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2022-09-15 14:32:14
OPA2681 with a dual broadband with the function of the ban, current feedback computing amplifier
Features
● Broadband+5V work: 225MHz (g u003d+2)
● Unit to stable: 280MHz (g u003d 1)
● High -output current : 150mA
● Output voltage switching: ± 4.0V
● High conversion rate: 2100V/μs
● Low power supply current: 6MA/CH
] ● Low -loss current: 200 μA/CH
● Enable/disable time: 25ns/100ns
Application
● XDSL line drive
● Matching matching I/Q channel amplifier
● Broadband video buffer
● High -speed imaging channel
● Portable instrument
● Differential ADC drive
] ● Active filter
● Broadband inverter and
Instructions
OPA2681 set a new performance level for the broadband dual -current feedback amplifier. Working at a very low 6MA/CH power supply, OPA2681 provides a conversion rate and output power, which is usually related to higher power currents. A new output -level structure provides a low -output voltage and cross distortion of the minimum output current. This provides a special single power operation. Using a single+5V power supply, OPA2681 can provide 1V to 4V output swing, the driving current exceeds 100mA, and the bandwidth is 150MHz. This function combination makes OPA2681 an ideal RGB line drive or a single power ADC input drive.
The low 6mA/CH power supply current of OPA2681 is accurately adjusted at 25 ° C. This fine -tuning and low temperature drift, Guaran Tee's maximum guarantee power supply current is lower than competitive products. The system power can be further reduced by using optional disable control pins (only SO-14). Keeping this ban on the open state is open, or keeps at a high position, you can work normally. If it is pulled down, the power current of OPA2681 will drop below 400 μA, and the output will enter a high impedance state at the same time. This function can be used for energy saving or videos multiple reuse applications.
OPA2681 related products
g u003d+2, RF u003d 402 # 8486;, RL u003d 100 unless there is another instructions (see Figure 1).
g u003d+2, RF u003d 499 , RL u003d 100 to+2.5V, unless otherwise There are instructions (see Figure 2).
Application information
Broadband current feedback operation
OPA2681 has high linear, high power, high power Export -level broadband current feedback calculation amplifier's excellent communication performance. Only a static current of 6mA/CH is required. OPA2681 will swing to the 1V range of any power rail, and provide a guarantee current of more than 135mA at room temperature. This low -output cleanliness requirement, plus partial voltage independent of the power supply voltage, provides a significant single (+5V) power operation. OPA2681 will provide bandwidth greater than 200MHz, and drives 2VP-P output on a single+5V power supply to 100 The previous enhanced output -level amplifier usually suffers a very poor cross distortion due to excessive output current. OPA2681 realizes a considerable power gain and better linear. The main advantage of the current feedback of the op amp voltage feedback is relatively independent of AC performance (bandwidth and distortion) and signal gain. For similar AC performance with improvement of DC accuracy, consider high conversion rate, stable unit gain, and voltage feedback OPA2680.
FIG. 1 shows DC coupling +2 gain dual -power circuit configuration as ± 5V specifications and typical performance curves. For the purpose of testing, the input impedance is used to set the input impedance to 50 , and the output impedance is used to set the output impedance to 50 The voltage fluctuation reported in the specification is obtained directly at the input and output pin, while the load power (DBM) is defined under the matching 50 For the circuit in Figure 1, the total effective load is 100 || 804 u003d 89 Disable control line (DIS) usually keeps opening (only SO-14) to ensure the normal work of the amplifier. Figure 1 contains an optional component. In addition to the normal power supply -to -ground decoupled power container, a 0.1 μF capacitor also includes a 0.1 μF capacitor. In the actual PC plate layout, this optional additional capacitor usually increases the two harmonic distortion performance by 3 to 6 decibels.
