8

SIGNAL PATH

designer

with respect to the wanted signal can prevent dis-

tortion products appearing all over the baseband.

In applications involving multiple closely-spaced-

frequency tones, good Intermodulation Distortion

(IMD) and related third-order Output Intercept

Power (OIP3) are required from the amplifier. This

minimizes difference-frequency distortion products

that are otherwise created too close to the signal of

interest to be filtered out. Ideally, any distortion

specifications quoted for the amplifier should be

for the signal level and load conditions presented

by the application. In many applications where

the dynamic range is increased by processing gain,

distortion will be the number one concern in maxi-

mizing resolution.

The final stage before the ADC is the noise filter.

The simplest solution for a DC-coupled baseband

application is a passive first-order low-pass RC. For

this simple first-order filter, the -3 dB frequency,

effective-noise bandwidth for noise calculations is

to meet specific passband-flatness needs based on

various filter polynomials such as Butterworth,

Bessel, and Chebyshev. These will give sharper roll

off and lower noise bandwidths in addition to mak-

ing it easier to meet the sharp roll-off requirements of

Figure 2c. An example of a first-order low-pass filter is

shown in Figure 7 where National’s new LMH6552

1 GHz fully-differential amplifier drives one half

of a dual ADC12DL080 12-bit 80 MSPS ADC

via a 65 MHz first-order low-pass filter, formed by

the two series 125Ω-output resistors and the

2.2 pF-output capacitor in parallel with the ADC’s

input capacitance.

Figure 8 shows the LMH6552 and ADC12DL080

Spurious Free Dynamic Range (SFDR) and SNR

performance versus frequency.

The LMH6552 amplifier is based on a CFB

architecture and consequently delivers relatively

constant bandwidth as the gain is varied. For example,

the unity gain LSBW at 2 Vp-p output is 950 MHz,

and for higher gains the BW reduction is small with

820 MHz at G = 2, 740 MHz at G = 4, and 590 MHz

at G = 8. A VFB device would require almost 5 GHz

gain-bandwidth product to achieve 590 MHz BW at

G = 8.

The LMH6552 is ideal for a range of 8- to

14-bit applications depending on the specific

speed, distortion, and noise requirements of the end

application. Optimum performance is delivered on

split ±5V supplies but the LMH6552 will also run

on single supplies as low as single 5V. The amplifier

input-voltage noise is 1nV/ Hz and the input-

current noise is 19.5 pA/ Hz. The output noise

is strongly influenced by the input-current noise

so strongly by the input-voltage noise and closed-

loop gain, as would be the case for voltage-feedback

amplifiers. Consequently, the LMH6552 device

can operate at much higher values of gain without

12-bit 80 MSPS

ADC12DL080

125

Ω

2.2 pF

LMH6552

125

Ω

0

5

10

15

202530

35

40

Input Frequency (MHz)

50

55

60

65

70

75

80

85

90

SFDR (dBc)

SNR (dBFs)

Selecting Amplifiers, ADCs, and Clocks for High-Performance Signal Paths

Figure 7. The LMH6552 Amplifier driving the ADC12DL080

converter

Figure 8. LMH6552 and ADC12DL080 SFDR and SNR

performance vs frequency

1

2πRC

=

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