2

Application Information

HS-1212RH Advantages

The HS-1212RH features a novel design which allows the

user to select from three closed loop gains, without any

external components. The result is a more flexible product,

fewer part types in inventory, and more efficient use of board

space. Implementing a dual, gain of 2, cable driver with this

IC eliminates the four gain setting resistors, which frees up

board space for termination resistors.

Like most newer high performance amplifiers, the

HS-1212RH is a current feedback amplifier (CFA). CFAs

offer high bandwidth and slew rate at low supply currents,

but can be difficult to use because of their sensitivity to

feedback capacitance and parasitics on the inverting input

(summing node). The HS-1212RH eliminates these

concerns by bringing the gain setting resistors on-chip. This

yields the optimum placement and value of the feedback

resistor, while minimizing feedback and summing node

parasitics. Because there is no access to the summing node,

the PCB parasitics do not impact performance at gains of +2

or -1 (see “Unity Gain Considerations” for discussion of

parasitic impact on unity gain performance).

The HS-1212RH’s closed loop gain implementation provides

better gain accuracy, lower offset and output impedance,

and better distortion compared with open loop buffers.

Closed Loop Gain Selection

This “buffer” operates in closed loop gains of -1, +1, or +2, with

gain selection accomplished via connections to the inputs.

Applying the input signal to +IN and floating -IN selects a gain

of +1 (see next section for layout caveats), while grounding -IN

selects a gain of +2. A gain of -1 is obtained by applying the

input signal to -IN with +IN grounded through a 50

Ω resistor.

The table below summarizes these connections:

Unity Gain Considerations

Unity gain selection is accomplished by floating the -Input of

the HS-1212RH. Anything that tends to short the -Input to

GND, such as stray capacitance at high frequencies, will

cause the amplifier gain to increase toward a gain of +2. The

result is excessive high frequency peaking, and possible

instability. Even the minimal amount of capacitance

associated with attaching the -Input lead to the PCB results

in approximately 6dB of gain peaking. At a minimum this

requires due care to ensure the minimum capacitance at the

-Input connection.

Table 1 lists five alternate methods for configuring the

HS-1212RH as a unity gain buffer, and the corresponding

performance. The implementations vary in complexity and

involve performance trade-offs. The easiest approach to

implement is simply shorting the two input pins together,

and applying the input signal to this common node. The

amplifier bandwidth decreases from 430MHz to 280MHz,

but excellent gain flatness is the benefit. A drawback to this

approach is that the amplifier input noise voltage and input

offset voltage terms see a gain of +2, resulting in higher

noise and output offset voltages. Alternately, a 100pF

capacitor between the inputs shorts them only at high

frequencies, which prevents the increased output offset

voltage but delivers less gain flatness.

Another straightforward approach is to add a 620

Ω resistor

in series with the amplifier’s positive input. This resistor and

the HS-1212RH input capacitance form a low pass filter

which rolls off the signal bandwidth before gain peaking

occurs. This configuration was employed to obtain the data

sheet AC and transient parameters for a gain of +1.

Pulse Overshoot

The HS-1212RH utilizes a quasi-complementary output stage

to achieve high output current while minimizing quiescent

supply current. In this approach, a composite device replaces

the traditional PNP pulldown transistor. The composite device

switches modes after crossing 0V, resulting in added distortion

for signals swinging below ground, and an increased overshoot

on the negative portion of the output waveform (see Figure 6,

Figure 9, and Figure 12). This overshoot isn’t present for small

bipolar signals (see Figure 4, Figure 7, and Figure 10) or large

positive signals (see Figure 5, Figure 8 and Figure 11).

PC Board Layout

This amplifier’s frequency response depends greatly on the

care taken in designing the PC board (PCB). The use of low

inductance components such as chip resistors and chip

capacitors is strongly recommended, while a solid

ground plane is a must!

Attention should be given to decoupling the power supplies.

A large value (10

µF) tantalum in parallel with a small value

(0.1

µF) chip capacitor works well in most cases.

GAIN

CONNECTIONS

+INPUT

-INPUT

-1

50

Input

+1

Input

NC (Floating)

+2

Input

GND

HS-1212RH