FIG. 2 shows the basis of AC coupling,+2 gain, single power circuit configuration, as the basis of+5V specifications and typical performance curves. Although it is not ""Railto-Rail"" design, compared with other very wide-frequency band current feedback amplifiers, OPA2681 requires the minimum input and output voltage balance. It will provide a 3VP-P output swing single+5V bandwidth greater than 150MHz. The key to the operation of broadband single power supplyThe requirements are to keep the input and output signal swing in the available voltage range of input and output. The circuit in FIG. 2 uses a simple resistance division of the+5V power supply (two 806 resistors) to establish an input midpoint bias. Then enter the signal to be coupled to the midpoint voltage bias. The input voltage can swing within the 1.5V range of any power supply foot, and a 2VP-P input signal range is provided between the power pins. Adjust the input impedance matching resistor (57.6 ) for testing to provide input matching of 50 The gain resistance (RG) is a AC coupling, a +1 DC gain to the circuit, which will also apply the DC bias voltage (2.5V) to the output terminal. The feedback resistance value has been adjusted from the dual -pole power supply conditions to re -optimize the flat frequency response in the case of+5V and the gain to +2 (see setting the resistance value to optimize the bandwidth). Similarly, on a single+5V power supply, the output voltage can swing within the 1V range of any one power supply, while providing an output current of more than 75mA. It is required to use 100 load to the midpoint partially pressed in this characteristic circuit. In the third -level output, the bipolar current infusion of the 81 -level input is the smallest.
Single power differential A/D converter driver
Broadband video multi -road reuse
A common application of video speed amplifier (including disable pipe foot) is to connect multiple amplifiers together, and then from several possible video input Choose which one is input to a line. This simple ""wired or video multi -road reuse"" can easily be implemented in OPA2681N, as shown in Figure 4.
Generally, the synchronization or retrospective time of the video signal executes channel switching. At this time, the two inputs are equal. OPA2681's disabled features of ""first pass and then break"" ensure that when using the wired or circuit shown in Figure 4, there is always a amplifier control line. Because the two inputs may be connected for a short time during the conversion process between the channels, the output is combined through the output impedance matching resistor (in this example 82.5 ). When a channel is disabled, its feedback network forms a part of the output impedance, and the signal is slightly attenuated when the output to the cable. The gain and output matching resistance slightly increased to obtain +1 signal gain under the matching load, and provide the output impedance of 75 for the cable. Video Multi -path reuse connection (Figure 4) can also ensure that the maximum differential voltage of the unseensed channel input end does not exceed the rated ± 1.2V maximum value of the standard video signal level.
The disable operation part shows the opening and closing switch fault of the use of single -channel grounding inputs is usually less than ± 50mV. When the two outputs are switched (as shown in Figure 4), the output cable is always controlled by one or the other amplifier due to the disabled time of ""first pass through"". In this case, the two 0V input switch faults dropped to lt; 20mv.
High -speed active filter 
In this case, 51 input matching resistors are optional. The input signal is coupled from the+5V power supply to the 2.5V DC reference voltage through the resistor division. The first level is used as a +1 voltage of signalThe buffer, which requires 600 feedback resistors to maintain stability. This first level is easy to drive the required low input resistance to the input of this high -frequency filter. The second level is set to +1 DC gain, and the 2.5V working point is transmitted to the output pin, and the communication gain is +4. The feedback resistance has been adjusted to optimize the bandwidth of the amplifier itself. As shown in the single -electric source frequency response diagram, the OPA2681 in this configuration will provide a small signal bandwidth of gt; 200MHz. The capacitance value is selected as low as possible, but it is enough to submerge the parasitic input capacitor of the amplifier. Slightly adjust the resistance value to obtain the required filter frequency response. At the same time, considering the transmission delay of about 1NS through each channel of OPA2681.
A high -power twisted tie drive
A very harsh application of a high -speed amplifier is driving low load impedance, while maintaining a high output voltage to high frequency. Using dual-current feedback computing amplifier OPA2681, the 15VP-P output signal can be implemented into a typical twist line with a typical impedance of 100Ω. The configuration is as shown in the homepage, the two amplifiers of OPA2681 are pushed to configure the driver output transformer, so that the peak signal of the peak signal of each op amp doubled to 15VP-P. The turning ratio ratio of the transformer is 2. In order to provide the source of matching, this requires a one -sided 25Ω source impedance (RS) to give the transformer equation n2 u003d RL/RS. The average distribution between the output needs to be installed at 12.4 #8486 at each output end; a series matching resistor is installed at each output. Take 25 total resistance load (for differential output signals) and draw a load cable on the output voltage and current restriction diagram. It can be seen that 1.5V is required at the current peak current of 150mA, and at 150mA at 150mA Under the negative peak current, it requires 2.5V to be cleaned. By moving the DC working point 0.5V to 6.5V, the 7.5VP-P of each amplifier is implemented through a+12V power supply, as shown in the upstream drive of the homepage.
