Use current-feedback amplifiers to speed up data-acquisition and high-end video systems

Many high-speed applications require driving a circuit element that is capacitive in nature. In order to gain bandwidth, capacitive loads are often driven by a buffer; however, most high-speed buffers do not tolerate capacitive loads connected directly to their outputs. In this article we will see how data-acquisition systems and high-performance video systems can benefit from the use of a current-feedback amplifier.

Current-feedback amplifiers (CFB) offer a number of unique characteristics. One of the most exploited attributes is their ability to drive large signals at high gains without the limitations of the gain bandwidth (GBW) product. This ability to drive large signals at high speeds and high gain can help to overcome bandwidth loss that can occur when trying to accommodate a capacitive load. This article will discuss two different applications where this technique can be used, namely data acquisition and a high-end video-crosspoint switch.

Data acquisition
Capturing analog signals is the key function of digital oscilloscopes, communications systems, and video capture. In nearly every case, there is a need to condition the signal with amplifiers before capturing it with an analog-to-digital converter (ADC).

Driving ADCs is one of the leading applications for high speed amplifiers. This is a demanding application when high speed is desired. The input of many of the best ADCs is almost purely capacitive. This kind of load is difficult to drive with a high-speed amplifier.

One simple solution for driving a capacitive load, shown in Figure 1a , is to use a series resistor between the amplifier and the ADC. This solution is both simple and effective; however, the RC time constant set up between the resistor and the ADC input sets an upper limit on speed, no matter how fast the driving amplifier is.

Figure 1a: Gain of one circuit with Rout isolation resistor.

(Click on image to enlarge)

One simple solution that is available with CFB amplifiers is to set up a “times two, divide by two” circuit between the amplifier and the ADC, Figure 1b .

Figure 1b: Example circuit using “times two, divide by two” circuit.

(Click on image to enlarge)

The most basic version is when the op amp gain is originally unity gain as shown in Figure 1a. The modified circuit is shown in Figure 1b. The technique can be applied to circuits where the desired gain is two, as well.

In this case, the original circuit would be a gain of two and the modified circuit would be a gain of four. The configuration only requires that the original gain be increased by a factor of two, followed by a divide-by-two resistive divider between the amplifier and the capacitive load. The resistors Rout and RL in Figure 1b accomplish this divide-by-two function.

The increase in available bandwidth can be considerable, especially compared to a similar circuit with a voltage-feedback amplifier (VFB). As shown later. the VFB amplifier will lose bandwidth by a 2× factor when increasing gain, due to the gain bandwidth limitation.

Current-feedback amplifiers do not have this limitation. For a CFB amplifier, the higher gain does not decrease available bandwidth very much over the unity-gain case. This allows the amplifier to exploit the far lower RC time constant of the driving resistors. In Figure 1a, the RC time constant is (1/(2 × π × Rout × C)) = 235 MHz, whereas in Figure 1b the time constant is reduced to (1/(2 × π × (Rout || RL ) × C)) = 880 MHz.

A lower value of Rout makes the RC time constant formed by the ADC input and the Rout resistor higher in frequency. The divide-by-two portion of the circuit is adding a resistor in parallel with the capacitive load. This further decreases the RC time constant, by forming another discharge path for the capacitor charge.

There is a practical limit to the lower value of the Rout and RL , however, and this is set by the distortion levels that are required. Most amplifiers produce more harmonic distortion as the load current they must source and sink increases. For this reason, the two resistors should be made as low as necessary to get the desired bandwidth and no lower, unless distortion products are not an issue, for example, in very narrow-band applications.

One example of the potential bandwidth enhancement is shown in Figure 2 and Figure 3 . One interesting note is that the current feedback amplifier offers substantially flatter response in the gain-of-1 (unity gain) configuration. This is because with a CFB op amp, the loop gain can be controlled with the feedback resistor.

