Features

● Broadband+5V work: 225MHz (g u003d+2)

● Unit to stable: 280MHz (g u003d 1)

● High -output current : 150mA

● Output voltage swing: ± 4.0V

● High conversion rate: 2100V/μs

● Low dg/dlue: .001%/. 01 °

● Low power supply current: 6MA

● Low -loss current: 320 μA

Application

● XDSL line drive

● Broadband Video buffer

● High -speed imaging channel

● Portable instrument

● ADC buffer

● Active filter

● Broadband inverter and

● High SFDR medium frequency amplifier

Instructions

OPA681 set a new performance level for the broadband current feedback amplifier. Running at a very low 6MA power supply, OPA681 provides a conversion rate and output power, which is usually related to higher power supply. A new output -level structure provides a low -output voltage and cross distortion of the minimum output current. This provides a special single supply operation. Using a single+5V power supply, OPA681 can provide 1V to 4V output width, the driving current exceeds 100mA, and the bandwidth is 150MHz. This function combination makes OPA681 an ideal RGB line drive or single -power ADC input drive.

The low 6MA power current of OPA681 is accurately adjusted at 25 ° C. This fine -tuning and low -temperature drift ensures the maximum power supply current than the lower competitive products. Can further reduce the system power by using optional disable control pins. 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 OPA681 will drop below 320 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.

OPA681 Related Products

200MHz radio frequency Seeking and amplifier

Typical performance curve: vs u003d ± 5V

g u003d+2, RF u003d 402 , RL u003d 100 , unless there is another instructions (see Figure 1).

g u003d+2, RF u003d 499 , RL u003d 100 to+2.5V, unless there is another instructions (see Figure 2).

Application information Broadband current feedback operation OPA681 provides broadband current feedback operations The excellent communication performance of the amplifier has a highly linear and high -power output level. Only the static current of 6MA is required. OPA681 will swing to the 1V range of any power rail, and guarantee a 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. OPA681 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. OPA681 realizes a considerable power gain and better linearity. 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 communication performances under low gain, in the case of improving DC accuracy, considering high conversion rates, stable unit gain, and voltage feedback OPA680.

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 the control line (DIS) usually keeps opening 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, OPA681 requires the minimum input and output voltage balance. It will provide a 3VP-P output swing single+5V bandwidth greater than 150MHz. The key requirement for the operation of the broadband single power is to keep the input and output signal swing in the available voltage range of input and output. Figure 2 circuit to makeCreate an input midpoint bias with a simple resistor division of the+5V power supply (two 806 resistors). 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 80mA. A load that requires a 100Ω in this characteristic circuit to a midpoint bias. The new output stage used in OPA681 can pass the large bipolar output current into this mid -point load with the smallest cross distortion, such as+5V power supply, three harmonic distortion diagrams.

Single power A/D converter interface

Most modern high -performance A/D converters (such as the ADS8XX of Texas Instruments Company And ADS9XX series) work on a+5V (or lower) power supply. For a single power supply amplifier, it is a considerable challenge to provide low distortion input signals at the ADC input terminal with a signal frequency of more than 5MHz. OPA681's high conversion rate, abnormal output swing, and high lineivity make it an ideal single -power ADC drive. Figure 3 shows an input interface example of a very high performance 10 -bit 60MSPS CMOS converter. The OPA681 in FIG. 3 provides gt; 180MHz bandwidth, the signal gain is +4, and the output swing is 2VP-P. One of the main advantages of the current feedback internal structure used in OPA681 is that as the signal gain increases, it can maintain high bandwidth. By dividing the top and bottom of the internal ADC reference trapezoidal diagram, the non -inverted input bias voltage refer to the midpoint of the ADC signal range. In the case of an exchanges (RG) AC coupling, the bias to the output gain is +1, and at the same time, it also makes the output voltage swing center. The performance of the 20MHz analog input frequency and the 60MSPS clock frequency gives the performance of gt; 58dbc sfdr.

Broadband inverter and amplifier

Because the signal bandwidth of the current feedback computing amplifier can be controlled by noise gain (NG, which is usually the same as the non -counter -phase signal gain), it can be used by OPA681 to achieve a very broad -strange anti -phase demand and grade. The circuit on the homepage of this data table shows an example of reverse and amplifier, where the resistance value has been adjusted to maintain the maximum bandwidth and input impedance matching. If each RF signal is driven by 50 source, the NG of the circuit will be (1+100 /(100 5) u003d 6. Total feedback impedance (from VO to inverter error current) is the sum of RF+(RI X NG). Among them, RI is the impedance of the input of inverter input (see the resistance value to optimize the performance part). Use 100 feedback (the signal gain from each input to the output pin is -2) requires an additional 20 with the inverter input series to increase the feedback impedance. When adding this resistor to a typical internal RI u003d 41 total feedback impedance is 100 +(65 x 6) u003d 490 The value required for frequency response. The performance of the test shows that under the matching 50 load, through 100MHz, the small signal bandwidth exceeds 200MHz, and the 15DBM – 1DBM is compressed.