The application of the wire drive usually has high requirements for low -disturbed signal transmission. A current feedback amplifier like OPA2681 is an ideal choice to provide low distortion performance to obtain higher gain. The sample is set to a differential gain of 7.5. The circuit can transmit a maximum 15VP-P signal with a bandwidth of more than 60MHz.
160MHz broadband amplifier
As mentioned earlier, as the signal gain increases, the current feedback topology of OPA2681 provides almost constant bandwidth. The three computing amplifier broadband instrument amplifiers shown in Figure 6 use this to achieve the differential bandwidth of 160MHz. The signal is applied to the high impedance non -inverse input terminal of OPA2681. Differential gain is set by (1+2RF/RG), and the value shown in Figure 6 is equal to 5. Feedback resistance, RF, optimized under this specific gain. Can increase by adjusting RGAdjustment. The voltage feedback amplifier OPA680 of the voltage feedback amplifier that is configured as a standard differential amplifier is performed. In order to maintain OPA2681 good distortion performance, it is set to match the load of each amplifier output by setting R3+R4 u003d R1, instead of using the same resistance value in the differential amplifier.
Design Tool 
In the two encapsulation styles of OPA2681, several PC boards can be used to assist circuits to assist circuits Preliminary evaluation of performance. Both of them are free, as a descriptive file provided by an unpopular personal computer board. The following table shows the summary information of these boards.
Please contact the TI application to support the hotline to request any these boards. 
When analyzing the performance of the simulation circuit and system, it is very useful to use SPICE to simulate the computer performance. This is especially true for video and RF amplifier circuits, because parasitic capacitors and inductors will have a significant impact on circuit performance. OPA2681's SPICE model can obtain (website) or a model (1-800-548-6132) on the D disk of Ti application department. The application department can also get design help through this number. These models can well predict small signal communication and transient performance under various operating conditions. They do not do well in predicting harmonic distortion or DG/DP characteristics. These models do not try to distinguish the packaging type in their small signal communication performance, nor do they try to simulate the coupling of channels to channels.
Operation suggestion
Set the resistance value to optimize the bandwidth
current feedback computing amplifier like OPA2681 can maintain the almost constant signal gain bandwidth by adjusting the external resistance value appropriately. The typical performance curve shows this; as the gain increases, the small signal bandwidth has only decreased slightly. These curves also show that the feedback resistance has changed each gain settings. The resistance ""value"" of the current feedback on the circuit inverter side can be considered as a frequency response compensation element, and their ""ratio"" sets the signal gain. Figure 7 shows the analysis circuit of the small signal frequency of OPA2681.
The key element of this current feedback of the op amp modeling is:
α → from non -counter -phase input to the buffer gain input
Ri → buffer output impedance
Feedback error current signal
z (s) → frequency from IERR to VO Related open -loop cross -resistance gain
Cushorer The gain is usually very close to 1.00, and it is usually ignored from the signal gain consideration. But it willSet CMRR for the configuration of the single transportation split amplifier. For buffer gain lt; 1.0, CMRR u003d -20 x log (1 -α) db.
The output impedance RI of the buffer is a key part of the bandwidth control equation. OPA2681 is usually about 45 .
The current feedback operational amplifier sensor sensor's error current (opposite to the difference in input error voltage of the voltage feedback computing amplifier) u200bu200bis passed to the output terminal through a transmissile gain related to the internal frequency. The typical performance curve shows this kind of opening and cross -block response. This is similar to the opening voltage gain curve of the voltage feedback op amp. The transmission function of the Development 7 Circuit obtains the equations 1:
This is written in a circuit gain analysis format, which is displayed by the non -unlimited open loop gain. middle. If Z (S) is infinitely frequent, the denominator of equation 1 will be reduced to 1, and the ideal expectation signal gain displayed in the molecule will be obtained. The score in Formula 1 denim determines the frequency response. Formula 2 is displayed as a circuit gain equation:
If the number of 20X pairs (RF+NGX RI) on the top of the ring -opening cross -ring diagram, the difference between the two will be the difference between the two Is the loop gain at a given frequency. In the end, Z (s) was rolled to the denominator of equivalent equation 2. At this time, the loop gain decreased to 1 (curve intersection). The closed frequency response of the amplifier given by the equal form 1 starts attenuation, which is exactly similar to the frequency of the noise gain of the voltage feedback computing amplifier equal to the opening voltage gain. The difference here is that the total impedance in the equivalent 2 denim can be slightly controlled by the expected signal gain (or NG).