Figure 2: Gain-of-one response.
LMH6703: Rf = 402 Ω, Rout = 75 Ω, 3 dB BW = 610 MHz;
LMH6609: Rf = 0 &#937, Rout = 86.6 &#937, 3 dB BW = 380 MHz

(Click on image to enlarge)

Figure 3: Op amp gain = 2, net gain =1.
LMH6703: Rf = 412 Ω, Rout = 40.2, 3 dB BW = 760 MHz;
LMH6609: Rf = 301 Ω, Rout = 30.1 Ω, 3 dB BW = 245 MHz

(Click on image to enlarge)

(Note that in Figure 3, the CFB amplifier still beats the VFB amplifier as well as the predicted bandwidth as set by the Rout and Cin time constant. This is because the CFB amplifier can compensate for the capacitive loading by adjusting the feedback resistor value. See the LMH6703 datasheet and application notes OA-13 and OA-31 for more information on choosing a CFB feedback-resistor value.)

While the CFB circuit gains 150 MHz with the “times two divide by two” circuit, the VFB amplifier is actually faster at unity gain. Voltage-feedback amplifiers typically lose half of their bandwidth by going from unity gain to a gain of two, due to the gain bandwidth limitation. So, this circuit offers no advantage with VFB amplifiers.

Differential current feedback
The benefits of current feedback can be used for differential applications as well. The LMH6552 general-purpose CFB differential amplifier can be used in the “times two divide by two” configuration, as shown in Figure 4 .

Figure 4: Using a differential CFB amplifier in the “times two divide by two” configuration.

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A similar opportunity arises with differential op amps as shown in Figure 5 . In this case, the ADC input is resistive rather than capacitive; however, driving a matched impedance filter costs 6 dB of gain, and the filter will have some loss as well. Using a current-feedback amplifier allows this signal loss to be regained with minimal loss of bandwidth. An advantage of using an amplifier with externally set gain is that the filter loss can be easily recovered by adjusting the amplifier gain.

Figure 5: Differential ADC driver circuit:
termination and filter loss = 8 dB, amplifier gain = 8 dB.

(Click on image to enlarge)

As always, when trying to achieve high bandwidth, board layout is critical and will influence the optimal values for the feedback and termination resistors. Once a board is designed and built, it may require some experimentation to find the optimal component values.

One caution is in order, though, when using the “times two divide by two” circuit: the required signal swing is doubled. Using parts built on a relatively high supply-voltage process may be necessary. Many parts built on newer processes are limited to 5 V supplies and may not be able to provide the voltage swing required. High-voltage processes, such as National's VIP10 process, may provide an advantage in this case. The LMH6703 and LMH6552 are built on this process.

Crosspoint switch
Computer graphics have resolutions of more than 1600×1200 pixels and can have bandwidths over 300 MHz. Top-end video systems can distribute these high-resolution images to multiple monitors or projectors in classrooms or conference rooms. One key component in a video distribution system is a crosspoint switch. The LMH6584 or LMH6585 crosspoint switches, for example, have 32 inputs and 16 outputs and offer internal bandwidth of 400 MHz. The inputs have a maximum of 12 pF of input capacitance. Often two crosspoints are connected in parallel to gain more outputs.

Using two crosspoints gives a 32 x 32 matrix, however, the effective input capacitance is doubled when using a single chip. Using 75 Ω transmission lines to drive two chips will substantially reduce their bandwidth due to the RC time constant set up by the input capacitance and the termination and source resistors. The bandwidth limitation set by 24 pF of capacitance and the termination resistors is 175 MHz. Using an LMH6703 amplifier in the “times two divide by two” configurations restores the bandwidth lost by the capacitive loading, and allows the crosspoint to perform to its full potential.

Figure 6 shows an abbreviated schematic of the circuit. The 75 Ω input resistors preserve the 75 Ω environment expected in video systems, while the buffer allows the crosspoint inputs to see an effective source resistance of only 15 Ω.

Figure 6: Using a buffer to increase crosspoint-switch bandwidth.

(Click on image to enlarge)

Current-feedback amplifiers, by providing gain-independent bandwidth and high slew rates, and combined with low distortion products, can offer flexibility in circuits with capacitive loads and other circuits were flexible gain configurations are required. This enhanced flexibility may salvage a circuit that appears to be bandwidth limited by capacitive loading.

About the Author
Loren Siebert is an Applications Engineer with the High-Speed Signal Path Products Group at National Semiconductor Corporation. Prior to working at National Semiconductor, Loren spent 11 years working in the cellular communications industry as a radio systems engineer, a career that was sparked by his ham radio background (KI0DU).

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