7.2.1 Bias Current Cancellation

To cancel the bias current errors of the non-inverting configuration, the parallel combination of the gain setting (R_g) and feedback (R_f) resistors should equal the equivalent source resistance (R_seq) as defined in Figure 47. Combining this constraint with the non-inverting gain equation also seen in Figure 47, allows both R_f and R_g to be determined explicitly from the following equations:

When driven from a 0-Ω source, such as the output of an op amp, the non-inverting input of the LMH6624 and LMH6626 should be isolated with at least a 25-Ω series resistor.

As seen in Figure 48, bias current cancellation is accomplished for the inverting configuration by placing a resistor (R_b) on the non-inverting input equal in value to the resistance seen by the inverting input (R_f||(R_g+R_s)). R_b should to be no less than 25 Ω for optimum LMH6624 and LMH6626 performance. A shunt capacitor can minimize the additional noise of R_b.

Figure 47. Non-Inverting Amplifier Configuration

Figure 48. Inverting Amplifier Configuration

7.2.2 Total Input Noise vs. Source Resistance

To determine maximum signal-to-noise ratios from the LMH6624 and LMH6626, an understanding of the interaction between the amplifier’s intrinsic noise sources and the noise arising from its external resistors is necessary.

Figure 49 describes the noise model for the non-inverting amplifier configuration showing all noise sources. In addition to the intrinsic input voltage noise (e_n) and current noise (i_n = i_n⁺ = i_n⁻) source, there is also thermal voltage noise (e_t = √(4KTR)) associated with each of the external resistors. Equation 3 provides the general form for total equivalent input voltage noise density (e_ni). Equation 4 is a simplification of Equation 3 that assumes R_f||R_g = R_seq for bias current cancellation. Figure 50 illustrates the equivalent noise model using this assumption. Figure 51 is a plot of e_ni against equivalent source resistance (R_seq) with all of the contributing voltage noise sources of Equation 4. This plot gives the expected e_ni for a given (R_seq) which assumes R_f||R_g = R_seq for bias current cancellation. The total equivalent output voltage noise (e_no) is e_ni*A_V.

Figure 49. Non-Inverting Amplifier Noise Model

Equation 3.

Figure 50. Noise Model with R_f||R_g = R_seq

Equation 4.

As seen in Figure 51, e_ni is dominated by the intrinsic voltage noise (e_n) of the amplifier for equivalent source resistances below 26 Ω. Between 26 Ω and 3.1 kΩ, e_ni is dominated by the thermal noise (e_t = √(4kT(2R_seq)) of the equivalent source resistance R_seq. Above 3.1 kΩ, e_ni is dominated by the amplifier’s current noise (i_n = √2 i_nR_seq). When R_seq = 283 Ω (that is, R_seq = e_n/√2 i_n) the contribution from voltage noise and current noise of LMH6624 and LMH6626 is equal. For example, configured with a gain of +20V/V giving a −3 dB of 90 MHz and driven from R_seq = Rf || Rg = 25 Ω (e_ni = 1.3 nV√Hz from Figure 51), the LMH6624 produces a total output noise voltage (e_ni × 20 V/V × √(1.57 × 90 MHz)) of 309 μVrms.

Figure 51. Voltage Noise Density vs. Source Resistance

If bias current cancellation is not a requirement, then R_f || R_g need not equal R_seq. In this case, according to Equation 3, R_f || R_g should be as low as possible to minimize noise. Results similar to Equation 3 are obtained for the inverting configuration of Figure 48 if R_seq is replaced by R_b and R_g is replaced by R_g + R_s. With these substitutions, Equation 3 will yield an e_ni referred to the non-inverting input. Referring e_ni to the inverting input is easily accomplished by multiplying e_ni by the ratio of non-inverting to inverting gains.

7.2.3 Noise Figure

Noise Figure (NF) is a measure of the noise degradation caused by an amplifier.

Equation 5.

The Noise Figure formula is shown in Equation 5. The addition of a terminating resistor R_T, reduces the external thermal noise but increases the resulting NF. The NF is increased because R_T reduces the input signal amplitude thus reducing the input SNR.

Equation 6.

The noise figure is related to the equivalent source resistance (R_seq) and the parallel combination of R_f and R_g. To minimize "Noise Figure":

• Minimize R_f || R_g
• Choose the Optimum R_S (R_OPT)

R_OPT is the point at which the NF curve reaches a minimum and is approximated by:

Equation 7.

7.2.4 Low Noise Integrator

The LMH6624 and LMH6626 devices implement a deBoo integrator shown in Figure 52. Positive feedback maintains integration linearity. The low input offset voltage of the LMH6624 and LMH6626 devices and matched inputs allow bias current cancellation and provide for very precise integration. Keeping R_G and R_S low helps maintain dynamic stability.

Figure 52. Low Noise Integrator

7.2.5 High-gain Sallen-key Active Filters

The LMH6624 and LMH6626 devices are well suited for high gain Sallen-Key type of active filters. Figure 53 shows the 2^nd order Sallen-Key low pass filter topology. Using component predistortion methods discussed in Application Note OA-21, Component Pre-Distortion for Sallen Key Filters (SNOA369) will enable the proper selection of components for these high-frequency filters.

Figure 53. Sallen-Key Active Filter Topology

7.2.6 Low Noise Magnetic Media Equalizer

The LMH6624 and LMH6626 devices implement a high-performance low noise equalizer for such application as magnetic tape channels as shown in Figure 54. The circuit combines an integrator with a bandpass filter to produce the low noise equalization. The circuit’s simulated frequency response is illustrated in Figure 55.

Figure 54. Low Noise Magnetic Media Equalizer

Figure 55. Equalizer Frequency Response

7.3.1 Single Supply Operation

The LMH6624 and LMH6626 devices can be operated with single power supply as shown in Figure 56. Both the input and output are capacitively coupled to set the DC operating point.

Figure 56. Single Supply Operation