8.3.1 Feedback Resistor Selection

One of the key benefits of a current feedback operational amplifier is the ability to maintain optimum frequency response independent of gain by using appropriate values for the feedback resistor (R_F). The Electrical Characteristics and Typical Characteristics plots specify an R_F of 560 Ω (390 Ω for the SOIC package), a gain of 2 V/V, and ±5-V power supplies (unless otherwise specified). Generally, lowering R_F from it’s recommended value will peak the frequency response and extend the bandwidth while increasing the value of R_F will cause the frequency response to roll off faster. Reducing the value of R_F too far below it’s recommended value will cause overshoot, ringing and, eventually, oscillation.

Figure 27. Recommended R_F vs. Gain

Since a current feedback amplifier is dependant on the value of R_F to provide frequency compensation and since the value of R_F can be used to optimize the frequency response, different packages use different R_F values. As shown in Figure 27, the SOT-23-6 and the SOIC package use different values for the feedback resistor, R_F. Since each application is slightly different, it is worth some experimentation to find the optimal R_F for a given circuit. In general, a value of R_F that produces ≈0.1 dB of peaking is the best compromise between stability and maximum bandwidth. Note that it is not possible to use a current feedback amplifier with the output shorted directly to the inverting input. The buffer configuration of the LMH6703 requires a 560 Ω (390 Ω for SOIC package) feedback resistor for stable operation.

The LMH6703 was optimized for high speed operation. As shown in Figure 27, the suggested value for R_F decreases for higher gains. Due to the output impedance of the input buffer, there is a practical limit for how small R_F can go, based on the lowest practical value of R_G. This limitation applies to both inverting and non inverting configurations. For the LMH6703 the input resistance of the inverting input is approximately 30Ω and 20Ω is a practical (but not hard and fast) lower limit for R_G. The LMH6703 begins to operate in a gain bandwidth limited fashion in the region when R_G is nearly equal to the input buffer impedance. Note that the amplifier will operate with R_G values well below 20 Ω, however results may be substantially different than predicted from ideal models. In particular the voltage potential between the Inverting and Non-Inverting inputs cannot be expected to remain small.

Inverting gain applications that require impedance matched inputs may limit gain flexibility somewhat (especially if maximum bandwidth is required). The impedance seen by the source is R_G || R_T (R_T is optional). The value of R_G is R_F /Gain. Thus for a SOT-23 in a gain of —5V/V, an R_F of 460 Ω is optimum and R_G is 92 Ω. Without a termination resistor, R_T, the input impedance would equal R_G, 92 Ω. Using an R_T of 109Ω will set the input resistance to match a 50-Ω source. Note that source impedances greater then R_G cannot be matched in the inverting configuration.

For more information see Application Note OA-13 (SNOA366) which describes the relationship between R_F and closed-loop frequency response for current feedback operational amplifiers. The value for the inverting input impedance for the LMH6703 is approximately 30 Ω. The LMH6703 is designed for optimum performance at gains of 1 to 10 V/V and −1 to −9 V/V. Higher gain configurations are still useful, however, the bandwidth will fall as gain is increased, much like a typical voltage feedback amplifier.

The LMH6703 data sheet shows both SOT-23-6 and SOIC data in the Electrical Characteristic section to aid in selecting the right package. The Typical Characteristics section shows SOT-23-6 package plots only.

8.3.2 DC Accuracy and Noise

Example below shows the output offset computation equation for the non-inverting configuration (see Figure 29) using the typical bias current and offset specifications for A_V = 2:

Output Offset : V_O = (I_BN × R_IN ± V_OS) (1 + R_F/R_G) ± I_BI × R_F

Where R_IN is the equivalent input impedance on the non-inverting input.

Example computation for A_V = 2, R_F = 560 Ω, R_IN = 25 Ω:

V_O = (7 μA × 25 Ω ± 1.5 mV) (1 + 560/560) ± 2 μA × 560≈ −3.7 mV to 4.5 mV

A good design, however, should include a worst case calculation using Min/Max numbers in the data sheet tables, in order to ensure "worst case" operation.

Further improvement in the output offset voltage and drift is possible using the composite amplifiers described in Application Note OA-07 (SNOA365). The two input bias currents are physically unrelated in both magnitude and polarity for the current feedback topology. It is not possible, therefore, to cancel their effects by matching the source impedance for the two inputs (as is commonly done for matched input bias current devices).

The total output noise is computed in a similar fashion to the output offset voltage. Using the input noise voltage and the two input noise currents, the output noise is developed through the same gain equations for each term but combined as the square root of the sum of squared contributing elements. See Application Note OA-12 (SNOA375) for a full discussion of noise calculations for current feedback amplifiers.

8.3.3 Enable/Disable

Figure 28. SD Pin Simplified Schematic
(SOT-23 Pinout Shown)

For 5-V supplies only, the LMH6703 has a TTL logic compatible disable function. Apply a logic low (< 0.8 V) to the SD pin and the LMH6703 is disabled. Apply a logic high (> 2.0 V), or let the pin float and the LMH6703 is enabled. Voltage, not current, at the Shutdown pin (SD) determines the enable/disable state. Care must be exercised to prevent the shutdown pin voltage from going more than 0.8 V below the midpoint of the supply voltages (0V with split supplies, V⁺/2 with single supply biasing). Doing so could cause transistor Q1 to Zener resulting in damage to the disable circuit (See Figure 28). The core amplifier is unaffected by this, but the shutdown operation could become permanently slower as a result.

Disabled, the LMH6703 inputs and output become high impedances. While disabled the LMH6703 quiescent current is approximately 200 µA. Because of the pull up resistor on the shutdown circuit, the I_CC and I_EE currents (positive and negative supply currents respectively) are not balanced in the disabled state. The positive supply current (I_CC) is approximately 300 µA while the negative supply current (I_EE) is only 200 µA. The remaining I_EE current of 100 µA flows through the shutdown pin.

The disable function can be used to create analog switches or multiplexers. Implement a single analog switch with one LMH6703 positioned between an input and output. Create an analog multiplexer with several LMH6703s and tie the outputs together.