6445

- HDSL and ADSL Drivers
- Multimedia Broadcast Systems
- Professional Video Cameras
- Video Amplifiers
- Copiers/Scanners/Fax
- HDTV Amplifiers
- Pulse Amplifiers and Peak Detectors
- CATV/Fiber Optics Signal Processing

APPLICATION NOTES:

PERFORMANCE DISCUSSION: The LM7171 is a very high speed, voltage feedback amplifier. It consumes only 6.5mA supply current while providing a gain-bandwidth product of 170MHz (Minimum) and a slew rate of 2000V/uS (Minumum). It also has other great features such as low differential gain and phase and high output current.

The LM7171 is a true voltage feedback amplifier. Unlike current feedback amplifiers (CFAs) with a low inverting input impedance and a high non-inverting input impedance, both inputs of voltage feedback amplifiers (VFA's) have high impedance nodes. The low impedance inverting input in CFA's and a feedback capacitor create an additional pole that will lead to instability. As a result, CFA's cannot be used in traditional op amp circuits such as photodiode amplifiers, I-to-V converters and integrators, where a feedback capacitor is required.

CIRCUIT OPERATION: The class AB input stage in the LM7171 is fully symmetrical and has a similar slewing characteristic to the current feedback amplifiers. In the LM7171 Simplified Schematic, (see AN00006) Q1 through Q4 form the equivalent of the current feedback input buffer, RE the equivalent of the feedback resistor, and stage A buffers the inverting input. The triple-buffered output stage isolates the gain stage from the load to provide low output impedance.

SLEW RATE CHARACTERISTIC: The slew rate of LM7171 is determined by the current available to charge and discharge an internal high impedance node capacitor. This current is the differential input voltage divided by the total degeneration resistor RE. Therefore, the slew rate is proportional to the input voltage level, and the higher slew rates are achievable in the lower gain configurations. See the LM7171 Commercial Data Book for slew rate Vs input voltage level curve.

When a very fast, large signal, pulse is applied to the input of an amplifier, some overshoot or undershoot occurs. By placing an external resistor such as 1K Ohm in series with the input of the LM7171, the bandwidth is reduced to help lower the overshoot.

SLEW RATE LIMITATION: If the amplifier's input signal has too large of an amplitude at too high of a frequency, the amplifier is said to be slew rate limited; this can cause ringing in time domain, and peaking in frequency domain, at the output of the amplifier.

In the Commercial Data Book "Typical Performance Characteristics" section, there are several curves of Av = +2 and Av = +4 versus input power levels. For the Av = +4 curves,no peaking is present and the LM7171 responds identically to the different input power levels of 30 mV, 100 mV and 300mV.

For the Av = +2 curves, slight peaking occurs. This peaking at high frequency (>100MHz) is caused by a large input signal at high enough frequency, that it exceeds the amplifier's slew rate. The peaking in frequency response does not limit the pulse response in time domain. The LM7171 is stable with noise gain of > +2.

LAYOUT CONSIDERATION: PRINTED CIRCUIT BOARDS AND HIGH SPEED OP AMPS: There are many things to consider when designing PC boards for high speed op amps. Without proper caution, it is very easy to have excessive ringing, oscillation, and other degraded AC performance in high speed circuits. As a rule, the signal traces should be short and wide to provide low inductance and low impedance paths. Any unused board space must be grounded to reduce stray signal pickup. Critical components should also be grounded at a common point to eliminate voltage drop. Sockets add capacitance to the board and can affect high frequency performance. It is better to solder the amplifier directly into the PC board without using any socket.

USING PROBES: Active (FET) probes are ideal for taking high frequency measurements because they have wide bandwidth, high input impedance, and low input capacitance. However, the probe ground leads provide a long ground loop that will produce errors in measurement. Instead, the probes can be grounded directly by removing the ground leads and probe jackets and using scope probe jacks.

COMPONENT SELECTION & FEEDBACK RESISTOR: It is important in high speed applications to keep all component leads short. For discrete components, choose carbon composition-type resistors and mica-type capacitors. Surface mount components are preferred over discrete components for minimum inductive effect.

Large values of feedback resistors can couple with parasitic capacitance and cause undesirable effects such as ringing or oscillation in high speed amplifiers. For LM7171, a feedback resistor of 510 Ohms gives optimal performance.

COMPENSATION FOR INPUT CAPACITANCE: The combinations of an amplfier's input capacitance with the gain setting resistors adds a pole that can cause peaking or oscillation. To solve this problem, a feedback capacitor with a value Cf>(Rg X Cin)/Rf can be used to cancel that pole. For LM7171, a feedback capacitor of 2pF is recommended. AN00003 illustrates the compensation circuit.

POWER SUPPLY BYPASSING: Bypassing the power supply is necessary to maintain low power supply impedance across the frequency spectrum. Both positive and negative power supplies should be bypassed individually by placing 0.01uF ceramic capacitors directly to the power supply pins and 2.2uF tantalum capacitors close to the power supply pins. See AN00004.

TERMINATION: In high frequency applications, reflection occur if signals are not properly terminated. Figure 3, in the Commercial Data Book, shows a properly terminated signal,while Figure 4, in the Commercial Data Book, shows an improperly terminated signal.

To minimize reflection, coaxial cable with matching characteristic impedance to the signal source should be used. The other end of the cable should be terminated with the same value terminator or resistor. For the commonly used cables, RG59 has 75 Ohm characteristic impedance, and RG58 has 50 Ohm characteristic impedance.

