Radiation Hardened, High Speed, Low Power, Current Feedback Video Operational Amplifier with Output Disable
The is a high speed, low power current feedback amplifier built with Intersil's proprietary complementary bipolar UHF-1 (DI bonded wafer) process. These devices are QML approved and are processed and screened in full compliance with MIL-PRF-38535. This amplifier features a TTL/CMOS compatible disable control, pin 8, which when pulled low, reduces the supply current and forces the output into a high impedance state. This allows easy implementation of simple, low power video switching and routing systems. Component and composite video systems also benefit from this op amp's excellent gain flatness, and good differential gain and phase specifications. Multiplexed A/D applications will also find the HS-1145RH useful as the A/D driver/multiplexer. Specifications for Rad Hard QML devices are controlled by the Defense Supply Center in Columbus (DSCC). The SMD numbers listed here must be used when ordering. Detailed Electrical Specifications for these devices are contained in SMD 5962-96830. A "hot-link" is provided on our homepage for downloading. http://www.intersil.com/spacedefense/space.htm
· Electrically Screened to SMD # 5962-96830· QML Qualified per MIL-PRF-38535 Requirements· Low Supply Current. 5.9mA (Typ)· Wide -3dB Bandwidth.360MHz (Typ)· High Slew Rate.1000V/µs (Typ)· Excellent Gain Flatness (to 50MHz). ±0.07dB (Typ)· Excellent Differential Gain. 0.02% (Typ)· Excellent Differential Phase. 0.03 Degrees (Typ)· High Output Current.60mA (Typ)· Output Enable/Disable Time. 180ns/35ns (Typ)· Total Gamma Dose. 300kRAD(Si)· Latch Up. None (DI Technology)
· Multiplexed Flash A/D Driver· RGB Multiplexers/Preamps· Video Switching and Routing· Pulse and Video Amplifiers· Wideband Amplifiers· RF/IF Signal Processing
ORDERING NUMBER 5962F9683001VPA 5962F9683001VPC INTERNAL MKT. NUMBER HS7-1145RH-Q HS7B-1145RH-Q TEMP. RANGE (oC) to 125
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 321-724-7143 | Copyright © Intersil Corporation 1999
Although a current feedback amplifier's 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 RF, 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-1145RH design is optimized for at a gain of +2. Decreasing RF decreases stability, resulting in excessive peaking and overshoot (Note: Capacitive feedback will cause the same problems due to the feedback impedance decrease at higher frequencies). At higher gains, however, the amplifier is more stable so RF can be decreased in a trade-off of stability for bandwidth. The table below lists recommended RF values for various gains, and the expected bandwidth. For a gain +1, a resistor (+RS) in series with +IN is required to reduce gain peaking and increase stability.
The die version of the HS-1145RH provides the user with a GND pad for setting the disable circuitry GND reference. With symmetrical supplies the GND pad may be left unconnected, or tied directly to GND. If asymmetrical supplies (e.g., +10V, 0V) are utilized, and TTL compatibility is desired, die users must connect the GND pad to GND. With an external GND, the DISABLE input is TTL compatible regardless of supply voltage utilized.
The HS-1145RH utilizes a quasi-complementary output stage to achieve high output current while minimizing quiescent supply current. In this approach, a composite device replaces the traditional PNP pulldown transistor. The composite device switches modes after crossing 0V, resulting in added distortion for signals swinging below ground, and an increased undershoot on the negative portion of the output waveform (See Figures 5, 8, and 11). This undershoot isn't present for small bipolar signals, or large positive signals. Another artifact of the composite device is asymmetrical slew rates for output signals with a negative voltage component. The slew rate degrades as the output signal crosses through 0V (See Figures 5, 8, and 11), resulting in a slower overall negative slew rate. Positive only signals have symmetrical slew rates as illustrated in the large signal positive pulse response graphs (See Figures 4, 7, and 10).
This amplifier's frequency response depends greatly on the 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 device's input and output connections. Capacitance, parasitic or planned, connected to the output must be minimized, or isolated as discussed in the next section. Care must also be taken to minimize the capacitance to ground at the amplifier's inverting input (-IN), as this capacitance causes gain peaking, pulse overshoot, and if large enough, instability. To reduce this capacitance, the designer should remove the ground plane under traces connected to -IN, and keep connections to -IN as short as possible. An example of a good high frequency layout is the Evaluation Board shown in Figure 2.
For best operation, the DC source impedance seen by the non-inverting input should be 50. This is especially important in inverting gain configurations where the noninverting input would normally be connected directly to GND.
The HS-1145RH derives an internal GND reference for the digital circuitry as long as the power supplies are symmetrical about GND. With symmetrical supplies the digital switching threshold (VTH = (VIH is 1.4V, which ensures the TTL compatibility of the DISABLE input. If asymmetrical supplies (e.g., +10V, 0V) are utilized, the switching threshold becomes:
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 of 270MHz (for +1). By decreasing as CL increases (as illustrated in the curves), the maximum bandwidth is obtained without sacrificing stability. In spite of this, the bandwidth decreases as the load capacitance increases. For example, = 40pF, the overall bandwidth is limited to 180MHz, and bandwidth drops = 400pF.
FIGURE 1. RECOMMENDED SERIES OUTPUT RESISTOR vs LOAD CAPACITANCE FIGURE 2C. SCHEMATIC FIGURE 2. EVALUATION BOARD SCHEMATIC AND LAYOUT
The performance of the HS-1145RH may be evaluated using the HFA11XX Evaluation Board. The layout and schematic of the board are shown in Figure 2. The VH connection may be used to exercise the DISABLE pin, but note that this connection has no 50 termination. To order evaluation boards (part number HFA11XXEVAL), please contact your local sales office.