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LMH6554

0.1 PF

0.1 PF

LMH6554

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SNOSB30O – OCTOBER 2008 – REVISED MARCH 2013

When using the LMH6554 in single-to-differential mode, the complimentary output is forced to a phase inverted

replica of the driven output by the common mode feedback circuit as opposed to being driven by its own

complimentary input. Consequently, as the driven input changes, the common mode feedback action results in a

varying common mode voltage at the amplifier's inputs, proportional to the driving signal. Due to the non-ideal

common mode rejection of the amplifier's input stage, a small common mode signal appears at the outputs which

is superimposed on the differential output signal. The ratio of the change in output common mode voltage to

output differential voltage is commonly referred to as output balance error. The output balance error response of

the LMH6554 over frequency is shown in the Typical Performance Characteristics VS = ±2.5V.

These equations, along with the source matching condition, must be solved iteratively to achieve the desired gain

with the proper input termination. Component values for several common gain configuration in a 50

Ω

environment are given in Table 1.

Table 1. Gain Component Values for 50

Ω System

Gain

0dB

200

Ω

191

Ω

62

Ω

27.7

Ω

6dB

200

Ω

91

Ω

76.8

Ω

30.3

Ω

12dB

200

Ω

35.7

Ω

147

Ω

37.3

Ω

Single Supply Operation

Single 5V supply operation is possible: however, as discussed earlier, AC input coupling is recommended due to

input common mode limitations. An example of an AC coupled, single supply, single-to-differential circuit is

shown in Figure 32. Note that when AC coupling, both inputs need to be AC coupled irrespective of single-to-

differential or differential-differential configuration. For higher supply voltages DC coupling of the inputs may be

possible provided that the output common mode DC level is set high enough so that the amplifier's inputs and

outputs are within their specified operation ranges.

Figure 32. AC Coupled for Single Supply Operation

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