feature

Low cost single ( AD8055 ) and dual (AD8056); easy-to-use voltage feedback structure; high speed; 300 MHz, -3 dB bandwidth (G=+1); 1400 V/s slew rate; 20 ns sink to 0.1%; Low distortion: –72 dBc@10 MHz; low noise: 6 nV/√Hz; low DC error: 5 mV max VOS, 1.2 A max IB; small package; SOT-23-5 available for AD8055; 8-lead particle available for AD8056 Body; Excellent Video Specs (RL=150, G=+2); Gain Flatness 0.1dB to 40MHz; 0.01% Differential Gain Error; 0.02 Differential Phase Error; Drives Four Video Loads at 0.02% Speed (37.5) and 0.1 differential gain and differential phase; low power consumption, 5 V supply; 5mA typical/amplifier supply current; high output drive current: over 60mA.

application

Imaging; photodiode preamplifiers; video line drivers; differential line drivers; professional cameras; video switches; special effects; A-to-D drivers; active filters.

Product Description

The AD8055 (single) and AD8056 (dual) voltage feedback amplifiers offer the bandwidth and slew rate typically found in current feedback amplifiers. Furthermore, these amplifiers are easy to use and provide at very low cost.

Despite their low cost, the AD8055 and AD8056 provide excellent overall performance. For video applications, its differential gain and phase errors are 0.01% and 0.02°, and 0.02% and 0.1°, respectively, into a 150Ω load, driving four video loads (37.5Ω) simultaneously. Their 0.1 dB flat output to 40 MHz and broadband output to 300 MHz, as well as 1400 V/µs slew rate and 20 ns settling time make them useful for a variety of high-speed applications.

The AD8055 and AD8056 require only 5 mA/amplifier supply current and operate from dual ±5 V or single +12 V supplies, while being able to deliver over 60 mA of load current. All of these are available in a small 8-lead plastic dip, 8-lead SOIC package, 5-lead SOT-23-5 package (AD8055) and 8-lead particulate package (AD8056). These features make the AD8055/AD8056 ideal for portable and battery-powered applications where size and power are critical. These amplifiers are available over an industrial temperature range of -40°C to +85°C.

Maximum power consumption

The maximum power that can be safely dissipated by the AD805/AD8056 is limited by the associated rise in junction temperature. The maximum safe junction temperature for plastic packaged devices is determined by the glass transition temperature.

In plastics, above approximately 150°C, temporarily exceeding this limit may result in changes in parametric performance due to changes in the stress the package imposes on the mold. Junction temperatures exceeding +175°C for extended periods of time can cause device failure.

Although the AD805/AD8056 are internally short-circuit protected, this may not be sufficient to guarantee that the maximum junction temperature (±150°C) will not be exceeded under all conditions. To ensure correct operation, it is necessary to observe the maximum power derating curve.

Application Quad Video Driver

The AD8055 is a useful low-cost circuit that can drive up to four video lines. For this application, as shown in Figure 33, the amplifier is configured with a non-vertical gain of 2. The input video source is terminated in 75Ω for high impedance non-converting inputs.

Each output cable is connected to the op amp output through a 75Ω series rear termination resistor for proper cable termination. Termination resistors at the other end of the line divide the output signal by 2, which is then compensated by the gain of the two op amp stages.

For a single load, the circuit has a differential gain error of 0.01% and a differential phase error of 0.02 degrees. The two load measurements were 0.02% and 0.03 degrees, respectively. For four loads, the differential gain error is 0.02%, while the differential phase increases to 0.1 degrees.

Single-Ended to Differential Line Driver

Driving balanced twisted pair cables, differential input A/D converters, and other applications that require differential signaling require generating differential signals from single-ended signals. This is sometimes achieved by using inverting and non-inverting amplifier stages to generate complementary signals.

The circuit shown in Figure 34 shows how the AD8056 can be used to make a single-ended-to-differential converter that offers some advantages over the above architecture. Each op amp is configured for unity gain by a feedback resistor from the output to the inverting input. Additionally, each output drives an opposing op-amp with a gain of -1 through a crossover resistor. The result is that the outputs are complementary and have high gain in the overall configuration.

Feedback techniques similar to traditional op amps are used to control the gain of the circuit. From the non-converted input of Amplifier 1 to the output of Amplifier 2, there is an inverse gain. Between these points, a feedback resistor can be used to close the loop. As is the case with a traditional op amp inverting gain stage, adding an input resistor changes the gain.

The gain of this circuit from the input to the output of amplifier 1 is RF/RI, while the gain of the output of amplifier 2 is –RF/RI. Therefore, the circuit creates a balanced differential output signal from a single-ended input. The advantage of this circuit is that the gain can be changed by changing a single resistor and still maintain a balanced differential output.

Low noise, low power preamplifier

The AD8055 is a low cost, low noise, low power preamplifier. The gain of the 10 preamps can be achieved with a feedback resistor of 909 ohms and a gain resistor of 100 ohms, as shown in Figure 35. The circuit has a -3dB bandwidth of 20MHz.

At low source resistances (<100Ω), the main contributors to the input-referred noise of this circuit are the input voltage noise of the amplifier and the noise of the 100Ω resistor. 6 nV/√ and 1.2 nV/√, respectively.

These values yield a total input-referred noise of 6.1 nV/√. hertz

Power consumption limit

With a 10V supply (total VCC–VEE), the AD8055 in the SOT-23-5 package has a quiescent power of 65mW, while the AD8056 in Microsoft has a quiescent power of 120mW. This means that the temperature of the SOT-23-5 package is 15.6°C above ambient, while the temperature of the Microsoft package is 24°C above ambient.

The power dissipated under heavy load conditions is roughly equal to the supply voltage minus the output voltage, times the load current, plus the quiescent power described above. Then, multiplying the total power dissipation by the thermal resistance of the package gives the temperature rise of the part above ambient temperature. Joint temperature should be kept below 150°C.

The AD8055 in the SOT-23-5 package can dissipate 270 mW, while the AD8056 in the M80SOIC package can dissipate 325 mW (at 85°C ambient) without exceeding the maximum mold temperature. In the case of the AD8056, this is greater than 1.5 V rms into 50Ω, enough to accommodate a 4 V pp sine wave signal at both outputs simultaneously. However, since each output of the AD8055 or AD8056 can provide up to 110 mA into a short circuit, a continuous short circuit condition will exceed the maximum safe junction temperature.

Resistor selection

The following table serves as a guide for resistor selection to maintain gain flatness versus frequency at various gain values.

Driving capacitive loads

When driving capacitive loads, most op amps will peak in frequency response before the frequency decays. Figure 36 shows the response of the AD8056 with a gain of +2 and a load of 100Ω, shunted by different capacitor values. It can be seen that under these conditions, the part is stable up to 30pF capacitive load.

In general, to minimize peaking or ensure stability for larger capacitive load values, a small series resistor, RS, can be added between the op amp output and capacitor CL. For the setup shown in Figure 37, the relationship between RS and CL was derived empirically, as shown in Figure 38. Selecting RS produces less than 1db peaks in the frequency response. It is also important to note that after a sharp rise, RS settles around 25Ω very quickly.

Dimensions

Dimensions are in inches and (mm).