OPA2681 After internal compensation, it provides the maximum flat frequency response for RF u003d 402 The optimal feedback of the target function (49mpe2). With the change of signal gain, the contribution of NGX-RI items in feedback interruption will also change, but it can keep it unchanged by adjusting RF. Formula 3 gives the approximate equation of the best radio frequency signal gain:
As the expected signal gain increase, this equation will eventually predict a negative RF. You can also set the subjective limit of this adjustment by maintaining RG at the minimum value of 20 minimum value. The lower value will load the buffer level in the input level and output level. If the RF is too low, it will actually reduce the bandwidth. Figure 8 shows the recommendation RF and NG for ± 5V and a single+5V operation. The value of RF and GAIN displayed here is about the value of generating typical performance curves. The difference between them is that the optimized value used in the typical performance curve also corrects the plate parasites that have not been considered in the simplified analysis, so as to obtain equations 3. The value displayed in Figure 8 shows bandwidth optimizationThe design provides a good starting point.
The total impedance entering the reverse input can be used to adjust the width of the closed -loop signal. Inserting a series resistance between the inverter input and the knot and the knot will increase the feedback impedance (the denominator of Formula 2), thereby reducing the bandwidth. OPA2681's internal buffer output impedance is slightly impact, and the source impedance is viewed from the non -reversing input terminal. High source resistance will increase RI and reduce bandwidth. For those single -power applications that form a mid -point bias at the non -inverse input terminal through high -value resistance at the non -inverter input terminal, the off -coupled container to inhibit the power supply ripples, non -inverse input noise current diversion, and minimize the high frequency value of RI to 7 to 7 to 7 It is important.
Reverse amplifier operation
Since OPA2681 is a general broadband current feedback amplifier, most common operational amplifiers application circuits are available for designers. Because the feedback resistance is a compensation element for current feedback computing, those who need to feedback elements (such as points, cross -guided, and some filters) have considerable flexibility. It is for peace) is especially suitable for OPA2681. Figure 9 shows a typical inverter configuration. The I/O impedance and signal gain in FIG. 1 is retained in the configuration of the inverter circuit.
In the reverse configuration, you must pay attention to the two key design considerations. First, the gain resistance (RG) becomes part of the input impedance of the signal channel. If you need to input impedance matching (when the signal is coupled with cables, twisted wiring, long PC board wires, or other transmission wire conductors, this is beneficial), it is usually necessary to add an additional matching resistor to the ground. RG itself is usually not set to the required input impedance, because its value and the required gain will determine that it may be non -best RF from the perspective of frequency response. The total input impedance of the power supply becomes a parallel combination of RG and RM.
The second main consideration is mentioned in the previous paragraph that the signal source impedance becomes part of the noise gain equation, and it will have a slight impact on the bandwidth through equation 1. The values u200bu200bshown in Figure 9 Re -optimize the bandwidth of the noise gain (NG u003d 2.74) in Figure 9 to explain this. In the example in FIG. 9, the RM value and the external 50 source impedance parallel combination to generate 50 | | 68 The impedance is connected in series with RG to calculate the noise gain, that is, ng u003d 2.74. Insert the value of the Infantance 3 in the reverse input impedance of the RF and 45 in FIG.
Note that in this double -pole power supply inverter application, non -inverter input is directlyGround. It is generally recommended to add a resistor at the non -inverter input terminal to achieve the bias current error elimination of the output end. The input bias current of the current feedback computing amplifier is usually not matched in terms of amplitude or polarity. In Figure 9 circuit, connecting a resistor to the non -counter -phase input terminal of OPA2681 will actually provide additional gain for the bias current and noise current of the input end, but it will not reduce the output DC error because the input is partial The current is not matched.
Output current and voltage
The output voltage and current capacity provided by OPA2681 are unparalleled low -cost dual single -piece computing amplifiers. Under the air load conditions at 25 ° C, the output voltage is usually closer to 1V than the voltage fluctuation of any one of the power rails; the guaranteed swing is limited to the 1.2V range of any one power rail. In 15 load (minimum test load), the output is guaranteed to exceed ± 135mA.