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 use OPA681 to implement, 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. The disabled features of OPA681's ""first -through and then disconnection"" ensure that there is always an amplifier control line when using the wired or circuit shown in Figure 4. 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 input usually less than ± 50mv. When the two outputs are switched (as shown in Figure 6), the output cable is always controlled by one or the other amplifier due to the disabled time of ""first pass through"". In this case, two 0V lossesThe switch in the entry and switch is reduced to lt; 20mv.

Single power supply mid -frequency placing large -bandwidth provided by OPA681, at the same time, it runs a single+5V power supply itself is suitable for the application of the medium frequency amplifier. An advantage of using an operational amplifier like OPA681 as a medium -frequency amplifier is that compared with static power consumption, accurate signal gain and lower third -order interoperability can be achieved. In addition, OPA681 provides a very small package in the SOT23-6 package with a power shutdown function and is suitable for portable applications. A problem using the operation amplifier in the medium -frequency amplifier is that their noise coefficients are relatively high. Sometimes people recommend that the best source resistance can be used to minimize the noise coefficient of the amplifier. Adding a resistor to achieve this optimal value can increase the noise coefficient, but it will actually reduce the signal -to -noise ratio. A more effective way is to introduce the signal through the input transformer. Figure 5 shows a particularly useful example of OPA681.

Several advantages of OPA681 through the voltage transformer into the inverter input gain resistor. First of all, the decoupling capacitor of the non -inverted input end eliminates the contribution of non -inverter input current noise to output noise. Secondly, if the input noise voltage of the operation amplifier is reflected to the input side of the RG, it is actually attenuated. The voltage transformer with 1: 2 (number ratio ratio) will be used as a 50Ω source impedance from the first side to the secondary side as 200 source impedance (200 RG resistance through the transformer first as 50 input matching impedance impedance impedance reflection). The noise gain (NG) output of the amplifier is 1+600/400 u003d 2.5V/V. The 2.2NV/√Hz input voltage noise of the computing amplifier is multiplied by the output noise gain, and then the noise item is reflected to the input side of the RG resistor to divide it with 3. When the input point of reflection to the computing amplifier circuit, the net increase of the voltage noise of non -inverse input voltage is 0.833. When returning to the transformer at one side, this will be further reduced.

FIG. 5 The relatively low -gain medium -frequency placing large -frequency large -device circuit gives a 12DB noise coefficient at the input terminal of the transformer. Add RF resistance to 600 (once the RG is set to 200 for input impedance matching), it will slightly reduce the bandwidth. The measurement results show that the small signal bandwidth of the circuit 5 circuit is 150MHz, which is very high through 30MHz. Although OPA681 has not showed interception characteristics in terms of 2 tone and 3 -order interoperability distortion, it maintains a very high non -mixed dynamic range through high output power and high frequency. In Figure 5, the maximum single-sound power of the single power circuit matching the load is 1DBM (this requires 2.8VP-P swing at the output pins of OPA681). The 2 -tone SFDR measured by the circuit at this maximum load power is super at the frequency of 30MHz.Pass 55DBC.

Design Tool

Demonstration board

Under the three encapsulation styles of OPA681, several PC boards can be used for preliminary assessment of the performance of auxiliary circuits. All of these are free, as an unpopular personal computer board with explanation file. The following table shows the summary information of these boards.

Please contact the TI application to support the hotline to request any these boards.

Macro model and application support

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. The SPICE model of OPA681 can be obtained through the TI website or a model on disk (1-800-548-6132) on the D disk of TI application departments. 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. The effects of their prediction of harmonic distortion or DG/Dφ are not ideal. These models do not try to distinguish the encapsulation type in its small signal communication performance.

Operation suggestion

Set the resistance value to optimize the bandwidth

current feedback computing amplifier like OPA681 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 6 shows the analysis circuit of the small signal frequency of OPA681.

The key element of this current feedback of the op amp modeling is:

α → from non -inverse input to reverse input buffer gain

Ri → buffer output impedance

IERR → feedback error current signal

z (s) → The frequency of the frequency from IERR to VO Related open -loop cross -resistance gain

The buffer gain is usually very close to 1.00, and it is usually ignored from the signal gain consideration. However, it will set CMRR for a single transport difference amplifier configuration. For buffer gain α 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. OPA681 is usually about 41 .

current feedback operation releaseThe error current in the large sensor sensor inverter node (contrary to the difference in input error voltage of the voltage feedback computing amplifier), and passes it 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 Figure 6 Circuit obtains the equations 1:

This is written in a circuit gain analysis format. Among them, the error caused by non -infinitely open loop gain is expressed by denominator. 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 you draw 20X pairs (RF+NGX RI) on the top of the ring -on -rire cross -diagram, the difference between the two is the difference between the two is the difference between the two is The ring 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 -loop 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).

