Inputs

The LMH6518 evaluation board is designed to be used with either differential and single-ended input configurations, however, the LMH6518 evaluation board is shipped out in a single-ended input configuration. In this configuration, the evaluation board is configured for single-ended drive (into the positive input, IN+ SMA connector) and the undriven input is biased to VCC/2 (= 2.5V). This single-ended input must be offset to 2.5V DC (< ±1mV of the undriven input voltage at pin 7 of the LMH6518). Here are possible input sources:

• Lab generator (or other signal source) capable of 2.5V offset in the presence of milli-volt level input signal.

• Using a ground referenced (AC-coupled or no offset adjust needed) generator with a Bias-Tee module that allows DC value to be set to 2.5V using a fine-tuned voltage lab DC power supply.

• A Hi-Z buffer that outputs the required milli-volt level AC signal centered at 2.5V from a ground referenced input signal. In oscilloscope applications, this Hi-Z buffer is what interfaces the high-impedance oscilloscope probe to the LMH6518.

For best performance, make sure that the LMH6518 inputs (pins 6 and 7) are both close to VCC/2 (= 2.5V) and there is less than 1mV of difference in voltage between them. On the board, use a DC voltmeter to confirm DC voltage of pins 6 and 7 (top pad of the R7 and right pad of the C23). The voltage potential of pin 6 (driven input) of the LMH6518 can be controlled by varying the incoming signal offset (2.5V nominal) at the SMA connector.

As the DC offset of the inputs go beyond 1mV, measured results can be affected. Some of the side-effects of input dc offset are: output clipping, excess distortion, loss of bandwidth, or step response anomalies. Potentiometer R4 is provided to null any remnant offset, especially useful when the operating conditions change of the device. For an effective method of input offset adjustment, especially when the LMH6518 is set for high gain (HG/ 0dB attenuation), monitor OUT+ and OUT- (1.2V nominal) with a DC voltmeter and adjust R4 to minimize the voltage difference between these two outputs to less than 50mV. This effectively reduces any input voltage offset.

To configure from single-ended to differential input, remove resistor R5 and solder a 0Ω 0603 resistor to R7.

Outputs

Both the main and auxiliary output differential impedance needs to be close to 100Ω with minimal parasitic effects when being measured. As these outputs are each set to approximately 1.2V common mode level, the outputs cannot be terminated in DC coupled 50Ω directly. (similar to an oscilloscope input) AC coupled 50Ω terminated instrumentation (such as S parameter analyzers) can be connected to these SMA outputs directly for observation. With a single-ended Hi-Z input instrument, the LMH6518 outputs must be terminated in 100Ω differential, as shown in Figure 2-8.

Figure 2-8 LMH6518 Output Observation With Hi-Z Instrumentation

The auxiliary outputs must be terminated in 100Ω on board by installing R16 when observing the main output. The evaluation board comes with R16 populated.

The outputs of the device need to be biased near 1.2V by applying the appropriate voltage to the VOCM (VOCM_Aux) pin for the main (auxiliary) output of the LMH6518. The board has the option of biasing these pins using onboard 1.2V reference circuitry or can be tied to an external bias such as the common mode output control of an ADC. How to configure the shunt jumpers to the appropriate positions for either configuration is shown in Section 2.3.