Although the above specifications are familiar with the industry, voltage and current restrictions are considered. In many applications, it is a voltage X current, or V-I product, which is more related to the circuit operation. Refer to the ""output voltage and current limit"" diagram in the typical performance curve. The X and Y axis of this figure show the zero voltage output current limit and zero current output voltage limit, respectively. These four quadrants give more detailed views of OPA2681 output drive capabilities. It is noticed that the chart is bounded by the ""security operation area"" with the largest internal power consumption of 1W (in this case, only one channel). Overlapped the resistance lines to the figure show that OPA2681 can drive ± 2.5V to 25 or ± 3.5V to 50 100 load line (standard test circuit load) display completely ± 3.9V output swing capacity, as shown in typical technical specifications.
The minimum output voltage and current excess temperature are simulated at the worst situation at the worst case. Only when the cold starts, the output current and voltage will be reduced to the value shown in the guarantee table. When the output transistor provides power, their knot temperature will increase, reducing their VBE (increasing the available output voltage swing) and increasing current gain (increased output current). In the steady -state operation, because the output level temperature is higher than the lowest working environment temperature, the output voltage and current can always be greater than the value shown in the ultra -temperature specification.
In order to maintain the maximum output level linearity, it does not provide short -circuit protection. This is usually not a problem, because most applications include a series matching resistor at the output end. If the output end of the resistor is short -circuited, it will limit the internal power consumption. However, in most cases, the output pin is directly connected to the adjacent positive power supply foot (8 pins packaging) will damage the amplifier. If you need additional short -circuit protection, consider the small string wound resistor in the power cord. This will be reduced under the overput loadAvailable output voltage swing. The 5 series resistors in each power cord will limit the internal power consumption of the short circuit when the output is less than 1W. At the same time, it only reduces the available output voltage swing of 0.5V, which is suitable for the ideal load current of up to 100mA. Always put the 0.1UF power supply container directly behind these power supply restricted flow resistors on the power pins.
Drive capacitance load
For the operational amplifier, the most demanding and most common load conditions are the capacitor load. Generally, the capacitance load is an input terminal of the A/D converter, including additional external capacitors, which may be recommended to improve A/D linearity. When the capacitance load is directly applied to the output pin, a high -speed and high -open -ring gain amplifier like OPA2681 is easily affected by the decrease in stability and the peak of the closed -loop response. When considering the opening resistance of the amplifier, this capacitance load will add a pole to the signal pathway to reduce the phase margin. Some people have proposed several external solutions to solve this problem. When the main consideration of frequency response flat, pulse response and/or distortion, the simplest and most effective solution is to insert a series of isolation resistance between the amplifier output and the capacitance load. Feedback circuit isolation. This does not eliminate the pole from the ring response, but shift it and add zero at a higher frequency. The effect of additional zero is to eliminate the phase lag of the container characteristics, thereby increasing the phase margin and improving the stability.
The typical performance curve shows the recommended RS and the frequency response generated by the recommended RS and the capacitance load and the frequency generated under the load. Parasitic capacitance load greater than 2PF will begin to reduce the performance of OPA2681. The long PC board trajectory, unsatisfactory cables, and connection with multiple devices can easily lead to exceeding this value. Always consider this impact carefully, and use as close as possible to the OPA2681 output pins to add a recommended series resistor (see the circuit board layout guide).
distortion performance
OPA2681 provides good distortion performance at 100 #8486 at ± 5V power supply. Compared with other solutions, it provides excellent performance on a lighter load and/or on a single+5V power supply. Generally speaking, the second harmonic will dominate the distortion before the base wave signal reaches a very high frequency or power level, and the three harmonic components can be ignored. Then focus on the second harmonic to increase the load impedance directly to improve the distortion. Keep in mind that the total load includes the feedback network-in non-counter-phase configuration (Figure 1), this is the sum of RF+RG, and in the reverse configuration, this is just RF. In addition, additional power supply -coupled capacitors (0.1 μF) (for bipolar operations) are provided between the power pins, which slightly improves the second -order distortion (3 to 6dB).
In most operational amplifiers, increasing the output voltage swing will directly increase the harmonic distortion. Typical performance curve display, 2 harmonicsThe growth rate is slightly lower than the expected 2x rate, while the growth rate of the three harmonics is slightly lower than the expected 3x rate. When the test power is doubled, the difference between the difference between it and the second harmonic is less than the expected 6DB, and the d