higher gains. This decrease may be minimized by taking advantage of the current feedback amplifier's unique relationship between bandwidth and RF. All current feedback amplifiers require a feedback resistor, even for unity gain applications, and , in conjunction with the internal compensation capacitor, sets the dominant pole of the frequency response. Thus, the amplifier's bandwidth is inversely proportional to RF. The HS-1120RH design is optimized for at a gain of +1. Decreasing in a unity gain application decreases stability, resulting in excessive peaking and overshoot. At higher gains the amplifier is more stable, so RF can be decreased in a tradeoff of stability for bandwidth. The table below lists recommended RF values for various gains, and the expected bandwidth.
Capacitive loads, such as an A/D input, or an improperly terminated transmission line will degrade the amplifier's phase margin resulting in frequency response peaking and possible oscillations. In most cases, the oscillation can be avoided by placing a resistor (RS) in series with the output prior to the capacitance. Figure 1 details starting points for the selection of this resistor. The points on the curve indicate the RS and CL combinations for the optimum bandwidth, stability, and settling time, but experimental fine tuning is recommended. Picking a point above or to the right of the curve yields an overdamped response, while points below or left of the curve indicate areas of underdamped performance. RS and CL form a low pass network at the output, thus limiting system bandwidth well below the amplifier bandwidth 850MHz. By decreasing RS as CLincreases (as illustrated in the curves), the maximum bandwidth is obtained without sacrificing stability. Even so, bandwidth does decrease as you move to the right along the curve. For example, = 30pF, the overall bandwidth is limited to 300MHz, and bandwidth drops = 340pF.
The frequency response of this amplifier depends greatly on the amount of care taken in designing the PC board. The use of low inductance components such as chip resistors and chip capacitors is strongly recommended, while a solid ground plane is a must! Attention should be given to decoupling the power supplies. A large value (10µF) tantalum in parallel with a small value (0.1µF) chip capacitor works well in most cases. Terminated microstrip signal lines are recommended at the input and output of the device. Capacitance directly on the output must be minimized, or isolated as discussed in the next section. Care must also be taken to minimize the capacitance to ground seen by the amplifier's inverting input (-IN). The larger this capacitance, the worse the gain peaking, resulting in pulse overshoot and possible instability. To this end, it is recommended that the ground plane be removed under traces connected to -IN, and connections to -IN should be kept as short as possible. An example of a good high frequency layout is the Evaluation Board shown in Figure 2.
FIGURE 1. RECOMMENDED SERIES OUTPUT RESISTOR vs LOAD CAPACITANCE
The performance of the HS-1120RH may be evaluated using the HFA11XXEVAL Evaluation Board. The layout and schematic of the board are shown in Figure 2. To order evaluation boards, please contact your local sales office.
The output offset voltage of the HS-1120RH may be nulled via connections to the BAL pins. Unlike a voltage feedback amplifier, offset adjustment is accomplished by varying the sign and/or magnitude of the inverting input bias current (-IBIAS). With voltage feedback amplifiers, bias currents are matched and bias current induced offset errors are nulled by matching the impedances seen at the positive and negative inputs. Bias
currents are uncorrelated on current feedback amplifiers, so this technique is inappropriate. -IBIAS flows through RF causing an output offset error. Likewise, any change in -IBIAS forces a corresponding change in output voltage, providing the capability for output offset adjustment. By nulling -IBIAS to zero, the offset error due to this current is eliminated. In addition, an adjustment limit greater than the -IBIAS limit allows the user to null the contributions from other error sources, such as VIO, or +IN source impedance. For example, the excess adjust current of 50µA [IBNADJ (Min) - IBSN (Max)] allows for the nulling of an additional 26mV of output offset error (with 510) at room temperature. The amount of adjustment is a function , so adjust range increases with increased RF. If allowed by other considerations, such as bandwidth and noise, RF can be increased to provide more adjustment range. The recommended offset adjustment circuit is shown in Figure 3.