DRIVING CAPACITIVE LOADS: Amplifiers driving capactive loads can oscillate or have ringing at the output. To eliminate oscillation or reduce ringing, an isolation resistor can be placed as shown on AN00005. The combination of the isolation resistor and the load capacitor forms a pole to increase stability by adding more phase margin to the overall system. The desired performance depends upon the value of the isolation resistor; the bigger the isolation resistor, the more damped the pulse response becomes. For LM7171, a 50 Ohm isolation resistor is recommended for initial evaluation. Figure 6, in the Commercial Data Book, shows the LM7171 driving a 150pF load with the 50 Ohm isolation resistor.

POWER DISSIPATION: The maximum power allowed to dissipate in a device is defined as: Pd =[Tj(max) - TA]/ThetaJA, where Pd (is the power dissipation in a device), Tj(max) (is the maximum junction temperature), TA (is the ambient temperature), ThetaJA (is the thermal resistance of a particular package).

For example, for the LM7171 in a J-8 package, the maximum power dissipation at 25 C ambient temperature is 730mW.

The total power dissipation in a device can be calculated as: Pd = Pq + Pl

Pq is the quiescent power dissipated in a device with no load connected at the output. Pl is the power dissipated in the device with a load connected at the output; it is not the power dissipated by the load.

Furthermore, Pq = supply current x total supply voltage with no load, Pl = output current x (voltage difference between supply voltage and output voltage of the same side of supply voltage).

For example, the total power dissipated by the LM7171 with Vs = <15V and output voltage of 10V into 1K Ohm is:
Pd = Pq + Pl
= (6.5mA)x(30V)+(10mA)x(15V - 10V)
= 195mW + 50mW
= 245mW

Supply Voltage (V+ - V-)
                                                                      36V
Differential Input Voltage
(Note 6)
                                                                      ±10V
Maximum Junction Temperature
                                                                      150 C
Power Dissipation
(Note 2, 3)
                                                                      730mW
Output Short Circuit to Ground
(Note 4)
                                                                      Continuous
Operating Temperature Range
                                                                       -55 C TA +125 C
Thermal Resistance
(Note 7)
ThetaJA
8-Pin CERAMIC DIP (Still Air)                            106 C/W
(500LF/Min Air flow)                                         53 C/W
10-Pin CERPAK (Still Air)                                   182 C/W
(500LF/Min Air flow)                                         105 C/W
10-Pin CERAMIC SOIC (Still Air)                        182 C/W
(500LF/Min Air flow)                                         105 C/W
ThetaJC
(Note 3)
8-Pin CERAMIC DIP                                           3 C/W
10-Pin CERPAK                                                  5 C/W
10-Pin CERAMIC SOIC                                       5 C/W
Package Weight
(Typical)
8-Pin CERAMIC DIP                                            965mg
10-Pin CERPAK                                                   235mg
10-Pin CERAMIC SOIC                                        230mg
Storage Temperature Range
                                                                          -65 C to +150 C
ESD Tolerance
(Note 5)
                                                                          3000V
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur.Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test conditions.

Note 2: The maximum power dissipation must be derated at elevated temperatures and is dictated by Tjmax (maximum junction temperature), ThetaJA (package junction to ambient thermal resistance), and TA (ambient temperature). The maximum allowable power dissipation at any temperature is Pdmax = (Tjmax - TA)/ThetaJA or the number given in the Absolute Maximum Ratings, whichever is lower.


Note 3: The package material for these devices allows much improved heat transfer over our standard ceramic packages. In order to take full advantage of this improved heat transfer, heat sinking must be provided between the package base (directly beneath the die), and either metal traces on, or thermal vias through, the printed circuit board. Without this additional heat sinking, device power dissipation must be calculated using junction-to-ambient, rather than junction-to-case,thermal resistance. It must not be assumed that the device leads will provide substantial heat transfer out of the package, since the thermal resistance of the leadframe material is very poor, relative to the material of the package base. The stated junction-to-case thermal resistance is for the package material only, and does not account for the additional thermal resistance between the package base and the printed circuit board. The user must determine the value of the additional thermal resistance and must combine this with the stated value for the package, to calculate the total allowed power dissipation for the device.

Note 4: Applies to both single-supply and split-supply operation. Continuous short circuit operation at elevated ambient temperature can result in exceeding the maximum allowed junction temperature of 150 C.

Note 5: Human body model, 1.5k Ohms in series with 100pF.

Note 6: Input differential voltage is measured at Vs = +15V.




Note 7: All numbers apply for packages soldered directly into a PC board.

The 6445 is a high speed voltage feedback amplifier that has the slewing characteristic of a current feedback amplifier; yet it can be used in all traditional voltage feedback amplifier configurations. The 6445 is stable for gains as low as + 2 or -1. It provides a very high slew rate at 2000V/uS (Minimum) and a wide gain-bandwidth product of 170MHz (Minimum) while consuming only 6.5mA of supply current. It is ideal for video and high speed signal processing applications such as HDSL and pulse amplifiers. With 100mA output current, the 6445 can be used for video distribution, as a transformer driver, or as a laser diode driver.

Operation on +15V power supplies allows for large signal swings and provides greater dynamic range and signal-to-noise ratio. The 6445 is ideal for ADC/DAC systems. In addition, the 6445 is specified for +5V operation for portable applications.

The 6445 is built on National's advanced VIP(TM)III(Vertically integrated PNP)complementary bipolar process.