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the datasheet-specified, input common-mode-volt-

age-range limits. For AC-coupled input operation,

the input common mode will be at the same

pin voltage. For DC-coupled single-to-differential

operation, the input common-mode voltage is the

output common-mode voltage divided down by

supplies such as ±5V. However, for single-supply

operation such as GND and +10V, the divided-

down-output common-mode voltage, appearing as

the input common-mode voltage, must not exceed

the amplifier’s rated operating-input common-

mode-voltage range. Lack of headroom on the input

can force the use of a negative rail lower than ground.

For the best distortion performance, unlimited by

amplifier input or output headroom, split ±5V

supplies are recommended.

An ideal amplifier for ADC driving would be

completely transparent to the ADC and not degrade

its performance. Although this is a challenge, it is

possible to minimize performance degradation. From

a DC-specification perspective, the most fundamental

amplifier requirement is that the output-voltage range

of the amplifier supports the ADC-input-voltage

range for full-scale output. From an AC perspective,

the amplifier must have flat bandwidth and gain

such that the wanted signal is not attenuated by the

amplifier’s frequency response, as well as low-enough

noise and distortion levels that they do not impact

the ADC’s performance.

The required amplifier bandwidth is dictated by the

input-signal-frequency spectrum to be processed and

requirements for good distortion performance at high

frequencies. The maximum signal frequency and the

ADC’s full-scale input-voltage level determine the

required amplifier slew rate and Large Signal Band-

width (LSBW). These specifications determine the

channel bandwidth when driving the ADC input at

full scale.

Due to the roll off of the amplifier open-loop gain,

amplifier distortion starts to degrade at frequen-

cies much lower than the amplifier LSBW. In the

case of VFB amplifiers with a fixed-gain-bandwidth

product, the amount of available gain for very-high-

frequency signals can be limited. A CFB amplifier

with relatively wide-gain independent bandwidth

and excellent gain flatness is a good choice for

very-high-frequency signals. The actual gain flat-

ness required will depend on the application

requirements.

Assuming all of the other AC and DC specifications

can be met, noise and distortion will ultimately be the

two main specifications of interest for a given ADC

and amplifier combination since these determine

the SINAD. The ENOB can be calculated from the

SINAD using the equation:

ENOB = (SINAD - 1.76)/6.02, where SINAD is in dB

Since distortion and noise are specified separately

for the amplifier and ADC, it is necessary to look

at how combining the amplifier with the ADC

affects the overall subsystem’s performance. The

noise of the driving amplifier and the noise of the

ADC are uncorrelated and can be rss-summed

together for the purpose of analysis. In order

for the amplifier noise not to degrade the ADC

performance, the amplifier output noise over the

frequency band of interest ideally should be at least

6 dB less than the ADC input noise.

The amplifier output-noise voltage spectral density

measured in V/ Hz, is calculated by root-sum-squaring

the output-noise voltage contributions arising from the

amplifier’s input-voltage noise and current noise, with

the additional noise of any external resistors around

the amplifier. The total noise seen at the ADC input

depends on the channel bandwidth, so it is critical to

optimize the design for minimum acceptable band-

width in order to maximize noise performance. Unless

limited by a channel filter, noise and distortion prod-

ucts from the entire amplifier bandwidth will all be

sampled by the ADC and aliased back down into the

the channel filter is chosen to limit the amplifier-noise

bandwidth and attenuate any distortion products.

Ideally, any in-band, amplifier-distortion products

should be 6 dB lower than the ADC’s own distortion

products. Choosing the sample frequency carefully

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9/5/07 3:24:35 PM

9/5/07 3:24:35 PM