8.1.1 Input Characteristics

The LMH6881 has internally terminated inputs. The INMD and INPD pins are intended to be the differential input pins and have an internal 100-Ω resistive termination. An example differential circuit is shown in Figure 37. When using the differential inputs, the single-ended inputs should be left disconnected.

The INMS and INPS pins are intended to be used for single-ended inputs and have been designed to support single-ended termination of 50 Ω working as an active termination. For single-ended signals an external 50-Ω resistor is required as shown in Figure 38. When using the single-ended inputs, the differential inputs should be left disconnected.

All of the input pins are self biased to 2.5 V. When using the LMH6881 for DC-coupled applications it is possible to externally bias the input pins to voltages from 1.5 V to 3.5 V. Performance is best at the 2.5-V level specified. Performance will degrade slightly as the common mode shifts away from 2.5 V.

The first stage of the LMH6881 is a low-noise amplifier that can accommodate a maximum input signal of 2 V_ppd on the differential input pins and 1 V_pp on either of the single-ended pins. Signals larger than this will cause severe distortion. Although the inputs are protected against ESD, sustained electrical overstress will damage the part. Signal power over 13 dBm should not be applied to the amplifier differential inputs continuously. On the single-ended pins the power limit is 10 dBm for each pin.

8.1.2 Output Characteristics

The LMH6881 has a low-impedance output very similar to a traditional Op-amp output. This means that a wide range of loads can be driven with good performance. Matching load impedance for proper termination of filters is as easy as inserting the proper value of resistor between the filter and the amplifier (See Figure 47 for example.) This flexibility makes system design and gain calculations very easy. By using a differential output stage the LMH6881 can achieve large voltage swings on a single 5-V supply. This is illustrated in Figure 42. This figure shows how a voltage swing of 4 V_PPD is realized while only swinging 2 V_PP on each output. A 1-V_P signal on one branch corresponds to 2 V_PP on that branch and 4 V_PPD when looking at both branches (positive and negative).

Figure 42. Differential Output Voltage

The LMH6881 has been designed for both AC-coupled and DC-coupled applications. To give more flexibility in DC-coupled applications, the common mode voltage of the output pins is set by the OCM pin. The OCM pin needs to be driven from an external low-noise source. If the OCM pin is left floating, the output common mode is undefined, and the amplifier will not operate properly.

There is a DC gain of 2 between the OCM pin and the output pins so that the OCM voltage should be from 1 V to 1.5 V. This will set the output common mode voltage from 2 V to 3 V. Output common mode voltages outside the recommended range will exhibit poor voltage swing and distortion performance. The amplifier will give optimum performance when the output common mode is set to half of the supply voltage (2.5 V or 1.25 V at the OCM pin).

The ability of the LMH6881 to drive low-impedance loads while maintaining excellent OIP3 performance creates an opportunity to greatly increase power gain and drive low-impedance filters. This gives the system designer much needed flexibility in filter design. In many cases using a lower impedance filter will provide better component values for the filter. Another benefit of low-impedance filters is that they are less likely to be influenced by circuit board parasitic reactances such as pad capacitance or trace inductance. The output stage is a low-impedance voltage amplifier, so voltage gain is constant over different load conditions. Power gain will change based on load conditions. See Figure 43 for details on power gain with respect to different load conditions. The graph was prepared for the 26-dB voltage gain. Other gain settings will behave similarly.

All measurements in this data sheet, unless specified otherwise, refer to voltage or power at the device output pins. For instance, in an OIP3 measurement the power out will be equal to the output voltage at the device pins squared, divided by the total load voltage. In back terminated applications, power to the load would be 3 dB less. Common back terminated applications include driving a matched filter or driving a transmission line.

Figure 43. Power Gain as a Function of the Load

Printed-circuit-board (PCB) design is critical to high-frequency performance. To ensure output stability the load-matching resistors should be placed as close to the amplifier output pins as possible. This allows the matching resistors to mask the board parasitics from the amplifier output circuit. An example of this is shown in Figure 47. Also note that the low-pass filters in Figure 45 and Figure 46 use center-tapped capacitors. Having capacitors to ground provides a path for high-frequency, common-mode energy to dissipate. This is equally valuable for the ADC, so there are also capacitors to ground on the ADC side of the filter. The LMH6881EVAL evaluation board is available to serve as a guide for system board layout. See SNOA869 for more details.

