LMH6550 datasheet(13/17 Pages) NSC

13 / 17 page

Application Section (Continued)

SINGLE ENDED INPUT TO DIFFERENTIAL OUTPUT

The LMH6550 provides excellent performance as an active

balun transformer. Figure 3 shows a typical application

where an LMH6550 is used to produce a differential signal

from a single ended source. It should be noted that com-

pared to differential input, using a single ended input will

reduce gain by 1/2. So that the closed loop gain will be; Gain

= Av=0.5*R

F/RG.

In single ended input operation the output common mode

voltage is set by the V

CMpin as in fully differential mode.

Also, In this mode the common mode feedback circuit must

recreate the signal that is not present on the unused differ-

ential input pin. The performance chart titled “Balance Error”

is the measurement of the effectiveness of this process. The

common mode feedback circuit is responsible for ensuring

balanced output with a single ended input. Balance error is

defined as the amount of input signal that couples into the

output common mode. It is measured as a the undesired

output common mode swing divided by the signal on the

input. Balance error can be caused by either a channel to

channel gain error, or phase error. Either condition will pro-

duce a common mode shift. The chart titled “Balance Error”

measures the balance error with a single ended input as that

is the most demanding mode of operation for the amplifier.

Supply and V

CMpin bypassing are also critical in this mode of

operation. See the above section on FULLY DIFFERENTIAL

OPERATION for bypassing recommendations also see Fig-

ure 4 and Figure 5 for recommended supply bypassing

configurations.

SINGLE SUPPLY OPERATION

The input stage of the LMH6550 has a built in offset of 0.7V

towards the lower supply to accommodate single supply

operation with single ended inputs. As shown in Figure 6, the

input common mode voltage is less than the output common

voltage. It is set by current flowing through the feedback

network from the device output. The input common mode

range of 0.4V to 3.2V places constraints on gain settings.

Possible solutions to this limitation include AC coupling the

input signal, using split power supplies and limiting stage

gain. AC coupling with single supply is shown in Figure 7.

In Figure 6 below closed loop gain = A

V=RF/RG. Please note

that in single ended to differential operation V

IN is measured

single ended while V

OUT is measured differentially. This

means that gain is really 1/2 or 6 dB less when measured on

either of the output pins separately.

ICM= Input common mode voltage = (V

IN+V

−

IN)/2.

DRIVING ANALOG TO DIGITAL CONVERTERS

Analog to digital converters (ADC) present challenging load

conditions. They typically have high impedance inputs with

large and often variable capacitive components. As well,

there are usually current spikes associated with switched

capacitor or sample and hold circuits. Figure 8 shows a

typical circuit for driving an ADC. The two 56

Ω resistors

serve to isolate the capacitive loading of the ADC from the

amplifier and ensure stability. In addition, the resistors form

part of a low pass filter which helps to provide anti alias and

noise reduction functions. The two 39 pF capacitors help to

smooth the current spikes associated with the internal

switching circuits of the ADC and also are a key component

in the low pass filtering of the ADC input. In the circuit of

Figure 8the cutoff frequency of the filter is 1/ (2*

π*56Ω *(39

pF + 14pF)) = 53MHz (which is slightly less than the sam-

pling frequency). Note that the ADC input capacitance must

be factored into the frequency response of the input filter,

and that being a differential input the effective input capaci-

tance is double. Also as shown in Figure 8 the input capaci-

tance to many ADCs is variable based on the clock cycle.

See the data sheet for your particular ADC for details.

20130111

FIGURE 6. Relating A

Vto Input/Output Common Mode

Voltages

20130109

FIGURE 7. AC Coupled for Single Supply Operation

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