OPA681 After internal compensation, the frequency response of RF u003d 402 at the NG u003d 2 of the ± 5V power supply is maximum flat. Calculating Formula 2's denominator (that is, the feedback) has the best goal of 484 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 7 shows the recommendation RF and NG of ± 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 7 provides a good starting point for the design that needs to be optimized by bandwidth.

The total impedance of entering the inverter input can be used to adjust the closed -loop signal bandwidth. 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. This bandwidth control method is used for the inverter of the homepageCircuit. OPA681's internal buffer output impedance is less affected by the source impedance from non -counter -phase input terminals. High source resistance will increase RI and reduce bandwidth. For those single -power applications that generate mid -point bias at the non -counter -phase input at the non -counter -phase input at a high -value resistance, the off -coupled container to the power supply noise inhibition, non -inverse input noise current diversion, and minimum high frequency value in RI 6 important.

Reverse amplifier operation

Since OPA681 is a general broadband current feedback amplifier, most common operational amplifiers application circuits are available for designers. Because the feedback resistance is the compensation element of the current feedback computing amplifier, the application of feedback elements (such as integration device, cross resistance, and some filters) requires considerable flexibility for unit gain stable voltage feedback OPA680. Broadband inverter operation (especially harmony) is particularly suitable for OPA681. Figure 8 shows a typical inverter configuration. The input/output impedance and signal gain in FIG. 1 retains 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 FIG. 8 By slightly reduced the RF (Figure 1) to re -optimize the bandwidth of Figure 8 (ng u003d 2.74) to explain this. In the example of FIG. 8, the RM value and the external 50 source impedance parallel combination, generate 50 | | 68 u003d 28.8 The impedance is connected in series with RG to calculate the noise gain, that is, ng u003d 2.74. The reverse input impedance of the RF and 41 in Figure 8 Insert the Integrated Equation 3 in the reverse input impedance of the RF and 41 to obtain the feedback of the best value of 484 the best value.

Note that in this double -pole power supply inverter application, non -inverter input is directly grounded. 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 8, connect the resistor to the non -inverse input terminal of OPA681, which will actually be the bias current and the bias current and the input terminal and the input terminalThe noise current provides additional gain, but because the input bias current does not match, the output DC error will not be reduced.

Output current and voltage

OPA681's output voltage and current capacity are unparalleled low -cost single -chip operations 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 a more detailed view of the OPA681 output drive capability, and pointed out that the figure is bound to the ""security operation area"" with the largest internal power consumption of 1W. Overlapped the resistance lines to the figure show that OPA681 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 stage temperature will be 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 reduce the available output voltage swing under the overput load. The 5 series resistors in each power cord will limit the internal power consumption when the output is short -circuited than 1W, and at the same time, the available output voltage swing is reduced to 0.5V to reach the required load current of 100mA. Always place the 0.1 μF power supply container directly behind these power supply restricted flow resistors on the power pins.

DrivePower capacitance load

For the computing amplifier, one of the most demanding and most common load conditions is 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 foot pin pin, high -speed and high -open -rings gain amplifiers like OPA681 are easily affected by decreased stability and 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 OPA681. It is easy to cause the ultra -long connection of multiple device circuit boards, resulting in the ultra -long line board. Always consider this impact carefully, and close the recommended series resistor (see the circuit board layout guide) as possible to the OPA681 output pins.

distortion performance

OPA681 provides good distortion performance under the 100Ω load of ± 5V power. Compared with other solutions, it provides excellent performance on a lighter load and/or on a single+5V power supply. The second harmonic or third harmonic usually dominate, until the third harmonic signal reaches the level that can be ignored. Then focus on the second harmonic to increase the load impedance directly to improve the distortion. Remember, in Figure 1, RF is the total load in the reverse configuration. In addition, additional power supply -coupled capacitors (0.1 μF) (for bipolar operation) are provided between power pins (3dB to 6DB).

In most operational amplifiers, increasing the output voltage swing will directly increase the harmonic distortion. The typical performance curve shows that the growth rate of two harmonics is slightly lower than the expected 2x rate, while the growth rate of 3 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 degree of reduction between the difference between it and the third harmonic is less than the expected 12DB. This also shows the 2 -tone, the 3 -order interoperability (IM3) response curve. The third -order mixture level is extremely low at the low output power level. Even when the basic power reaches a very high level, the output level still keeps it at a low level. The typical performance curve indicates th