8.1.3 Interfacing to an ADC

The LMH6881 is an excellent choice for driving high-speed ADCs such as the ADC12D1800RF, ADC12D1600RF or the ADS5400. The following sections will detail several elements of ADC system design, including noise filters, and AC- and DC-coupling options.

8.1.3.1 ADC Noise Filter

When connecting a broadband amplifier to an analog-to-digital converter, it is nearly always necessary to filter the signal before sampling it with the ADC. Figure 44 shows a schematic of a second order Butterworth filter, and Table 6 shows component values for some common IF frequencies. These filters offer a good compromise between bandwidth, noise rejection and cost. This filter topology is the same as is used on the ADC14V155KDRB High IF Receiver reference design board. This filter topology is adequate for reducing aliasing of broadband noise and will also provide rejection of harmonic distortion and many of the images that are commonly created by mixers.

Figure 44. ADC Noise Filter Schematic

Table 6. Filter Component Values⁽¹⁾

CENTER FREQUENCY	BANDWIDTH	R1, R2	L1, L2	C1, C2	C3	L5	R3, R4
75 MHz	40 MHz	90 Ω	390 nH	10 pF	22 pF	220 nH	100 Ω
150 MHz	60 MHz	90 Ω	370 nH	3 pF	19 pF	62 nH	100 Ω
180 MHz	75 MHz	90 Ω	300 nH	2.7 pF	15 pF	54 nH	100 Ω
250 MHz	100 MHz	90 Ω	225 nH	1.9 pF	11 pF	36 nH	100 Ω

(1) Resistor values are approximate, but have been reduced due to the internal 10 Ω of output resistance per pin.

8.1.3.3 DC Coupling to ADC

The LMH6881 supports DC-coupled signals. In order to successfully implement a DC-coupled signal chain the common-mode voltage requirements of every stage need to be met. This will require careful planning, and in some cases there will be signal level, gain or termination compromises required to meet the requirements of every part. Figure 45 and Figure 46 show a method using resistors to change the 2.5-V common mode of the amplifier output to a common mode compatible for the input of a low-input-voltage ADC such as the ADC12D1800RF. This DC level shift is achieved while maintaining an AC impedance match with the filter in Figure 45, while in Figure 46 there is a small mismatch between the amplifier termination resistors and the ADC input. Because there is no universal ADC input common mode and some ADCs have impedance controlled input, each design will require a different resistor ratio. For high-speed data conversion systems it is very important to keep the physical distance between the amplifier and the ADC electrically short. When connections between the amplifier and the ADC are electrically short, termination mismatches are not critical.

Figure 45. DC-Coupled ADC Driver Example 1, High-Input Impedance ADC

Figure 46. DC-Coupled ADC Driver Example 2, Terminated Input ADC

8.1.4 Figure of Merit: Dynamic Range Figure

The dynamic range figure (DRF) as illustrated in Figure 4, is defined as the input third order intercept point (IIP3) minus the noise figure (NF). The combination of noise figure and linearity gives a good proxy for the total dynamic range of an amplifier. In some ways this figure is similar to the SFDR of an analog-to-digital converter. In contrast to an ADC, though, an amplifier will not have a full-scale input to use as a reference point. With amplifiers, there is no one point where signal amplitude hits “full scale”. Yet, there are real limitations to how large of a signal the amplifier can handle. Normally, the distortion products produced by the amplifier will determine the upper limit to signal amplitude. The intermodulation intercept point is an imaginary point that gives a well understood figure of merit for the maximum signal an amplifier can handle. For low-amplitude signals the noise figure gives a threshold of the lowest signal that the amplifier can reproduce. By combining the third-order input intercepts point and the noise figure the DRF gives a very good indication of the available dynamic range offered.

Table 7. Compatible High Speed ADCs

8.2.1 LMH6881 Typical Application

Figure 47. LMH6881 Typical Application

8.2.1.3 Application Curves

Figure 48. OIP3 vs Frequency

Figure 49. Noise Figure vs Voltage Gain

8.2.2 LMH6881 Used as Twisted-Pair Cable Driver

Figure 50. LMH6881 Used as Twisted-Pair Cable Driver

8.2.2.3 Application Curves

Figure 51. Output Power vs Input Power

Figure 52. Common Mode Rejection (Sdc21) vs Frequency